phase: 167_api_payload_diet

All gates green. Final report:

**Verified (phase 167 final pass — all 6 tasks already in `complete/`)**
- Live re-measure of the headline criterion on the **1,000-doc live-replica corpus** (restored `data/bor_live_kb_replica.dump.sql` into a scratch DB `bor_audit`, alembic 0011→0027, dropped after; the shared dev DB was never touched): `GET /api/docs/tree` default depth=1 = **8,474 identity → 1,128 B with `Content-Encoding: gzip`** (< 40 KB bar; ~2 MB defect fixed), repeat load **304 / 0 bytes**; depth=8 265,147→23,515; node drill 20,844→1,065; `/api/docs` 246,366→45,104; `/api/documents/content` 892,124→60,776 (`LOOK`, accepted-with-why). Reproduces `AUDIT.md` to within 1 byte.
- Transport contracts proven live: `POST /api/chat` **no** `Content-Encoding` + frames arriving live + budget line `… enc=identity duration_ms=7380 stream=1`; HTML still `no-cache` with the `?v=` rewrite (and gzipped); A20 headers on 200 **and** 304; ETag identical across identity/gzip; `depth=0|9|abc`→422; anonymous tree/node→403; unknown folder/source→404 `unknown node`.

**Gates (all 100% green)**
- `uv run pytest` → **3980 passed**; `uv run pytest --cov=app --cov-report=term-missing` → **TOTAL 99%** (`compression.py`, `http_cache.py`, `response_budget.py` 100%; `api/docs.py` 99%, one defensive dedupe line).
- `uv run ruff check .` → All checks passed; `uv run pyright` → 0 errors.
- Phase + regression E2E **in isolation** (`--junitxml`, exit codes): `test_kb_tree_lazy_load` 11, `test_kb_tree` 8, `test_kb_tree_nav` 4, `test_navbar_refresh` 7, `test_document_dates` 6, `test_git_source_dates` 4, `test_sync_summary_visibility` 3, `test_table_ux_polish` 6, `test_sources_midstream_bug` 6, `test_ls_tree_drilldown` 3, `test_smoke` 3 — **0 failures/errors** (the earlier `-q -q` run had simply suppressed pytest's summary line).

**Documentation** — README.md + AGENTS.md reviewed in full (1,436 / 80 lines):
- Fixed (pre-existing, phase-167-exposed): README *Caching & deploys* claimed "One Starlette middleware (`app/core/caching.py`) applies the rule" while the section now describes three transport layers → rewritten to name caching/compression/http_cache, the real outermost→innermost order, and why `bytes=` is uncompressed.
- AGENTS.md rule 5 (UI Structure Check) gained the phase-167 clause: a lazily-drilled level keeps the current level rendered, its in-flight `role="status"` / `role="alert"`+Retry row lives in the same bounded scroll panel, and stat cards read server `totals` (never a walk of a depth-limited tree).
- Cross-checked and consistent: every `BOR_*` in `.env.example` is in the README table, every `Settings` field is documented, all 15 README-cited API routes exist in the app, no stale "one fetch / drill client-side" claim remains anywhere.

**Notable decisions / deviations**
- `compression.py` adds a documented `_RebufferSmallResponses` seam beyond the D1 sketch (the caching layer's `BaseHTTPMiddleware` re-framing otherwise defeats Starlette's `minimum_size` exemption and inflates tiny responses) — integration-pinned, math still Starlette's.
- A node-route **404 = vanished folder → honest reset to top level** (per `00_phase.md` D4/D6), not task-06's "alert + Retry"; 500 shows alert+Retry with the level preserved — both pinned, nothing weakened.
- Retrieval battery not measurable (shared E2E DB holds no corpus), but `git diff --name-only -- app/rag` is empty, so retrieval is unchanged by construction. `AUDIT.md` left byte-identical (it lives under `todo/`, off-limits to this pass); no git index/history commands run.

**Next pending phase:** none — `167_api_payload_diet` is the only entry in `.agents/phases/todo/`; the harness moves it to `complete/` and commits.

.agents/phases/todo/167_api_payload_diet/AUDIT.md

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+# Phase 167 · Task 05 — Measured API payload audit (the byte budget)
+
+Owner TODO L5: "GET `/api/docs/tree` loads 2 MB in production … Fix that **any any
+other endpoints** that could load a ton of data." This file is the measured answer —
+every number below is pasted from `scripts/api_payload_audit.py` (or the `curl` /
+server-log captures), never hand-written. LOCKED A5: the budget is a log line plus a
+report, never a 413 — nothing was rejected, nothing was capped.
+
+## How it was run
+
+| | |
+|---|---|
+| Date | 2026-10-08 |
+| Corpus | the **1,000-document live-replica snapshot** (`data/bor_live_kb_replica.dump.sql`, the `restore-test-db` skill) — fingerprint verified `docs=1000`, `chunks=8866`, `sources=deploy,homelab,homepage,ServMon,skills,vibe-bot`; the snapshot's `alembic=0011` was upgraded to head `0027` afterwards (the phase-119/162 precedent) |
+| App | `uv run uvicorn app.main:app --port 8000` (local dev, this working tree — tasks 01-04 of this phase included) |
+| BEFORE run | the same app with `BOR_COMPRESS_MIN_BYTES=0` — the gzip layer is **not added at all**, i.e. byte-for-byte the transport the pre-phase app had |
+| AFTER run | the same app with the shipped defaults (`BOR_COMPRESS_MIN_BYTES=860`, depth default 1, ETag on the three catalogue reads) |
+| Command | `uv run python -m scripts.api_payload_audit --base-url http://localhost:8000` (budget default `262144`) |
+| Budget log capture | `BOR_LOG_LARGE_RESPONSE_BYTES=0` (the loud mode — every `/api/*` response at INFO), used only for the log excerpts below |
+
+The script is **read-only**: it signs in through `POST /api/login` (the session
+cookie — the single-admin login is password-only, PLAN A10, so there is no
+`--username`) and GETs. It never mutates, so it is safe against a production target
+(`--base-url https://…`).
+
+### Corpus caveat (read this before comparing with the 2 MB report)
+
+The 2026-09-05 snapshot carries **0 `folder_summaries` rows and 0 image documents**
+(verified: `select count(*) from folder_summaries` → `0`,
+`select count(*) from documents where is_image` → `0`). The production 2 MB payload
+was 1,000 documents **plus** the hundreds of inlined AI folder descriptions
+(phase 127/155) and the image documents' full vision descriptions (phase 122/124) —
+none of which exist in the snapshot. **So the identity bytes below are a lower bound
+on the production payload**, and what transfers from this measurement is the
+*structure*, i.e. the ratios: the depth limit cut the tree by **31×** (it ships the
+ONE level the RAG view renders instead of the whole recursion below it — and every
+level of a summary-carrying folder is where the production payload's fat lives),
+gzip bought a further **5.5× to 20×** across these payloads (**7.5×** on the depth-1
+tree, **11×** on the full tree), and a repeat load costs **0 bytes**. The verdict
+table below names the live re-run as the follow-up.
+
+## BEFORE — no gzip middleware (`BOR_COMPRESS_MIN_BYTES=0`), full tree available
+
+> Read the BEFORE table's `gzip` column as "no compressor existed": ratio
+> `1.000` on every row, the route's bytes ARE the transfer. Its `304` verdicts
+> are NOT a pre-phase property — the conditional re-requests hit the validators
+> tasks 01-04 added, which this run left in place because only the transport was
+> being switched off (the honest way to measure "the same bytes, uncompressed").
+> Pre-phase there were no validators at all, so every repeat load cost the
+> identity column again.
+
+```
+api_payload_audit: base=http://localhost:8000 budget=262,144 bytes (LOOK threshold)
+api_payload_audit: corpus documents=1000 chunks_total=8866 sources=6 tree totals documents=1000 chunks=8866 saved_chats=0 biggest_doc=tests/fixtures/advisory/bodhi_exiv2.json
+  endpoint                                                                identity        gzip  ratio      ms  verdict
+  ----------------------------------------------------------------------------------------------------------------
+  tree depth=1 (default)                                                     8,474       8,474  1.000      12  304
+  tree depth=8 (full)                                                      265,147     265,147  1.000      37  304
+  tree node homelab/active                                                  20,844      20,844  1.000      11  304
+  docs catalogue                                                           246,366     246,366  1.000      14  304
+  document content (biggest: tests/fixtures/advisory/bodhi_exiv2.json)     892,124     892,124  1.000       8  LOOK (> 262144)
+      note: content field 772573 chars
+  saved chats list                                                              12          12  1.000       1  ok
+  saved chat (one)                                                               —           —      —       —  n/a
+      note: no saved chats
+  git sources                                                                  188         188  1.000       1  ok
+  suggestions                                                                   74          74  1.000       1  ok
+  sync status (polled)                                                         272         272  1.000       1  ok
+  tokens                                                                        13          13  1.000       2  ok
+  config                                                                       188         188  1.000       1  ok
+  whoami                                                                        37          37  1.000       1  ok
+api_payload_audit: ms is the gzip (real-client) variant; identity is the route's own bytes — the pair the ' :: api ' log line reports.
+```
+
+What the pre-phase browser actually paid for the **Sources page first paint** is the
+sum of the rows it fetches on load, and the pre-phase tree route answered the FULL
+recursive tree by default (`tree depth=8 (full)` is that shape):
+`265,147 + 246,366 + 188 + 188 + 37 = 511,926 bytes identity`, and the pre-phase app
+published **no validators at all**, so every repeat load re-paid the same 512 KB.
+
+## AFTER — shipped defaults (depth-1 default + gzip + ETag)
+
+api_payload_audit: base=http://localhost:8000 budget=262,144 bytes (LOOK threshold)
+api_payload_audit: corpus documents=1000 chunks_total=8866 sources=6 tree totals documents=1000 chunks=8866 saved_chats=0 biggest_doc=tests/fixtures/advisory/bodhi_exiv2.json
+  endpoint                                                                identity        gzip  ratio      ms  verdict
+  ----------------------------------------------------------------------------------------------------------------
+  tree depth=1 (default)                                                     8,474       1,129  0.133      13  304
+  tree depth=8 (full)                                                      265,147      23,520  0.089      41  304
+  tree node homelab/active                                                  20,844       1,064  0.051      11  304
+  docs catalogue                                                           246,366      45,107  0.183      23  304
+  document content (biggest: tests/fixtures/advisory/bodhi_exiv2.json)     892,124      60,776  0.068      14  LOOK (> 262144)
+      note: content field 772573 chars
+  saved chats list                                                              12          12  1.000       2  ok
+  saved chat (one)                                                               —           —      —       —  n/a
+      note: no saved chats
+  git sources                                                                  188         188  1.000       1  ok
+  suggestions                                                                   74          74  1.000       2  ok
+  sync status (polled)                                                         272         272  1.000       1  ok
+  tokens                                                                        13          13  1.000       2  ok
+  config                                                                       188         188  1.000       1  ok
+  whoami                                                                        37          37  1.000       1  ok
+api_payload_audit: ms is the gzip (real-client) variant; identity is the route's own bytes — the pair the ' :: api ' log line reports.
+```
+
+Sources-page first paint, same sum against the AFTER gzip column:
+`1,129 + 45,107 + 188 + 188 + 37 = 46,649 bytes on the wire` (from 511,926 identity,
+and every one of those five rows revalidates to **304 / 0 bytes** on the next load).
+The tree itself — the endpoint TODO L5 named — goes **265,147 → 1,129 = 235×**
+(31× from the depth limit × 7.5× from gzip), and the second visit is free.
+
+## `curl` proof on the audited corpus
+
+```
+$ curl -s -H 'Accept-Encoding: identity' -D- -o b1.json http://localhost:8000/api/docs/tree
+  etag: W/"53c86d5d554efd8487ad52ca6cd6f913"
+  cache-control: private, must-revalidate
+  content-length: 8474
+  vary: Cookie, Accept-Encoding
+  8474 bytes                                     # wc -c b1.json
+
+$ curl -s -H 'Accept-Encoding: gzip' -D- -o b2.gz http://localhost:8000/api/docs/tree
+  etag: W/"53c86d5d554efd8487ad52ca6cd6f913"     # the validator survives the encoding
+  content-encoding: gzip
+  1129 bytes                                     # wc -c b2.gz
+
+$ curl -s -H 'If-None-Match: W/"53c86d5d554efd8487ad52ca6cd6f913"' -o /dev/null \
+       -w 'status=%{http_code} size=%{size_download}\n' http://localhost:8000/api/docs/tree
+status=304 size=0
+```
+
+## The byte-budget log line (the answer to "which endpoint is heavy, right now")
+
+Captured with `BOR_LOG_LARGE_RESPONSE_BYTES=0` (every `/api/*` response at INFO);
+the shipped default is `65536`, so at rest only the big ones reach the log.
+`grep ' :: api ' ` isolates them. `bytes=` is what the ROUTE produced, `enc=` what
+went on the wire — the two together are the "how fat, and how fat did the client see
+it" pair.
+
+```
+INFO app.response_budget :: api method=GET path=/api/docs/tree status=200 bytes=8474 enc=identity duration_ms=14
+INFO app.response_budget :: api method=GET path=/api/docs/tree status=200 bytes=8474 enc=gzip duration_ms=12
+INFO app.response_budget :: api method=GET path=/api/docs/tree status=304 bytes=0 enc=identity duration_ms=11
+INFO app.response_budget :: api method=GET path=/api/docs/tree status=200 bytes=265147 enc=identity duration_ms=34
+INFO app.response_budget :: api method=GET path=/api/docs/tree status=200 bytes=265147 enc=gzip duration_ms=40
+INFO app.response_budget :: api method=GET path=/api/docs/tree/node status=200 bytes=20844 enc=gzip duration_ms=10
+INFO app.response_budget :: api method=GET path=/api/docs status=200 bytes=246366 enc=gzip duration_ms=22
+INFO app.response_budget :: api method=GET path=/api/documents/content status=200 bytes=892124 enc=gzip duration_ms=12
+INFO app.response_budget :: api method=GET path=/api/sync/status status=200 bytes=272 enc=identity duration_ms=0
+INFO app.response_budget :: api method=GET path=/api/whoami status=200 bytes=37 enc=identity duration_ms=0
+```
+
+`POST /api/chat` (SSE, A15) is not in the audit table — a stream has no bounded
+payload to budget — and it is skipped by the gzip layer by exact path (which is why
+its line reads `enc=identity`). One real turn against this corpus, measured:
+
+```
+INFO app.response_budget :: api method=POST path=/api/chat status=200 bytes=39527 enc=identity duration_ms=45002 stream=1
+```
+
+39,527 bytes of SSE frames streamed, `duration_ms` the stream's life (this capture
+ended because the measuring `curl --max-time 45` hung up — which is itself the
+other half of the contract: a stream the client walks away from still writes
+exactly ONE line, with the bytes that actually got out, never one line per frame
+and never a missing line). Pinned by
+`tests/unit/test_response_budget.py::test_a_cancelled_stream_still_logs_one_line_with_what_got_out`
+and by the integration pin
+`tests/integration/test_chat_api.py::test_chat_turn_logs_one_budget_line_with_the_stream_flag`.
+
+## Verdict per candidate endpoint
+
+| Endpoint | Measured (identity / wire) | Verdict |
+|---|---|---|
+| `GET /api/docs/tree` (default `depth=1`) | 265,147 → **8,474** identity, **1,129** gzip, repeat **304 / 0** | **FIXED** — the TODO L5 defect. Depth-limited default (task 02) + selective gzip (task 01) + ETag (task 03). Phase criterion "< 40 KB gzipped" met at 1,129 B with a 1,000-document corpus. |
+| `GET /api/docs/tree?depth=8` | 265,147 / 23,520 gzip, repeat 304 / 0 | **FIXED (bounded by the route's own contract)** — the explicit full tree stays available for tooling and the migrated date suites; it compresses 11× and revalidates to 0. Nothing asks for it in the UI anymore (`grep` of `frontend/` shows only `depth=1` + the node route). |
+| `GET /api/docs/tree/node` (the drill) | 20,844 / 1,064 gzip, repeat 304 / 0 | **FIXED** — one level per click (task 03/04), so the level the RAG view renders is ~1 KB on the wire instead of a re-ship of the corpus. |
+| `GET /api/docs` (the catalogue) | 246,366 / 45,107 gzip, repeat 304 / 0 | **FIXED within scope**, and a **watch item**: 246 KB identity is 6 KB under the 256 KB LOOK bar. It already carries the ETag pair (task 03) and gzips 5.5×. Follow-up if the corpus grows: `depth`-style paging or a per-source filter param — named, not applied (A5: raise nothing, reject nothing). |
+| `GET /api/documents/content` (biggest doc, 772 KB JSON) | 892,124 / 60,776 gzip | **ACCEPTED with why** — one document, on demand, when the owner opens it from a citation; the wire cost is 60 KB gzipped, and LOCKED A5 says the app's own documents are legitimately large. **Follow-up (named, not applied):** task 03 recorded this route as deliberately un-validated; if the `:: api ` log shows repeat 200s for the same document, give it the ETag pair keyed on `(source, path, content_hash)` (the row is already loaded, so the hash is free). |
+| `GET /api/chats` | 12 (empty corpus) | **ok** — one row per saved chat, no payloads in the list (phase 50 shape). Re-measure on the live corpus with the saved chats. |
+| `GET /api/chats/{id}` | n/a — `no saved chats` in the snapshot | **ACCEPTED with why** — the owner opening ONE saved conversation; the payload is that chat's raw `bor.chat.v1` JSONB, which is exactly what the page must render. Not measurable on this snapshot (the replica carries 0 saved chats) — **follow-up**: re-run this script against the live app, where the chats exist. |
+| `GET /api/git-sources` | 188 / 188 (under the 860 floor → identity, correctly) | **ok** — the registry is a handful of rows; no remedy needed, and task 05 item 6's conditional ETag is therefore NOT applied (it grows past the budget only with hundreds of sources). |
+| `GET /api/suggestions` | 74 | **ok** — three opening questions (phase 103). |
+| `GET /api/sync/status` | 272 | **ACCEPTED with why (polled)** — tiny, but the UI polls it every 2 s, which is precisely why the byte-budget line exists: the log shows a chatty poll, and 272 B says the poll is not a payload problem. |
+| `GET /api/tokens` | 13 | **ok** — secret-free list, one row per token (phase 79 A4). |
+| `GET /api/config` | 188 | **ok** — display strings only. |
+| `GET /api/whoami` | 37 | **ok** — two fields. |
+| `POST /api/chat` (SSE) | not auditable as a payload (skipped by gzip, A15) | **UNCHANGED by design** — the log line covers it with ` stream=1`; frame-by-frame delivery is pinned by `tests/unit/test_compression.py` and `tests/integration/test_chat_api.py`. |
+
+## What the audit changed, and what it proved
+
+* **No new remedy was needed beyond tasks 01-04.** The only over-budget row
+  (`/api/documents/content`) is the documented accepted case, and
+  `/api/git-sources` measures 188 bytes, so task 05 item 6's conditional ETag
+  was **not** applied — a validator on a sub-860-byte response costs a hash
+  and saves nothing (it would not even compress).
+* **The browser confirms the wire.** `tests/e2e/test_kb_tree_lazy_load.py` (task 06)
+  drives the real Sources view in Chromium and pins what this file measured: the
+  boot is ONE `?depth=1` request whose body carries `totals` and no deep children,
+  the response arrives `Content-Encoding: gzip` with `Accept-Encoding` in `Vary`
+  (Chromium's own negotiation — no stub), a reload re-sends `If-None-Match` and the
+  wire answers `304` for a fraction of the first transfer while the table still
+  renders from the cache, and a level the payload only names costs exactly one
+  `/api/docs/tree/node` fetch (zero on every re-entry). Because Chromium hands
+  `fetch` the cached 200 on a revalidation, that last pair is asserted on the CDP
+  Network events — the DevTools-panel truth, not Playwright's post-cache view.
+* **The depth limit is where the win is, not gzip.** depth-1 vs depth-8 on
+  the same corpus is 8,474 vs 265,147 identity bytes (**31×**) — gzip is the
+  second ~11× on top of it (265,147 → 23,520), and the ETag makes the repeat
+  load free. Had the phase shipped gzip alone, the Sources page would still
+  have paid 23 KB/request forever, and on the summary-inflated production
+  corpus proportionally more.
+* **`/api/docs` is the next one to watch** (246 KB identity, 6 KB under the
+  LOOK bar, already validated + compressed). The byte-budget line at its
+  default `65536` threshold logs it at INFO on every uncached load, so the
+  corpus growth that pushes it over shows up in the log before it shows up
+  for the owner.
+* **Two limits of this measurement, stated instead of hidden:** (1) the
+  snapshot has no folder summaries and no image docs, so the identity
+  numbers are a lower bound on the production payload (see the caveat
+  above); (2) it has no saved chats, so `/api/chats/{id}` is `n/a` rather
+  than measured — the script is read-only by design (it must stay safe
+  against the deployed app), so it does not invent a chat to size. Both are
+  re-measurable with one command against the live corpus.
+
+## Re-running this audit
+
+```bash
+# 1. the corpus (destructive to the local DB — see the skill's own backup step)
+#    restore-test-db skill: data/bor_live_kb_replica.dump.sql → brain_of_reese
+uv run alembic upgrade head                      # the snapshot carries an old alembic version
+# 2. the app
+uv run uvicorn app.main:app --port 8000
+# 3. the measurement (BOR_ADMIN_PASSWORD from .env; --password to override)
+uv run python -m scripts.api_payload_audit --base-url http://localhost:8000
+# the BEFORE transport, for a before/after pair like the tables above:
+BOR_COMPRESS_MIN_BYTES=0 uv run python -m scripts.api_payload_audit
+```

.agents/reports/167_api_payload_diet/167_api_payload_diet__00_phase.a1.md

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+All gates green. Final report:
+
+**Verified (phase 167 final pass — all 6 tasks already in `complete/`)**
+- Live re-measure of the headline criterion on the **1,000-doc live-replica corpus** (restored `data/bor_live_kb_replica.dump.sql` into a scratch DB `bor_audit`, alembic 0011→0027, dropped after; the shared dev DB was never touched): `GET /api/docs/tree` default depth=1 = **8,474 identity → 1,128 B with `Content-Encoding: gzip`** (< 40 KB bar; ~2 MB defect fixed), repeat load **304 / 0 bytes**; depth=8 265,147→23,515; node drill 20,844→1,065; `/api/docs` 246,366→45,104; `/api/documents/content` 892,124→60,776 (`LOOK`, accepted-with-why). Reproduces `AUDIT.md` to within 1 byte.
+- Transport contracts proven live: `POST /api/chat` **no** `Content-Encoding` + frames arriving live + budget line `… enc=identity duration_ms=7380 stream=1`; HTML still `no-cache` with the `?v=` rewrite (and gzipped); A20 headers on 200 **and** 304; ETag identical across identity/gzip; `depth=0|9|abc`→422; anonymous tree/node→403; unknown folder/source→404 `unknown node`.
+
+**Gates (all 100% green)**
+- `uv run pytest` → **3980 passed**; `uv run pytest --cov=app --cov-report=term-missing` → **TOTAL 99%** (`compression.py`, `http_cache.py`, `response_budget.py` 100%; `api/docs.py` 99%, one defensive dedupe line).
+- `uv run ruff check .` → All checks passed; `uv run pyright` → 0 errors.
+- Phase + regression E2E **in isolation** (`--junitxml`, exit codes): `test_kb_tree_lazy_load` 11, `test_kb_tree` 8, `test_kb_tree_nav` 4, `test_navbar_refresh` 7, `test_document_dates` 6, `test_git_source_dates` 4, `test_sync_summary_visibility` 3, `test_table_ux_polish` 6, `test_sources_midstream_bug` 6, `test_ls_tree_drilldown` 3, `test_smoke` 3 — **0 failures/errors** (the earlier `-q -q` run had simply suppressed pytest's summary line).
+
+**Documentation** — README.md + AGENTS.md reviewed in full (1,436 / 80 lines):
+- Fixed (pre-existing, phase-167-exposed): README *Caching & deploys* claimed "One Starlette middleware (`app/core/caching.py`) applies the rule" while the section now describes three transport layers → rewritten to name caching/compression/http_cache, the real outermost→innermost order, and why `bytes=` is uncompressed.
+- AGENTS.md rule 5 (UI Structure Check) gained the phase-167 clause: a lazily-drilled level keeps the current level rendered, its in-flight `role="status"` / `role="alert"`+Retry row lives in the same bounded scroll panel, and stat cards read server `totals` (never a walk of a depth-limited tree).
+- Cross-checked and consistent: every `BOR_*` in `.env.example` is in the README table, every `Settings` field is documented, all 15 README-cited API routes exist in the app, no stale "one fetch / drill client-side" claim remains anywhere.
+
+**Notable decisions / deviations**
+- `compression.py` adds a documented `_RebufferSmallResponses` seam beyond the D1 sketch (the caching layer's `BaseHTTPMiddleware` re-framing otherwise defeats Starlette's `minimum_size` exemption and inflates tiny responses) — integration-pinned, math still Starlette's.
+- A node-route **404 = vanished folder → honest reset to top level** (per `00_phase.md` D4/D6), not task-06's "alert + Retry"; 500 shows alert+Retry with the level preserved — both pinned, nothing weakened.
+- Retrieval battery not measurable (shared E2E DB holds no corpus), but `git diff --name-only -- app/rag` is empty, so retrieval is unchanged by construction. `AUDIT.md` left byte-identical (it lives under `todo/`, off-limits to this pass); no git index/history commands run.
+
+**Next pending phase:** none — `167_api_payload_diet` is the only entry in `.agents/phases/todo/`; the harness moves it to `complete/` and commits.

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+ok

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+....................                                                     [100%]
+=============================== warnings summary ===============================
+.venv/lib/python3.13/site-packages/fastapi/testclient.py:1
+  /var/home/ducoterra/Projects/Personal/brain_of_reese/.venv/lib/python3.13/site-packages/fastapi/testclient.py:1: StarletteDeprecationWarning: Using `httpx` with `starlette.testclient` is deprecated; install `httpx2` instead.
+    from starlette.testclient import TestClient as TestClient  # noqa
+
+-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
+================================ tests coverage ================================
+_______________ coverage: platform linux, python 3.13.13-final-0 _______________
+
+Name                           Stmts   Miss  Cover
+--------------------------------------------------
+app/__init__.py                    1      0   100%
+app/api/__init__.py                0      0   100%
+app/api/auth.py                   52      0   100%
+app/api/chat.py                  277      1    99%
+app/api/chat_images.py            50      0   100%
+app/api/chats.py                 110      0   100%
+app/api/config.py                 13      0   100%
+app/api/doc_drafts.py             99      0   100%
+app/api/docs.py                  234      1    99%
+app/api/git_sources.py           259      0   100%
+app/api/health.py                 10      0   100%
+app/api/steering.py               42      0   100%
+app/api/suggestions.py            33      0   100%
+app/api/sync.py                  204      0   100%
+app/api/tokens.py                 40      0   100%
+app/api/ui_settings.py            55      0   100%
+app/config.py                    280      0   100%
+app/core/__init__.py               0      0   100%
+app/core/auth.py                  45      0   100%
+app/core/caching.py              124      0   100%
+app/core/compression.py           59      0   100%
+app/core/debugging.py             29      2    93%
+app/core/docs_push.py             39      0   100%
+app/core/errors.py                 5      0   100%
+app/core/http_cache.py            22      0   100%
+app/core/logging.py               13      0   100%
+app/core/rate_limit.py            44      0   100%
+app/core/response_budget.py       63      0   100%
+app/core/security_headers.py      20      0   100%
+app/core/theming.py               38      0   100%
+app/core/tokens.py                44      0   100%
+app/db.py                         22      0   100%
+app/main.py                       73      0   100%
+app/models.py                    136      0   100%
+app/rag/__init__.py                0      0   100%
+app/rag/agent.py                 454      1    99%
+app/rag/archive_upload.py        134      0   100%
+app/rag/chunker.py               220      4    98%
+app/rag/doc_dates.py              18      0   100%
+app/rag/folder_summaries.py      134      0   100%
+app/rag/git_sources.py            77      0   100%
+app/rag/importer.py              841      4    99%
+app/rag/llm.py                   246      1    99%
+app/rag/md_images.py              63      0   100%
+app/rag/overview.py               78      0   100%
+app/rag/pdf.py                   192      0   100%
+app/rag/prompts.py               156      0   100%
+app/rag/retriever.py             252      3    99%
+app/rag/scaffolding.py            55      0   100%
+app/rag/source_removal.py         41      0   100%
+app/rag/sources_meta.py           16      0   100%
+app/rag/suggestions.py            27      0   100%
+app/rag/summarizer.py             58      0   100%
+app/schemas.py                   505      0   100%
+--------------------------------------------------
+TOTAL                           6102     17    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+WARNING: there is a new pyright version available (v1.1.411 -> v1.1.414).
+Please install the new version or set PYRIGHT_PYTHON_FORCE_VERSION to `latest`
+
+validation OK

.agents/reports/167_api_payload_diet/167_api_payload_diet__01_gzip_middleware.a1.md

@@ -0,0 +1,19 @@
+**Task 01 complete — all green.**
+
+**Implemented**
+- `app/core/compression.py`: `SelectiveGZipMiddleware` (pure ASGI, composes Starlette `GZipMiddleware`, `DEFAULT_SKIP_PATHS=("/api/chat",)` EXACT match — `/api/chat-images` still compresses) + `_RebufferSmallResponses` framing seam
+- `app/config.py`: `compress_min_bytes: int = 860` (`BOR_COMPRESS_MIN_BYTES`) with `agent_max_rounds`-pattern validator (`0` = off, `-1` refuses startup naming the field)
+- `app/main.py`: wired between `configure_caching` and `SecurityHeadersMiddleware` (A20 stays outermost); `0` adds nothing at all; `.env.example` entry with the three-value comment
+- `tests/unit/test_compression.py` (19 tests: gzip/identity/floor/pre-encoded skip, SSE frame-by-frame raw-ASGI interleaving proof + `list(stream)` idiom, exact-match pin, lifespan/websocket pass-through, wiring order, kill switch); `tests/integration/test_docs_api.py` + 4 pins (tree gzip→decompress-equal, A20 headers on gzipped body, small `/api/health` identity, A19 page contract through gzip)
+
+**Gates**
+- `uv run pytest` → **3903 passed**; `uv run pytest --cov=app --cov-report=term-missing` → **99%** overall, `app/core/compression.py` **100%**
+- `uv run ruff check .` → All checks passed; `uv run pyright` → 0 errors
+- E2E isolated (exit 0): `test_smoke`, `test_chat_rag`, `test_cache_busting`, `test_asset_cache_revalidation`, `test_kb_tree`
+- Measured: 50-doc tree 9,708 → 571 bytes (**17×**), `Vary: … Accept-Encoding` present
+
+**Docs**: README.md — gzip bullet in "Caching & deploys" + `BOR_COMPRESS_MIN_BYTES` config table row; AGENTS.md: no doc impact (no env/middleware content there).
+
+**Decisions**: (1) The `_RebufferSmallResponses` seam was required: starlette 1.x's `CachingMiddleware` (BaseHTTPMiddleware, below gzip per LOCKED order) re-frames every pass-through body (`more_body=True`), voiding Starlette's minimum-size exemption — `/api/health` came back gzip-inflated (33→71 B). The seam only re-delivers the buffered message shape Starlette's own exemption checks; no compression math forked. (2) Frame-by-frame proof is a deterministic raw-ASGI interleaving test (TestClient buffers bodies and cannot prove incremental arrival).
+
+**Next pending task**: `167_api_payload_diet/02_tree_depth_and_totals.md`.

.agents/reports/167_api_payload_diet/167_api_payload_diet__01_gzip_middleware.a1.md.status

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+ok

.agents/reports/167_api_payload_diet/167_api_payload_diet__01_gzip_middleware.a1.validate

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+...............                                                          [100%]
+=============================== warnings summary ===============================
+.venv/lib/python3.13/site-packages/fastapi/testclient.py:1
+  /var/home/ducoterra/Projects/Personal/brain_of_reese/.venv/lib/python3.13/site-packages/fastapi/testclient.py:1: StarletteDeprecationWarning: Using `httpx` with `starlette.testclient` is deprecated; install `httpx2` instead.
+    from starlette.testclient import TestClient as TestClient  # noqa
+
+-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
+================================ tests coverage ================================
+_______________ coverage: platform linux, python 3.13.13-final-0 _______________
+
+Name                           Stmts   Miss  Cover
+--------------------------------------------------
+app/__init__.py                    1      0   100%
+app/api/__init__.py                0      0   100%
+app/api/auth.py                   52      0   100%
+app/api/chat.py                  277      1    99%
+app/api/chat_images.py            50      0   100%
+app/api/chats.py                 110      0   100%
+app/api/config.py                 13      0   100%
+app/api/doc_drafts.py             99      0   100%
+app/api/docs.py                  196      1    99%
+app/api/git_sources.py           259      0   100%
+app/api/health.py                 10      0   100%
+app/api/steering.py               42      0   100%
+app/api/suggestions.py            33      0   100%
+app/api/sync.py                  204      0   100%
+app/api/tokens.py                 40      0   100%
+app/api/ui_settings.py            55      0   100%
+app/config.py                    273      0   100%
+app/core/__init__.py               0      0   100%
+app/core/auth.py                  45      0   100%
+app/core/caching.py              124      0   100%
+app/core/compression.py           59      0   100%
+app/core/debugging.py             29      2    93%
+app/core/docs_push.py             39      0   100%
+app/core/errors.py                 5      0   100%
+app/core/logging.py               13      0   100%
+app/core/rate_limit.py            44      0   100%
+app/core/security_headers.py      20      0   100%
+app/core/theming.py               38      0   100%
+app/core/tokens.py                44      0   100%
+app/db.py                         22      0   100%
+app/main.py                       71      0   100%
+app/models.py                    136      0   100%
+app/rag/__init__.py                0      0   100%
+app/rag/agent.py                 454      1    99%
+app/rag/archive_upload.py        134      0   100%
+app/rag/chunker.py               220      4    98%
+app/rag/doc_dates.py              18      0   100%
+app/rag/folder_summaries.py      134      0   100%
+app/rag/git_sources.py            77      0   100%
+app/rag/importer.py              841      4    99%
+app/rag/llm.py                   246      1    99%
+app/rag/md_images.py              63      0   100%
+app/rag/overview.py               78      0   100%
+app/rag/pdf.py                   192      0   100%
+app/rag/prompts.py               156      0   100%
+app/rag/retriever.py             252      3    99%
+app/rag/scaffolding.py            55      0   100%
+app/rag/source_removal.py         41      0   100%
+app/rag/sources_meta.py           16      0   100%
+app/rag/suggestions.py            27      0   100%
+app/rag/summarizer.py             58      0   100%
+app/schemas.py                   487      0   100%
+--------------------------------------------------
+TOTAL                           5952     17    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+WARNING: there is a new pyright version available (v1.1.411 -> v1.1.414).
+Please install the new version or set PYRIGHT_PYTHON_FORCE_VERSION to `latest`
+
+validation OK

.agents/reports/167_api_payload_diet/167_api_payload_diet__02_tree_depth_and_totals.a1.md

@@ -0,0 +1,15 @@
+**Task 02 complete — depth-limited KB tree + server totals (phase 167 D2, LOCKED A2).**
+
+Implemented:
+- `app/schemas.py`: new `KbTreeTotals` (documents/chunks/last_indexed, ge=0-bounded); `KbTree.totals` (additive top-level key); `truncated: bool = False` on `KbTreeFolder`/`KbTreeSource` with a `_drop_truncated_when_false` wrap serializer (the `_drop_image_fields` idiom) — untruncated nodes stay wire byte-identical; `KbTreeFile` untouched.
+- `app/api/docs.py`: `_folder_counts` → `(folders, counts, max_dates)` (recursive max `created_at` map, same pass); `_level_children(..., depth)` — budget-exhausted subfolders ship `truncated=true`, `children=[]`, recursive count + `max_dates`-derived `updated_at` (D9-proof), `summary`, `summary_pending`; `build_kb_tree(..., depth=None)` (None = byte-identical unlimited); source never truncated at depth≥1; endpoint gains `depth: Query(1, ge=1, le=8)` + `_kb_totals()` computed from the same rows (no extra query). `GET /api/docs` / agent `ls` untouched.
+- Tests: 11 new unit pins (depth matrix, byte-identity at None, truncated field set, `_subtree_max`↔`max_dates` cross-check at every depth, phase-94/98 invariants per depth, totals math/empty/validation, image affordance at depth 1); 4 new integration tests (default depth=1 + totals vs `/api/docs`, depth=3, 422 matrix, pre-phase wire-shape pin); migrated deep-walking pins to explicit `?depth=8` (`test_docs_api.py` ×6, `test_docs_api_dates.py` + new truncated-date test, `test_git_source_dates.py`, `test_document_dates.py`).
+- Docs: README RAG section documents depth/truncated/totals; AGENTS.md: no command/env changes → untouched.
+
+Gates: `uv run pytest --cov=app` → **3919 passed, coverage 99%** (`app/api/docs.py` 99% — only the pre-existing defensive `continue` uncovered; `app/schemas.py` 100%); `uv run ruff check .` clean; `uv run pyright` 0 errors.
+
+Notable decisions:
+- **Deviation (plan gap):** `tests/e2e/test_document_dates.py` also fetched the tree with no depth (its `_find_file` recurses) — same repair class as item 9's two suites; migrated with `?depth=8` too.
+- **Expected interim E2E state:** client `sources.js` (task 04's scope) still walks the tree for stat cards/drill, so `test_kb_tree.py` stat/drill assertions now fail against the intentional depth=1 default (verified the failure is exactly `#stat-docs` under-counting, not a server defect). This is the owner-confirmed A3 supersede; task 04 (client lazy drill + `totals` cards) and task 06 (E2E gate) repair it. Unit/integration suites (the project's `testpaths`) are 100% green; sources.js pins stay green untouched.
+
+Next pending task: `167_api_payload_diet/03_tree_node_endpoint_etag.md`.

.agents/reports/167_api_payload_diet/167_api_payload_diet__02_tree_depth_and_totals.a1.md.status

@@ -0,0 +1 @@
+ok

.agents/reports/167_api_payload_diet/167_api_payload_diet__02_tree_depth_and_totals.a1.validate

@@ -0,0 +1,127 @@
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+...............................                                          [100%]
+=============================== warnings summary ===============================
+.venv/lib/python3.13/site-packages/fastapi/testclient.py:1
+  /var/home/ducoterra/Projects/Personal/brain_of_reese/.venv/lib/python3.13/site-packages/fastapi/testclient.py:1: StarletteDeprecationWarning: Using `httpx` with `starlette.testclient` is deprecated; install `httpx2` instead.
+    from starlette.testclient import TestClient as TestClient  # noqa
+
+-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
+================================ tests coverage ================================
+_______________ coverage: platform linux, python 3.13.13-final-0 _______________
+
+Name                           Stmts   Miss  Cover
+--------------------------------------------------
+app/__init__.py                    1      0   100%
+app/api/__init__.py                0      0   100%
+app/api/auth.py                   52      0   100%
+app/api/chat.py                  277      1    99%
+app/api/chat_images.py            50      0   100%
+app/api/chats.py                 110      0   100%
+app/api/config.py                 13      0   100%
+app/api/doc_drafts.py             99      0   100%
+app/api/docs.py                  207      1    99%
+app/api/git_sources.py           259      0   100%
+app/api/health.py                 10      0   100%
+app/api/steering.py               42      0   100%
+app/api/suggestions.py            33      0   100%
+app/api/sync.py                  204      0   100%
+app/api/tokens.py                 40      0   100%
+app/api/ui_settings.py            55      0   100%
+app/config.py                    273      0   100%
+app/core/__init__.py               0      0   100%
+app/core/auth.py                  45      0   100%
+app/core/caching.py              124      0   100%
+app/core/compression.py           59      0   100%
+app/core/debugging.py             29      2    93%
+app/core/docs_push.py             39      0   100%
+app/core/errors.py                 5      0   100%
+app/core/logging.py               13      0   100%
+app/core/rate_limit.py            44      0   100%
+app/core/security_headers.py      20      0   100%
+app/core/theming.py               38      0   100%
+app/core/tokens.py                44      0   100%
+app/db.py                         22      0   100%
+app/main.py                       71      0   100%
+app/models.py                    136      0   100%
+app/rag/__init__.py                0      0   100%
+app/rag/agent.py                 454      1    99%
+app/rag/archive_upload.py        134      0   100%
+app/rag/chunker.py               220      4    98%
+app/rag/doc_dates.py              18      0   100%
+app/rag/folder_summaries.py      134      0   100%
+app/rag/git_sources.py            77      0   100%
+app/rag/importer.py              841      4    99%
+app/rag/llm.py                   246      1    99%
+app/rag/md_images.py              63      0   100%
+app/rag/overview.py               78      0   100%
+app/rag/pdf.py                   192      0   100%
+app/rag/prompts.py               156      0   100%
+app/rag/retriever.py             252      3    99%
+app/rag/scaffolding.py            55      0   100%
+app/rag/source_removal.py         41      0   100%
+app/rag/sources_meta.py           16      0   100%
+app/rag/suggestions.py            27      0   100%
+app/rag/summarizer.py             58      0   100%
+app/schemas.py                   505      0   100%
+--------------------------------------------------
+TOTAL                           5981     17    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+WARNING: there is a new pyright version available (v1.1.411 -> v1.1.414).
+Please install the new version or set PYRIGHT_PYTHON_FORCE_VERSION to `latest`
+
+validation OK

.agents/reports/167_api_payload_diet/167_api_payload_diet__03_tree_node_endpoint_etag.a1.md

@@ -0,0 +1,17 @@
+## Report — phase 167, task 03 (`167_api_payload_diet/03_tree_node_endpoint_etag`)
+
+**Implemented**
+- `app/core/http_cache.py` (new, 100% covered): `weak_etag` (`W/"sha256[:32]"`, str≡bytes), tolerant `etag_matches` (`*`, comma-lists, weak↔strong both directions, whitespace, `None`/empty→False), `revalidate_headers` (one place for `ETag` + `Cache-Control: private, must-revalidate`; docstrings name the A4/A19 scope).
+- `app/api/docs.py`: extracted `_kb_catalogue` (one catalogue query + the two bounded selects, shared by both tree routes — no N+1) and `_revalidated_json` (serialize once → tag → 304/200); `GET /api/docs/tree/node?source&path&depth` (admin gate, `path=""`→`KbTreeSource` via `_source_node`, else `KbTreeFolder` with `counts`/`max_dates`/summary/`summary_pending`; 404 `{"detail":"unknown node"}` incl. file-paths and traversal — row-only, no filesystem surface); ETag/304 wired on `/api/docs`, `/api/docs/tree`, `/api/docs/tree/node` (`response_model` kept for OpenAPI; node uses `response_model=None` + union annotation).
+- Tests: `tests/unit/test_http_cache.py` (12 pins) + 10 new integration tests in `tests/integration/test_docs_api.py` (node shape/depth/404-matrix/403/422s, node≡`depth=8` tree level equality, full ETag matrix on all 3 routes, A4 bust after folder-summary + document-date PATCHes with `sources_meta.version` provably unmoved, gzip coexistence).
+
+**Gates** — `uv run pytest` → **3941 passed**; `uv run pytest tests/integration/test_docs_api.py tests/unit/test_http_cache.py -v` → **71 passed**; `uv run pytest --cov=app --cov-report=term-missing` → **TOTAL 99%**, `app/core/http_cache.py` 100%, `app/api/docs.py` 99% (only the pre-existing defensive `continue`); `uv run ruff check .` clean; `uv run pyright` 0 errors; caching suites stay green (49 passed).
+
+**Docs** — README updated (Sources section: node route + revalidation; Caching section: the read-only-API validator arm of A19/A4). AGENTS.md: no doc impact. No new env vars.
+
+**Notable decisions**
+- The node-route node itself is never `truncated` (its direct children are always built — same rule as `KbTreeSource` at depth ≥1); deeper levels ride the children's flags.
+- `tests/e2e/test_kb_tree.py` has 2 failures (stat cards, 2nd-level drill) — **not introduced here**: they stem from task 02's `depth=1` default and are explicitly owned by task 04 (`sources.js` lazy drill + `totals`) and task 06 (E2E pin migration) per the phase plan; unit+integration (the project suite per `testpaths`) is 100% green.
+- ASSUMPTION kept per task text: `/api/documents/content` gets no validator this phase (task 05's audit decides).
+
+**Next pending task:** `167_api_payload_diet/04_client_lazy_drill.md`

.agents/reports/167_api_payload_diet/167_api_payload_diet__03_tree_node_endpoint_etag.a1.md.status

@@ -0,0 +1 @@
+ok

.agents/reports/167_api_payload_diet/167_api_payload_diet__03_tree_node_endpoint_etag.a1.validate

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+........................................................................ [  1%]
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+........................................................................ [ 95%]
+........................................................................ [ 96%]
+........................................................................ [ 98%]
+.....................................................                    [100%]
+=============================== warnings summary ===============================
+.venv/lib/python3.13/site-packages/fastapi/testclient.py:1
+  /var/home/ducoterra/Projects/Personal/brain_of_reese/.venv/lib/python3.13/site-packages/fastapi/testclient.py:1: StarletteDeprecationWarning: Using `httpx` with `starlette.testclient` is deprecated; install `httpx2` instead.
+    from starlette.testclient import TestClient as TestClient  # noqa
+
+-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
+================================ tests coverage ================================
+_______________ coverage: platform linux, python 3.13.13-final-0 _______________
+
+Name                           Stmts   Miss  Cover
+--------------------------------------------------
+app/__init__.py                    1      0   100%
+app/api/__init__.py                0      0   100%
+app/api/auth.py                   52      0   100%
+app/api/chat.py                  277      1    99%
+app/api/chat_images.py            50      0   100%
+app/api/chats.py                 110      0   100%
+app/api/config.py                 13      0   100%
+app/api/doc_drafts.py             99      0   100%
+app/api/docs.py                  234      1    99%
+app/api/git_sources.py           259      0   100%
+app/api/health.py                 10      0   100%
+app/api/steering.py               42      0   100%
+app/api/suggestions.py            33      0   100%
+app/api/sync.py                  204      0   100%
+app/api/tokens.py                 40      0   100%
+app/api/ui_settings.py            55      0   100%
+app/config.py                    273      0   100%
+app/core/__init__.py               0      0   100%
+app/core/auth.py                  45      0   100%
+app/core/caching.py              124      0   100%
+app/core/compression.py           59      0   100%
+app/core/debugging.py             29      2    93%
+app/core/docs_push.py             39      0   100%
+app/core/errors.py                 5      0   100%
+app/core/http_cache.py            22      0   100%
+app/core/logging.py               13      0   100%
+app/core/rate_limit.py            44      0   100%
+app/core/security_headers.py      20      0   100%
+app/core/theming.py               38      0   100%
+app/core/tokens.py                44      0   100%
+app/db.py                         22      0   100%
+app/main.py                       71      0   100%
+app/models.py                    136      0   100%
+app/rag/__init__.py                0      0   100%
+app/rag/agent.py                 454      1    99%
+app/rag/archive_upload.py        134      0   100%
+app/rag/chunker.py               220      4    98%
+app/rag/doc_dates.py              18      0   100%
+app/rag/folder_summaries.py      134      0   100%
+app/rag/git_sources.py            77      0   100%
+app/rag/importer.py              841      4    99%
+app/rag/llm.py                   246      1    99%
+app/rag/md_images.py              63      0   100%
+app/rag/overview.py               78      0   100%
+app/rag/pdf.py                   192      0   100%
+app/rag/prompts.py               156      0   100%
+app/rag/retriever.py             252      3    99%
+app/rag/scaffolding.py            55      0   100%
+app/rag/source_removal.py         41      0   100%
+app/rag/sources_meta.py           16      0   100%
+app/rag/suggestions.py            27      0   100%
+app/rag/summarizer.py             58      0   100%
+app/schemas.py                   505      0   100%
+--------------------------------------------------
+TOTAL                           6030     17    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+WARNING: there is a new pyright version available (v1.1.411 -> v1.1.414).
+Please install the new version or set PYRIGHT_PYTHON_FORCE_VERSION to `latest`
+
+validation OK

.agents/reports/167_api_payload_diet/167_api_payload_diet__04_client_lazy_drill.a1.md

@@ -0,0 +1,15 @@
+**Task 04 complete — client lazy drill (phase 167 D4/A3).**
+
+- `frontend/assets/sources.js`: load is now ONE `fetch("/api/docs/tree?depth=1")`; `levelCache` (Map, `source + "/" + folder`) cleared on every fresh tree; `needsLevelFetch` + `enterFolder`/`fetchLevel` fetch `GET /api/docs/tree/node?…` only for uncached `truncated` nodes, `Object.assign`-merge + cache only after a 200; `drillPending` double-click guard with `aria-busy`/`aria-disabled` on the row link; `role="status"` "Loading <folder>…" row (departing level stays rendered) and `role="alert"` "Couldn't load … — Retry" with a real `<button type="button">`; `loadSeq` extended to the drill (+ `navSeq` so navigation invalidates it, never a stale paint); node 404 → honest top-level reset; `folderExistsIn` → `folderKnownIn` (ancestor-chain over held levels); `treeStats()` prefers server `totals` (walk kept as fallback); popstate adopts rehydrate via `ensureCurrentLevel` (fixed the forward-nav E2E regression); module docstring rewritten for the depth-1 + per-level model.
+- `frontend/assets/styles.css`: one tokens-only `.kb-level-loading` rule (Retry reuses the existing `.retry-btn` pill).
+- Tests: migrated `tests/unit/test_kb_tree_ui.py` (depth=1/node fetch counts, levelCache lifecycle, race token, aria states, status/alert rows, 404 reset, totals-first stats, folderKnownIn, docstring pins — all phase-97/99 pins preserved) and `tests/unit/test_frontend_router.py:591`; migrated `tests/e2e/test_navbar_refresh.py` request-pin to `?depth=1`.
+
+Results:
+- `uv run pytest` → **3948 passed**; `--cov=app` → **99%** (>90%); `uv run ruff check .` clean; `uv run pyright` 0 errors.
+- E2E in isolation, all green: `test_kb_tree.py` (8), `test_kb_tree_nav.py` (4, incl. the back/forward walk), `test_navbar_refresh.py`, `test_document_dates.py`, `test_git_source_dates.py`, `test_sync_summary_visibility.py`, `test_table_ux_polish.py` (26), `test_sources_midstream_bug.py` + `test_ls_tree_drilldown.py` + `test_smoke.py` (12).
+
+Docs: README RAG-view paragraph extended (per-level cache, Loading/Retry states, 404 reset); no AGENTS.md impact.
+
+Notable: added `navSeq` + `ensureCurrentLevel` beyond the task sketch — popstate/forward adopts of a not-yet-refetched level would otherwise render a half-empty level (`test_kb_tree_nav.py::test_back_button_walks_the_breadcrumb` caught it; now green).
+
+Next pending task: `167_api_payload_diet/05_response_budget_audit.md`.

.agents/reports/167_api_payload_diet/167_api_payload_diet__04_client_lazy_drill.a1.md.status

@@ -0,0 +1 @@
+ok

.agents/reports/167_api_payload_diet/167_api_payload_diet__04_client_lazy_drill.a1.validate

@@ -0,0 +1,128 @@
+........................................................................ [  1%]
+........................................................................ [  3%]
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+........................................................................ [ 96%]
+........................................................................ [ 98%]
+............................................................             [100%]
+=============================== warnings summary ===============================
+.venv/lib/python3.13/site-packages/fastapi/testclient.py:1
+  /var/home/ducoterra/Projects/Personal/brain_of_reese/.venv/lib/python3.13/site-packages/fastapi/testclient.py:1: StarletteDeprecationWarning: Using `httpx` with `starlette.testclient` is deprecated; install `httpx2` instead.
+    from starlette.testclient import TestClient as TestClient  # noqa
+
+-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
+================================ tests coverage ================================
+_______________ coverage: platform linux, python 3.13.13-final-0 _______________
+
+Name                           Stmts   Miss  Cover
+--------------------------------------------------
+app/__init__.py                    1      0   100%
+app/api/__init__.py                0      0   100%
+app/api/auth.py                   52      0   100%
+app/api/chat.py                  277      1    99%
+app/api/chat_images.py            50      0   100%
+app/api/chats.py                 110      0   100%
+app/api/config.py                 13      0   100%
+app/api/doc_drafts.py             99      0   100%
+app/api/docs.py                  234      1    99%
+app/api/git_sources.py           259      0   100%
+app/api/health.py                 10      0   100%
+app/api/steering.py               42      0   100%
+app/api/suggestions.py            33      0   100%
+app/api/sync.py                  204      0   100%
+app/api/tokens.py                 40      0   100%
+app/api/ui_settings.py            55      0   100%
+app/config.py                    273      0   100%
+app/core/__init__.py               0      0   100%
+app/core/auth.py                  45      0   100%
+app/core/caching.py              124      0   100%
+app/core/compression.py           59      0   100%
+app/core/debugging.py             29      2    93%
+app/core/docs_push.py             39      0   100%
+app/core/errors.py                 5      0   100%
+app/core/http_cache.py            22      0   100%
+app/core/logging.py               13      0   100%
+app/core/rate_limit.py            44      0   100%
+app/core/security_headers.py      20      0   100%
+app/core/theming.py               38      0   100%
+app/core/tokens.py                44      0   100%
+app/db.py                         22      0   100%
+app/main.py                       71      0   100%
+app/models.py                    136      0   100%
+app/rag/__init__.py                0      0   100%
+app/rag/agent.py                 454      1    99%
+app/rag/archive_upload.py        134      0   100%
+app/rag/chunker.py               220      4    98%
+app/rag/doc_dates.py              18      0   100%
+app/rag/folder_summaries.py      134      0   100%
+app/rag/git_sources.py            77      0   100%
+app/rag/importer.py              841      4    99%
+app/rag/llm.py                   246      1    99%
+app/rag/md_images.py              63      0   100%
+app/rag/overview.py               78      0   100%
+app/rag/pdf.py                   192      0   100%
+app/rag/prompts.py               156      0   100%
+app/rag/retriever.py             252      3    99%
+app/rag/scaffolding.py            55      0   100%
+app/rag/source_removal.py         41      0   100%
+app/rag/sources_meta.py           16      0   100%
+app/rag/suggestions.py            27      0   100%
+app/rag/summarizer.py             58      0   100%
+app/schemas.py                   505      0   100%
+--------------------------------------------------
+TOTAL                           6030     17    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+WARNING: there is a new pyright version available (v1.1.411 -> v1.1.414).
+Please install the new version or set PYRIGHT_PYTHON_FORCE_VERSION to `latest`
+
+validation OK

.agents/reports/167_api_payload_diet/167_api_payload_diet__05_response_budget_audit.a1.md

@@ -0,0 +1,17 @@
+All green. Final state verified.
+
+**Implemented (task 05)**
+- `app/core/response_budget.py` — pure-ASGI byte-budget middleware: one `api method=… path=… status=… bytes=… enc=… duration_ms=…` line per `/api/*` response (`stream=1` for `text/event-stream`), INFO at/over budget, DEBUG below, `304`→`bytes=0`, non-`/api/`+non-HTTP silent, exceptions logged and re-raised unchanged; `bytes=` = route bytes, `enc=` = wire encoding (reads the shared start-message the gzip layer decorates in place — documented, integration-pinned).
+- `Settings.log_large_response_bytes=65536` + `agent_max_rounds`-style validator (`0`=all INFO, negative refuses startup); `.env.example` entry; `app/main.py` wires it **before** the gzip call → SecurityHeaders → gzip → response-budget → caching → session.
+- `scripts/api_payload_audit.py` (stdlib+httpx, read-only, exit 0 report / 2 unreachable-or-bad-login) + `AUDIT.md` with verbatim BEFORE (`BOR_COMPRESS_MIN_BYTES=0`) and AFTER tables, curl proof, real log lines incl. `POST /api/chat … stream=1`, and a verdict per endpoint.
+- Tests: new `tests/unit/test_response_budget.py` (24), new `tests/integration/test_response_budget_api.py` (7), one SSE `stream=1` pin in `tests/integration/test_chat_api.py`; migrated the two `test_compression.py` middleware-stack pins.
+
+**Live-infra used (local dev only)**: restored the 1,000-doc live replica via the `restore-test-db` skill (safety backup `/tmp/pre_restore_20261008_073453.dump`; fingerprint `docs=1000 chunks=8866`; alembic 0011→0027), ran the dev server for the audit, then stopped it. Key measured numbers: tree depth=1 **8,474 B → 1,129 B gzip → 304/0** (full depth-8 tree 265,147 B); `/api/docs` 246,366→45,107; `/api/documents/content` 892,124→60,776 (LOOK, accepted with why). AUDIT.md states the snapshot has **0 folder summaries / 0 image docs**, so identity bytes are a lower bound on the 2 MB production report.
+
+**Docs**: README "Operational notes: the response byte budget" (log format, `grep ' :: api '`, `BOR_LOG_LARGE_RESPONSE_BYTES`, audit run recipe, self-gzip/nginx deploy note) + config-reference row; AGENTS.md rule 10 + quick-reference line.
+
+**Gates (exact)**: `uv run pytest --cov=app --cov-report=term-missing` → **3980 passed**, TOTAL 99%, `app/core/response_budget.py` **100%**; `uv run ruff check .` → All checks passed; `uv run pyright` → 0 errors; `uv run pytest tests/e2e/test_smoke.py -q --no-cov` → green.
+
+**Deviations**: no `--username` flag (the single-admin `/api/login` is password-only, A10 — documented in the script); no new ETag remedies applied — `/api/git-sources` measures 188 B and `/api/documents/content` is the task's documented accepted case with a named follow-up.
+
+**Next pending task**: `167_api_payload_diet/06_e2e_payload_diet.md`.

.agents/reports/167_api_payload_diet/167_api_payload_diet__05_response_budget_audit.a1.md.status

@@ -0,0 +1 @@
+ok

.agents/reports/167_api_payload_diet/167_api_payload_diet__05_response_budget_audit.a1.validate

@@ -0,0 +1,130 @@
+........................................................................ [  1%]
+........................................................................ [  3%]
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+........................................................................ [ 94%]
+........................................................................ [ 95%]
+........................................................................ [ 97%]
+........................................................................ [ 99%]
+....................                                                     [100%]
+=============================== warnings summary ===============================
+.venv/lib/python3.13/site-packages/fastapi/testclient.py:1
+  /var/home/ducoterra/Projects/Personal/brain_of_reese/.venv/lib/python3.13/site-packages/fastapi/testclient.py:1: StarletteDeprecationWarning: Using `httpx` with `starlette.testclient` is deprecated; install `httpx2` instead.
+    from starlette.testclient import TestClient as TestClient  # noqa
+
+-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
+================================ tests coverage ================================
+_______________ coverage: platform linux, python 3.13.13-final-0 _______________
+
+Name                           Stmts   Miss  Cover
+--------------------------------------------------
+app/__init__.py                    1      0   100%
+app/api/__init__.py                0      0   100%
+app/api/auth.py                   52      0   100%
+app/api/chat.py                  277      1    99%
+app/api/chat_images.py            50      0   100%
+app/api/chats.py                 110      0   100%
+app/api/config.py                 13      0   100%
+app/api/doc_drafts.py             99      0   100%
+app/api/docs.py                  234      1    99%
+app/api/git_sources.py           259      0   100%
+app/api/health.py                 10      0   100%
+app/api/steering.py               42      0   100%
+app/api/suggestions.py            33      0   100%
+app/api/sync.py                  204      0   100%
+app/api/tokens.py                 40      0   100%
+app/api/ui_settings.py            55      0   100%
+app/config.py                    280      0   100%
+app/core/__init__.py               0      0   100%
+app/core/auth.py                  45      0   100%
+app/core/caching.py              124      0   100%
+app/core/compression.py           59      0   100%
+app/core/debugging.py             29      2    93%
+app/core/docs_push.py             39      0   100%
+app/core/errors.py                 5      0   100%
+app/core/http_cache.py            22      0   100%
+app/core/logging.py               13      0   100%
+app/core/rate_limit.py            44      0   100%
+app/core/response_budget.py       63      0   100%
+app/core/security_headers.py      20      0   100%
+app/core/theming.py               38      0   100%
+app/core/tokens.py                44      0   100%
+app/db.py                         22      0   100%
+app/main.py                       73      0   100%
+app/models.py                    136      0   100%
+app/rag/__init__.py                0      0   100%
+app/rag/agent.py                 454      1    99%
+app/rag/archive_upload.py        134      0   100%
+app/rag/chunker.py               220      4    98%
+app/rag/doc_dates.py              18      0   100%
+app/rag/folder_summaries.py      134      0   100%
+app/rag/git_sources.py            77      0   100%
+app/rag/importer.py              841      4    99%
+app/rag/llm.py                   246      1    99%
+app/rag/md_images.py              63      0   100%
+app/rag/overview.py               78      0   100%
+app/rag/pdf.py                   192      0   100%
+app/rag/prompts.py               156      0   100%
+app/rag/retriever.py             252      3    99%
+app/rag/scaffolding.py            55      0   100%
+app/rag/source_removal.py         41      0   100%
+app/rag/sources_meta.py           16      0   100%
+app/rag/suggestions.py            27      0   100%
+app/rag/summarizer.py             58      0   100%
+app/schemas.py                   505      0   100%
+--------------------------------------------------
+TOTAL                           6102     17    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+WARNING: there is a new pyright version available (v1.1.411 -> v1.1.414).
+Please install the new version or set PYRIGHT_PYTHON_FORCE_VERSION to `latest`
+
+validation OK

.agents/reports/167_api_payload_diet/167_api_payload_diet__06_e2e_payload_diet.a1.md

@@ -0,0 +1,14 @@
+Final verification pass complete. Everything is green.
+
+**Phase 167 task 06 — report**
+
+1. **New suite** `tests/e2e/test_kb_tree_lazy_load.py` — **11 passed in isolation**: one `?depth=1` boot request (zero node fetches, parsed body: `totals` 8 docs/12 chunks, recursive source counts, no node deeper than level 1, every level-1 folder `truncated: true`, D9 recursive date on the truncated `alpha`); gzip + `Vary: Accept-Encoding` + `W/"…"` ETag on the real Chromium negotiation; one node fetch per uncached level (source click = zero); two levels deep then back = zero new fetches, cache never re-fetched; D4 pending marker + image thumbnail + phase-156 sticky header/scroll panel/44 px Retry + 72rem container in a lazily rendered level; `role="status"`/"Loading alpha…" in-flight state where a second click issues no second request; 500 → alert + Retry with the level preserved, Retry recovers; 404 → honest reset to the top level.
+2. **Revalidation**: Chromium hands `fetch` the cached 200, so reload and History→Sources revalidation are pinned on the CDP Network events (DevTools-panel truth): `If-None-Match` sent, wire `304`, transfer < ½ the first load, table still renders; plus the never-stale half — a folder-description PATCH busts the tag → `200` + the new text renders.
+3. **Migrated, never deleted**: the superseded "ONE fetch / drill is client-side" claims re-pointed in `test_kb_tree.py` (module contract + `_wait_top_level` + `_drill` + the breadcrumb-back pin) and in 8 sibling suites' drill helpers (`test_kb_tree_nav`, `test_admin_auth`, `test_folder_summary_voice`, `test_sync_summary_visibility`, `test_import_documents`, `test_extensionless_import`, `test_import_extensions_env`, `test_quadlet_jinja_import`). All re-ran green in isolation.
+4. **Regression set, isolation**: kb_tree 8, kb_tree_nav 4, navbar_refresh 7, document_dates 6, git_source_dates 4, sync_summary_visibility 3, table_ux_polish 6, sources_midstream_bug 6, ls_tree_drilldown 3 (agent `ls` unchanged), smoke 3 — all pass.
+5. **Full gates**: `uv run pytest` **3980 passed**; `--cov=app` **TOTAL 99 %** (`compression.py`, `http_cache.py`, `response_budget.py` 100 %, `api/docs.py` 99 %); `ruff check .` and `pyright` clean.
+6. **Retrieval battery**: no `app/rag/**` file is touched by this phase (`git diff --name-only`), so retrieval is unchanged by construction; the battery itself cannot measure here — the shared E2E Postgres holds no corpus (the suites truncate it), so every question deflects at `best_cosine=0.0000`.
+7. **Defect found and fixed** (pre-existing, not phase 167): `test_sources_midstream_bug.py::_reset_db` truncated only `chunks, documents, query_log`, so a `kb_overview` row left by another suite's sync leaked into the prompt and the mock echoed `(kb: …)` into the answer, breaking its exact-text pin. It now truncates the same house list as `test_kb_tree._truncate_all`; verified failing before, 6/6 after, including directly after `test_kb_tree`. 27 other suites still truncate narrowly — same latent order coupling, left alone as out of scope.
+8. **Deviation flagged**: task 06's vanished-folder bullet says a `404` shows alert + Retry, but `00_phase.md` D4/D6 and the shipped client make the node route's 404 the vanished signal (reset to the top level). Both paths are pinned honestly — 404 → honest reset, 500 → alert + Retry with the level preserved — no assertion weakened.
+9. **Docs**: `AUDIT.md` gained the browser-verification paragraph (gzip, 304, one fetch per level); README's tree/node endpoint table, gzip + ETag deploy note, env var list and `AGENTS.md` rule 10 / quick-reference were reviewed and are already phase-167 consistent; `.env.example` carries both new vars.
+10. No git index/history commands run, no `todo/` phase file moved, renamed or edited.

.agents/reports/167_api_payload_diet/167_api_payload_diet__06_e2e_payload_diet.a1.md.status

@@ -0,0 +1 @@
+ok

.agents/reports/167_api_payload_diet/167_api_payload_diet__06_e2e_payload_diet.a1.validate

@@ -0,0 +1,130 @@
+........................................................................ [  1%]
+........................................................................ [  3%]
+........................................................................ [  5%]
+........................................................................ [  7%]
+........................................................................ [  9%]
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+........................................................................ [ 79%]
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+........................................................................ [ 83%]
+........................................................................ [ 85%]
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+........................................................................ [ 88%]
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+........................................................................ [ 92%]
+........................................................................ [ 94%]
+........................................................................ [ 95%]
+........................................................................ [ 97%]
+........................................................................ [ 99%]
+....................                                                     [100%]
+=============================== warnings summary ===============================
+.venv/lib/python3.13/site-packages/fastapi/testclient.py:1
+  /var/home/ducoterra/Projects/Personal/brain_of_reese/.venv/lib/python3.13/site-packages/fastapi/testclient.py:1: StarletteDeprecationWarning: Using `httpx` with `starlette.testclient` is deprecated; install `httpx2` instead.
+    from starlette.testclient import TestClient as TestClient  # noqa
+
+-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
+================================ tests coverage ================================
+_______________ coverage: platform linux, python 3.13.13-final-0 _______________
+
+Name                           Stmts   Miss  Cover
+--------------------------------------------------
+app/__init__.py                    1      0   100%
+app/api/__init__.py                0      0   100%
+app/api/auth.py                   52      0   100%
+app/api/chat.py                  277      1    99%
+app/api/chat_images.py            50      0   100%
+app/api/chats.py                 110      0   100%
+app/api/config.py                 13      0   100%
+app/api/doc_drafts.py             99      0   100%
+app/api/docs.py                  234      1    99%
+app/api/git_sources.py           259      0   100%
+app/api/health.py                 10      0   100%
+app/api/steering.py               42      0   100%
+app/api/suggestions.py            33      0   100%
+app/api/sync.py                  204      0   100%
+app/api/tokens.py                 40      0   100%
+app/api/ui_settings.py            55      0   100%
+app/config.py                    280      0   100%
+app/core/__init__.py               0      0   100%
+app/core/auth.py                  45      0   100%
+app/core/caching.py              124      0   100%
+app/core/compression.py           59      0   100%
+app/core/debugging.py             29      2    93%
+app/core/docs_push.py             39      0   100%
+app/core/errors.py                 5      0   100%
+app/core/http_cache.py            22      0   100%
+app/core/logging.py               13      0   100%
+app/core/rate_limit.py            44      0   100%
+app/core/response_budget.py       63      0   100%
+app/core/security_headers.py      20      0   100%
+app/core/theming.py               38      0   100%
+app/core/tokens.py                44      0   100%
+app/db.py                         22      0   100%
+app/main.py                       73      0   100%
+app/models.py                    136      0   100%
+app/rag/__init__.py                0      0   100%
+app/rag/agent.py                 454      1    99%
+app/rag/archive_upload.py        134      0   100%
+app/rag/chunker.py               220      4    98%
+app/rag/doc_dates.py              18      0   100%
+app/rag/folder_summaries.py      134      0   100%
+app/rag/git_sources.py            77      0   100%
+app/rag/importer.py              841      4    99%
+app/rag/llm.py                   246      1    99%
+app/rag/md_images.py              63      0   100%
+app/rag/overview.py               78      0   100%
+app/rag/pdf.py                   192      0   100%
+app/rag/prompts.py               156      0   100%
+app/rag/retriever.py             252      3    99%
+app/rag/scaffolding.py            55      0   100%
+app/rag/source_removal.py         41      0   100%
+app/rag/sources_meta.py           16      0   100%
+app/rag/suggestions.py            27      0   100%
+app/rag/summarizer.py             58      0   100%
+app/schemas.py                   505      0   100%
+--------------------------------------------------
+TOTAL                           6102     17    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+WARNING: there is a new pyright version available (v1.1.411 -> v1.1.414).
+Please install the new version or set PYRIGHT_PYTHON_FORCE_VERSION to `latest`
+
+validation OK

.env.example

@@ -12,6 +12,24 @@ BOR_ENVIRONMENT=development
 #  a local .env is ignored.)
 # BOR_LOG_LEVEL=INFO
 # BOR_STATIC_DIR=frontend          # dev default; container sets /app/static
+# BOR_COMPRESS_MIN_BYTES=860       # gzip responses of >= this many bytes when the
+                                   # client accepts it (phase 167, TODO L5 — the 2 MB
+                                   # /api/docs/tree defect; ~8-12x on this JSON).
+                                   # 0 = feature OFF (no middleware at all);
+                                   # negative refuses startup naming the field;
+                                   # 860 = the classic nginx default, below which
+                                   # compression can inflate a body instead of
+                                   # shrinking it. POST /api/chat (SSE, A15) is
+                                   # NEVER compressed.
+# BOR_LOG_LARGE_RESPONSE_BYTES=65536  # INFO/DEBUG cut for the per-response
+                                   # "api method=… bytes=… enc=…" line the
+                                   # byte budget logs for every /api/* response
+                                   # (phase 167, TODO L5 — read it with
+                                   # grep ' :: api '):
+                                   # >= 65536 (64 KB) -> INFO, below -> DEBUG
+                                   # (invisible at BOR_LOG_LEVEL=INFO);
+                                   # 0 = log EVERY /api/* response at INFO;
+                                   # negative refuses startup naming the field.
 
 # --- Database (matches `podman compose` db service) ---
 BOR_DATABASE_URL=postgresql+psycopg://reese:reese@localhost:5432/brain_of_reese

AGENTS.md

@@ -28,7 +28,12 @@
    the primary class shows the strip in place of the text, the
    additive class never hides the text: Text/Pages toggle group
    (`aria-pressed`, Text default) with the strip behind it
-   (phase 163)) and
+   (phase 163); a lazily-drilled KB-tree level keeps the level the
+   user is ON rendered while its node fetch runs, and its ONE
+   in-flight row (`role="status"`) / alert + Retry row
+   (`role="alert"`, a real `<button>`) sits inside that same bounded
+   scroll panel — and the stat cards read the server's `totals`,
+   never a walk of a depth-limited tree (phase 167)) and
    meets WCAG 2.1 AA basics (semantic
    landmarks, labels, contrast ≥4.5:1, focus-visible, aria-live for the
    stream).
@@ -51,8 +56,13 @@
    Playwright E2E file passes **in isolation**
    (`uv run pytest tests/e2e/test_<story>.py -v --no-cov`).
 10. **Amply log, visibly feed back:** server-side, log the required
-    per-turn line (PLAN §9); UI-side, honor the "never stale" feedback
-    contract (PLAN §7.4).
+    per-turn line (PLAN §9) **and** the per-response byte-budget line
+    (`app/core/response_budget.py` — one
+    `api method=… path=… status=… bytes=… enc=… duration_ms=…` line per
+    `/api/*` response, INFO at/over `BOR_LOG_LARGE_RESPONSE_BYTES`, read it
+    with `grep ' :: api '`, audit it with
+    `uv run python -m scripts.api_payload_audit`, phase 167); UI-side,
+    honor the "never stale" feedback contract (PLAN §7.4).
 
 ## Quick reference
 ```bash
@@ -66,4 +76,5 @@ uv run playwright install chromium           # once
 uv run pytest tests/e2e/test_smoke.py -v --no-cov
 uv run ruff check . && uv run pyright        # lint + types
 uv run python -m scripts.eval_retrieval --from-file tests/fixtures/retrieval_battery.txt   # retrieval regression battery (phases 119/162)
+uv run python -m scripts.api_payload_audit                      # endpoint byte audit vs the 256 KB budget (phase 167; app must be running)
 ```

README.md

@@ -159,6 +159,30 @@ document's date from the document viewer. The date also feeds the small
 recency boost in retrieval ranking (`BOR_RECENCY_BOOST`). Admin-only —
 anonymous visitors see a sign-in gate instead.
 
+The tree payload is bounded (phase 167): `GET /api/docs/tree` builds only
+`depth` levels (default **1** — the sources plus one level, the page's first
+paint; `depth=2..8` builds deeper, anything else is a 422) instead of the old
+~2 MB full recursive dump. A node whose children were not built carries
+`"truncated": true` and ships with its **recursive** document count, updated
+date, description and pending marker (`children: []`); untruncated nodes are
+byte-for-byte what they were before. The three stat cards read the
+server-computed top-level `totals` (`documents`, `chunks`, `last_indexed`) so
+they stay honest whatever depth was requested. Responses also ship gzipped
+(see *Caching & deploys*). Deeper levels load on demand through
+`GET /api/docs/tree/node?source=X&path=Y&depth=N` (admin-only, phase 167):
+one node — the source root for `path=""` — answered in exactly the shape the
+full tree produces for it (unknown source or folder → 404 `unknown node`),
+built from the same single catalogue query. The RAG view fetches each drilled
+level once and caches it (re-opening a folder costs no network), showing one
+*Loading…* row while a level arrives and a *Couldn't load … — Retry* line on
+failure with the current level staying rendered; a folder that vanished since
+it was listed (the node 404) resets the view to the top level. The
+breadcrumb, the bounded scroll panel, and the description editors behave
+exactly as before. All three catalogue reads
+(`/api/docs`, `/api/docs/tree`, `/api/docs/tree/node`) also carry a weak
+`ETag` + `Cache-Control: private, must-revalidate`, so reloading a page you
+just looked at costs a bodiless 304 (see *Caching & deploys*).
+
 ### Git sources (`/git-sources.html`)
 
 The admin-managed source registry: add or remove git repositories, upload
@@ -942,8 +966,13 @@ uploaded archives):
 ## Caching & deploys
 
 A deploy is a commit — and the browser must see it **without a hard refresh**.
-One Starlette middleware (`app/core/caching.py`) applies the rule at the
-transport layer:
+The first three bullets below are that A19 rule, applied at the transport layer
+by `app/core/caching.py`; the last two are phase 167 — gzip from
+`app/core/compression.py` and per-route validators from `app/core/http_cache.py`.
+Outermost to innermost, the stack is security-headers → gzip (only when
+`BOR_COMPRESS_MIN_BYTES > 0`) → the byte-budget logger → caching → routes, so
+security headers land on every response and `bytes=` in the budget line is the
+route's own, uncompressed payload (see *Operational notes*):
 
 - **HTML pages** ship `Cache-Control: no-cache`, **no `etag`**, **no
   `last-modified`** — each visit always gets a fresh 200 body.
@@ -954,6 +983,26 @@ transport layer:
   `HEAD` — so every commit/deploy flips the token and the versioned asset
   URLs change with it. A checkout without `.git` falls back to a stable
   content hash of the `frontend/` tree.
+- **Responses are gzipped (phase 167).** Every HTTP response except the
+  `POST /api/chat` SSE stream goes through selective gzip
+  (`app/core/compression.py`): when the client accepts gzip and the body
+  is at least `BOR_COMPRESS_MIN_BYTES` (default `860`; `0` = the feature
+  is OFF), KB JSON ships ~8-12x smaller — the fix for the ~2 MB
+  `/api/docs/tree` load. The SSE stream is skipped by EXACT path (A15:
+  frames keep arriving one by one, uncompressed), small responses stay
+  identity, and an already-encoded body is never double-encoded, so a
+  fronting nginx may gzip too.
+- **Catalogue reads revalidate (phase 167).** `GET /api/docs`,
+  `GET /api/docs/tree` and `GET /api/docs/tree/node` ship a content-exact
+  weak `ETag` (SHA-256 of the served body, `app/core/http_cache.py`) with
+  `Cache-Control: private, must-revalidate` — a repeat load costs a 304
+  with no body, and a sync or an admin edit (folder summary, document
+  date) changes the body, hence the tag, so it is seen on the next load.
+  The tag is deliberately NOT `sources_meta.version` (bumping that marks
+  saved chats stale, and those edits never bump it). This is a scoped
+  extension of the rule above: validators live on these three read-only
+  routes only — HTML pages keep publishing none, and the SSE stream and
+  every mutating route are untouched.
 
 No CDN, no new services, no build-step change: the middleware rewrites the
 asset references of the known pages in flight.
@@ -962,6 +1011,61 @@ asset references of the known pages in flight.
 > page visit, so the browser revalidates the HTML once and starts requesting
 > the versioned assets; every commit after that is picked up automatically.
 
+### Operational notes: the response byte budget (phase 167)
+
+Every `/api/*` response writes ONE parseable line, so "which endpoint is
+heavy, right now?" is answerable from the app's own log:
+
+```
+2026-10-08 07:40:57 INFO     app.response_budget :: api method=GET path=/api/docs/tree status=200 bytes=8474 enc=gzip duration_ms=12
+```
+
+```bash
+grep ' :: api ' server.log                    # every measured response
+grep ' :: api ' server.log | grep 'bytes=[0-9]\{6,\}'   # the six-figure ones
+```
+
+- **`bytes=`** is what the ROUTE produced (uncompressed), **`enc=`** is what
+  went on the wire (`gzip` or `identity`) — "how fat is it, and how fat did
+  the client see it". A revalidated `304` logs `bytes=0` on purpose: that is
+  the win you want to see.
+- **Level:** `INFO` at or over `BOR_LOG_LARGE_RESPONSE_BYTES` (default
+  `65536`), `DEBUG` below it, so the default log stays readable.
+  `BOR_LOG_LARGE_RESPONSE_BYTES=0` logs EVERY `/api/*` response at INFO —
+  the mode for a one-off investigation. A negative value refuses startup.
+- **`stream=1`** marks the SSE turn (`POST /api/chat`, A15): `bytes` is the
+  streamed total and `duration_ms` the stream's life. One line per stream,
+  written when it ends — including a stream the client cancels (with the
+  bytes that got out). The middleware is pure ASGI and never buffers.
+
+**Measuring the whole API** (the audit behind the fix):
+
+```bash
+# the corpus that matches production's order of magnitude
+#   (destructive to the local DB — see the restore-test-db skill)
+uv run uvicorn app.main:app --port 8000
+uv run python -m scripts.api_payload_audit --base-url http://localhost:8000
+```
+
+It signs in with `BOR_ADMIN_PASSWORD` (or `--password`; the single-admin
+login has no username), measures every candidate read route in BOTH
+`identity` and `gzip` forms plus one conditional revalidation, prints the
+corpus it measured, and marks anything over `--budget` (default `262144`)
+with `LOOK`. It is **read-only** (safe against the deployed app) and always
+exits 0 when it got its numbers — it is a report, not a gate (LOCKED A5:
+nothing in this phase rejects a large payload). Exit `2` means the app was
+unreachable or the login was rejected. The measured before/after tables and
+the verdict per endpoint live in the phase record — `167_api_payload_diet/AUDIT.md`
+under `.agents/phases/`.
+
+> **Deploy note (compression):** the app gzips its own responses, so nginx
+> `gzip on` is **optional** — leaving it off ships the same bytes, and
+> leaving it on is fine too: an already-encoded body is never double-encoded
+> by either side. Passing the client's header through untouched
+> (`proxy_set_header Accept-Encoding $http_accept_encoding;`, or simply not
+> overriding it) is correct; unsetting it would hand every client
+> uncompressed JSON.
+
 ---
 
 ## Configuration reference
@@ -982,6 +1086,8 @@ to `.env` and adjust.
 | `BOR_ENVIRONMENT` | `development` | `development` or `production` (the prod compose profile sets `production`) |
 | `BOR_LOG_LEVEL` | `INFO` | App log level |
 | `BOR_STATIC_DIR` | `frontend` | Where static files are served from (the container image sets `/app/static`) |
+| `BOR_COMPRESS_MIN_BYTES` | `860` | Selective-gzip floor (phase 167): responses ≥ this size compress when the client accepts gzip; `0` = OFF (no middleware), negative refuses startup. `POST /api/chat` (SSE) is never compressed |
+| `BOR_LOG_LARGE_RESPONSE_BYTES` | `65536` | INFO/DEBUG cut for the per-response `api method=… bytes=… enc=…` byte-budget line (phase 167, `grep ' :: api '`); `0` = log every `/api/*` response at INFO, negative refuses startup. See *Operational notes* |
 
 ### Database & concurrency
 

app/api/docs.py

@@ -18,10 +18,34 @@ date folds to today, D3) + flag it manual, or clear the manual flag
 write: NO LLM/embedding call — a date is never embedded (the
 deliberate contrast with the phase-57 ``is_summary`` re-embed).
 
-GET /api/docs/tree — the admin's full recursive KB tree in one fetch
-(phase 97, task 02): the same drill-down tree the agent's ``ls``
-walks, with the file metadata the RAG view's rows and stat cards need
-(the view drills client-side; ``GET /api/docs`` is untouched).
+GET /api/docs/tree — the admin's KB tree (phase 97, task 02; depth-
+limited since phase 167, task 02): the same drill-down tree the agent's
+``ls`` walks, with the file metadata the RAG view's rows and stat cards
+need. The default ``depth=1`` ships the sources plus ONE level with
+server-computed ``totals`` — a node whose children were not built
+carries ``truncated: true`` (the TODO L5 2 MB payload fix; ``GET
+/api/docs`` and the agent's ``ls`` path are untouched).
+
+GET /api/docs/tree/node — ONE tree node's direct children (phase 167,
+task 03, D3): the RAG view's per-level drill fetch — the same catalogue
+composition as ``/api/docs/tree`` (ONE catalogue query per drill, no
+N+1) resolved to a single ``KbTreeSource`` (``path=""``, the source
+root) or ``KbTreeFolder``. Admin-gated like the tree; 404
+``{"detail": "unknown node"}`` for a source that is neither registered
+nor indexed, or a folder failing the phase-94 existence rule. Row-only
+like every other route here: no request value ever reaches a
+filesystem path — ``../``-style values simply are not folders (the
+phase-10 traversal argument, reused).
+
+Revalidation (phase 167, task 03, LOCKED A4 — a scoped extension of
+A19): the three catalogue READ routes (``/api/docs``,
+``/api/docs/tree``, ``/api/docs/tree/node``) serialize once and carry a
+content-exact weak ``ETag`` + ``Cache-Control: private,
+must-revalidate`` (the :mod:`app.core.http_cache` helpers), so a repeat
+load costs a 304 with no body. The tag is a hash of the served bytes —
+NOT ``sources_meta.version`` (bumping that would mark saved chats
+stale, and the admin PATCHes do not bump it). ``sources_meta``, the
+SSE stream and the mutating surface are untouched.
 
 GET /api/documents/{doc_id}/image — the phase-122 (task 04) image
 BYTES route: the persistent copy behind an ``is_image`` document's
@@ -70,14 +94,16 @@ from collections.abc import Mapping, Sequence
 from datetime import UTC, datetime
 from pathlib import Path
 
-from fastapi import APIRouter, Depends, HTTPException
-from fastapi.responses import FileResponse
+from fastapi import APIRouter, Depends, HTTPException, Query, Request
+from fastapi.responses import FileResponse, Response
+from pydantic import BaseModel
 from sqlalchemy import func, select
 from sqlalchemy.orm import Session
 
 from app.api.sync import _sanitize_error
 from app.config import get_settings
 from app.core.auth import require_admin, require_user
+from app.core.http_cache import etag_matches, revalidate_headers, weak_etag
 from app.db import get_db
 from app.models import Chunk, Document, FolderSummary
 from app.rag.agent import list_source_names
@@ -99,6 +125,7 @@ from app.schemas import (
     KbTreeFile,
     KbTreeFolder,
     KbTreeSource,
+    KbTreeTotals,
     SummaryResult,
     SummaryUpdate,
 )
@@ -128,9 +155,10 @@ def doc_format(path: str, extensions: frozenset[str] = frozenset()) -> str:
 
 @router.get("/docs", response_model=DocList)
 def list_indexed_documents(
+    request: Request,
     db: Session = Depends(get_db),  # noqa: B008
     _admin: None = Depends(require_admin),  # noqa: B008
-) -> DocList:
+) -> Response:
     """All indexed documents with per-document chunk counts.
 
     Admin-only (phase 16 — the catalog is what the admin sign-in gates;
@@ -139,6 +167,16 @@ def list_indexed_documents(
     ``admin only`` and the Sources page renders its sign-in gate instead.
     An empty list means the knowledge base has not been imported yet —
     the Sources page renders its designed empty state in that case.
+
+    Phase 167 (task 03, D3/A4): the response carries the content-exact
+    weak ``ETag`` + ``Cache-Control: private, must-revalidate`` pair
+    (see :mod:`app.core.http_cache`) — the client is ``fetch``, which
+    serves a matched conditional request from its cache transparently,
+    and ``must-revalidate`` guarantees a sync is seen on the next load
+    (the tag changes because the body changes). The decorator keeps
+    ``response_model=DocList`` for the OpenAPI schema only — a returned
+    :class:`~fastapi.responses.Response` short-circuits FastAPI's
+    serialization, so the model is dumped exactly ONCE here.
     """
     rows = db.execute(
         select(
@@ -161,7 +199,7 @@ def list_indexed_documents(
         )
         .order_by(Document.source, Document.path)
     ).all()
-    return DocList(
+    model = DocList(
         documents=[
             DocSummary(
                 id=str(row.id),
@@ -175,6 +213,7 @@ def list_indexed_documents(
             for row in rows
         ]
     )
+    return _revalidated_json(request, model)
 
 
 @router.get("/documents/content", response_model=DocContent)
@@ -625,8 +664,9 @@ TreeFileRow = tuple[str, str, int, str, str]
 
 def _folder_counts(
     rows: Sequence[TreeFileRow],
-) -> tuple[set[str], dict[str, int]]:
-    """One source's folders (existence rule) + recursive counts (pure).
+) -> tuple[set[str], dict[str, int], dict[str, str]]:
+    """One source's folders (existence rule) + recursive counts + the
+    recursive max-date map (all pure).
 
     The phase-94 rules, reused verbatim from
     :func:`app.rag.agent.group_folder_listing` (ONE concept end to end —
@@ -640,9 +680,22 @@ def _folder_counts(
     ``folder + "/"`` — exactly the set the sync-time folder summary
     describes.
 
-    Returns ``(folders, counts)`` — the folder-name set and the
+    Returns ``(folders, counts, max_dates)`` — the folder-name set, the
     per-folder count map (every folder counts ≥ 1 by construction: its
-    own descendants, or the file wearing its name, exist).
+    own descendants, or the file wearing its name, exist), and the
+    per-folder RECURSIVE MAX document ``created_at`` map — the same
+    subtree the count walks, over the same ISO-8601 strings (one
+    ``isoformat()`` shape, so the lexicographic max is the
+    chronological one — the phase-106 D9 rule).
+
+    The max map is the phase-167 (task 02, D2) truncation-proof date
+    source: a depth-limited node has NO built children, so
+    :func:`_subtree_max` over its empty child list would read ``None``
+    and silently lose the D9 column; the aggregate — computed from the
+    rows in the same pass — keeps the date honest at every depth. The
+    expanded path keeps deriving ``updated_at`` via
+    :func:`_subtree_max` (one concept, two derivations — cross-pinned
+    equal by a unit test).
     """
     folders: set[str] = set()
     for path, _title, _chunks, _indexed_at, _created_at in rows:
@@ -651,14 +704,19 @@ def _folder_counts(
             folders.add(folder)
             folder = folder_of(folder)
     counts: dict[str, int] = {folder: 0 for folder in folders}
-    for path, _title, _chunks, _indexed_at, _created_at in rows:
+    max_dates: dict[str, str] = {}
+    for path, _title, _chunks, _indexed_at, created_at in rows:
         if path in folders:
             counts[path] += 1
+            if created_at > max_dates.get(path, ""):
+                max_dates[path] = created_at
         folder = folder_of(path)
         while folder:
             counts[folder] += 1
+            if created_at > max_dates.get(folder, ""):
+                max_dates[folder] = created_at
             folder = folder_of(folder)
-    return folders, counts
+    return folders, counts, max_dates
 
 
 #: One image-docs map value (phase 122, task 04):
@@ -673,9 +731,11 @@ def _level_children(
     folder: str,
     folders: set[str],
     counts: dict[str, int],
+    max_dates: dict[str, str],
     rows: Sequence[TreeFileRow],
     summaries: Mapping[tuple[str, str], str],
     images: Mapping[tuple[str, str], ImageDocInfo] | None = None,
+    depth: int | None = None,
 ) -> list[KbTreeFolder | KbTreeFile]:
     """One level's children (pure): subfolders in path order, then the
     direct files in input (catalog) order.
@@ -691,6 +751,20 @@ def _level_children(
     file list is NOT capped (the ``ls`` 50-line cap is a model-context
     budget — the UI is for humans). Recurses one level per call.
 
+    *depth* (phase 167, task 02, D2/A2) is the REMAINING build budget
+    for this level's subfolders: ``None`` = unlimited (the pre-phase
+    recursion, byte-identical); a positive budget is spent here and
+    ``depth - 1`` rides the recursion. When the budget is exhausted
+    (``depth == 1`` reaching its subfolders) the subfolder nodes are
+    TRUNCATED instead of recursed: ``children=[]``, ``truncated=True``,
+    and every field the level row still renders — the recursive
+    ``documents`` count, the recursive ``updated_at`` from *max_dates*
+    (NOT ``_subtree_max`` over the empty child list — the D9 trap), the
+    node's own ``summary`` (the row shows a SUBFOLDER's description),
+    and ``summary_pending`` under the unchanged rule. The level's
+    DIRECT files are always built — the budget limits DEPTH, never the
+    row set of the levels it does send.
+
     Each folder node also carries the phase-98 D3 ``summary_pending``
     flag: the recursive count ≥ :data:`MIN_DOCS_PER_FOLDER` AND no
     stored ``folder_summaries`` row for ``(source, sub)`` — the same
@@ -709,21 +783,40 @@ def _level_children(
     byte-identical).
     """
     children: list[KbTreeFolder | KbTreeFile] = []
+    expand = depth is None or depth > 1
+    next_depth = None if depth is None else depth - 1
     for sub in sorted(g for g in folders if folder_of(g) == folder):
-        sub_children = _level_children(
-            source, sub, folders, counts, rows, summaries, images
-        )
-        children.append(
-            KbTreeFolder(
-                path=sub,
-                documents=counts[sub],
-                updated_at=_subtree_max(sub_children),
-                summary=summaries.get((source, sub)),
-                summary_pending=counts[sub] >= MIN_DOCS_PER_FOLDER
-                and (source, sub) not in summaries,
-                children=sub_children,
+        if expand:
+            sub_children = _level_children(
+                source, sub, folders, counts, max_dates, rows, summaries, images, next_depth
+            )
+            children.append(
+                KbTreeFolder(
+                    path=sub,
+                    documents=counts[sub],
+                    updated_at=_subtree_max(sub_children),
+                    summary=summaries.get((source, sub)),
+                    summary_pending=counts[sub] >= MIN_DOCS_PER_FOLDER
+                    and (source, sub) not in summaries,
+                    children=sub_children,
+                )
+            )
+        else:
+            # Phase 167 (D2): budget exhausted — the node rides with its
+            # recursive aggregate fields and truncated=True; the client
+            # drills via the node route (task 03).
+            children.append(
+                KbTreeFolder(
+                    path=sub,
+                    documents=counts[sub],
+                    updated_at=max_dates[sub],
+                    summary=summaries.get((source, sub)),
+                    summary_pending=counts[sub] >= MIN_DOCS_PER_FOLDER
+                    and (source, sub) not in summaries,
+                    children=[],
+                    truncated=True,
+                )
             )
-        )
     for path, title, chunks, indexed_at, created_at in rows:
         if folder_of(path) == folder:
             image_info = images.get((source, path)) if images else None
@@ -761,7 +854,10 @@ def _subtree_max(children: Sequence[KbTreeFolder | KbTreeFile]) -> str | None:
     """A node's ``updated_at`` (phase 106, D9): the subtree's MAX
     document ``created_at``, computed from the node's direct children —
     files contribute their ``created_at``, subfolder nodes contribute
-    their (already recursive) ``updated_at``.
+    their (already recursive) ``updated_at``. UNCHANGED for the
+    expanded path since phase 167 (the truncated path uses the
+    :func:`_folder_counts` max map instead — one concept, two
+    derivations, cross-pinned equal by a unit test).
 
     All values share the same UTC ``isoformat()`` shape (the endpoint
     converts every ``created_at`` before the builder runs), so the
@@ -782,6 +878,7 @@ def build_kb_tree(
     doc_rows: Sequence[TreeDocRow],
     summaries: Mapping[tuple[str, str], str],
     images: Mapping[tuple[str, str], ImageDocInfo] | None = None,
+    depth: int | None = None,
 ) -> list[KbTreeSource]:
     """The pure tree builder behind ``GET /api/docs/tree`` (phase 97,
     task 02) — module-level and DB-free so unit tests drive it
@@ -847,6 +944,17 @@ def build_kb_tree(
       pending, single-file folders included — and a registered
       0-document source never is.
 
+    *depth* (phase 167, task 02, D2/A2) bounds how deep the tree is
+    BUILT: ``None`` (the default) = unlimited — every pre-phase call
+    and expectation is byte-identical; a positive budget stops the
+    recursion (see :func:`_level_children`) so the response carries the
+    sources plus *depth* levels, every node whose children were not
+    built flagged ``truncated=True`` with its recursive count, its
+    recursive ``updated_at`` (the max-date map — D9 survives
+    truncation), its ``summary`` and ``summary_pending``. A SOURCE node
+    is never truncated at ``depth ≥ 1`` — the view's top level is
+    source → level 1, so the source's direct children are always built.
+
     ONE concept end to end: the builder reuses
     :func:`app.rag.folder_summaries.folder_of` and the phase-94
     existence / count rules, so — for a single-source dataset — its
@@ -872,10 +980,14 @@ def build_kb_tree(
         if name in listed:  # defensive: list_source_names dedupes
             continue
         listed.add(name)
-        tree.append(_source_node(name, by_source.get(name, ()), summaries, images))
+        tree.append(
+            _source_node(name, by_source.get(name, ()), summaries, images, depth)
+        )
     for source in sorted(by_source):
         if source not in listed:
-            tree.append(_source_node(source, by_source[source], summaries, images))
+            tree.append(
+                _source_node(source, by_source[source], summaries, images, depth)
+            )
     return tree
 
 
@@ -884,6 +996,7 @@ def _source_node(
     rows: Sequence[TreeFileRow],
     summaries: Mapping[tuple[str, str], str],
     images: Mapping[tuple[str, str], ImageDocInfo] | None = None,
+    depth: int | None = None,
 ) -> KbTreeSource:
     """One source node (pure): whole-source count + the source-root
     summary + the root level's children (direct subfolders + direct
@@ -902,10 +1015,17 @@ def _source_node(
 
     ``updated_at`` (phase 106, D9): the source's subtree max —
     :func:`_subtree_max` over the root level's children; ``None`` for a
-    0-document source (no children, no dates).
+    0-document source (no children, no dates). Correct at every depth:
+    a TRUNCATED child still carries its recursive ``updated_at`` (the
+    max-date map), so the source's max never silently drops the levels
+    below the budget (phase 167, D2). The SOURCE node itself is never
+    marked ``truncated`` at ``depth ≥ 1`` — its direct children are
+    always built (the view's top level).
     """
-    folders, counts = _folder_counts(rows)
-    children = _level_children(source, "", folders, counts, rows, summaries, images)
+    folders, counts, max_dates = _folder_counts(rows)
+    children = _level_children(
+        source, "", folders, counts, max_dates, rows, summaries, images, depth
+    )
     return KbTreeSource(
         name=source,
         documents=len(rows),
@@ -916,32 +1036,56 @@ def _source_node(
     )
 
 
-@router.get("/docs/tree", response_model=KbTree)
-def list_kb_tree(
-    db: Session = Depends(get_db),  # noqa: B008
-    _admin: None = Depends(require_admin),  # noqa: B008
-) -> KbTree:
-    """The full recursive KB tree in ONE fetch (phase 97, task 02).
+def _kb_totals(doc_rows: Sequence[TreeDocRow]) -> KbTreeTotals:
+    """The catalogue totals beside the tree (phase 167, task 02, D2) —
+    computed from the SAME rows the builder consumes (no extra query):
+    ``documents = len(rows)``, ``chunks`` = the sum of the per-document
+    chunk counts, ``last_indexed`` = the MAX ``indexed_at`` stamp
+    (lexicographic max = chronological max over one ISO-8601 shape),
+    ``None`` for an empty KB. The RAG view's stat cards read these
+    because a depth-limited tree cannot be summed client-side (the
+    phase-97 ``treeStats()`` walk only sees the sent levels).
+    """
+    return KbTreeTotals(
+        documents=len(doc_rows),
+        chunks=sum(chunks for _s, _p, _t, chunks, _i, _c in doc_rows),
+        last_indexed=max((indexed for _s, _p, _t, _ch, indexed, _c in doc_rows), default=None),
+    )
 
-    Admin-only, like ``GET /api/docs`` — anonymous callers get 403
-    ``admin only`` (the RAG view's anonymous gate never fetches the
-    tree). The RAG view drills CLIENT-side: this is the view's single
-    fetch, zero per-level requests (the ``00_phase.md`` "The tree
-    endpoint" contract).
 
-    Composition: the registry source names (``list_source_names`` —
-    the superset rule's registry half, imported from ``app.rag.agent``
-    exactly as ``app/api/chat.py`` does) + the SAME outerjoin/grouped
-    catalogue query ``GET /api/docs`` runs (the document ``id``
-    excluded — the tree has no document ids) + ALL stored
-    ``folder_summaries`` rows (a bounded select — one row per
-    existing folder at the ≥ 1-doc rule; rows for sources the tree
-    does not list are never referenced by the builder) + the phase-122
-    (task 04) image-docs map (a second bounded select over the
-    ``is_image`` rows with a servable ``image_path`` — empty for every
-    pre-phase KB, so the response stays byte-identical to pre-phase)
-    — through the pure :func:`build_kb_tree`. ``GET /api/docs`` itself
-    is untouched.
+def _kb_catalogue(
+    db: Session,
+) -> tuple[
+    list[str],
+    list[TreeDocRow],
+    dict[tuple[str, str], str],
+    dict[tuple[str, str], ImageDocInfo],
+]:
+    """The catalogue composition the tree routes share (phase 167,
+    task 03) — ONE outerjoin/grouped catalogue query plus the two
+    bounded side selects, no N+1 anywhere (the drill pays exactly what
+    the first paint pays). Returns ``(names, doc_rows, summaries,
+    images)``:
+
+    * *names* — the registry source names (``list_source_names`` — the
+      superset rule's registry half, imported from ``app.rag.agent``
+      exactly as ``app/api/chat.py`` does);
+    * *doc_rows* — the SAME outerjoin/grouped catalogue query
+      ``GET /api/docs`` runs (the document ``id`` excluded — the tree
+      has no document ids), both stamps converted to ISO-8601 HERE so
+      the pure builder stays DB-free;
+    * *summaries* — ALL stored ``folder_summaries`` rows (a bounded
+      select — one row per existing folder at the ≥ 1-doc rule; rows
+      for sources the tree does not list are never referenced by the
+      builder);
+    * *images* — the phase-122 (task 04) image-docs map: a second
+      bounded select over the ``is_image`` rows with a servable
+      ``image_path`` only (a handful of rows at KB scale, never the
+      whole catalog; the catalogue query above stays byte-identical).
+      EMPTY for every pre-phase KB (no image rows), so the response
+      stays byte-identical to pre-phase — the fields are row-driven,
+      not toggle-driven (a surviving image doc keeps its thumbnail
+      through a toggle-off sync, the prune guard's UX side).
     """
     names = list_source_names(db)
     rows = db.execute(
@@ -974,14 +1118,7 @@ def list_kb_tree(
             select(FolderSummary.source, FolderSummary.folder_path, FolderSummary.summary)
         ).all()
     }
-    # Phase 122 (task 04): the image-docs affordance map — a bounded
-    # select over the ``is_image`` rows only (a handful of rows at KB
-    # scale, never the whole catalog; the catalogue query above stays
-    # byte-identical). EMPTY for every pre-phase KB (no image rows), so
-    # the response stays byte-identical to pre-phase — the fields are
-    # row-driven, not toggle-driven (a surviving image doc keeps its
-    # thumbnail through a toggle-off sync, the prune guard's UX side).
-    images: dict[tuple[str, str], tuple[str, str]] = {
+    images: dict[tuple[str, str], ImageDocInfo] = {
         (source, path): (str(doc_id), summary)
         for source, path, doc_id, summary in db.execute(
             select(
@@ -992,4 +1129,163 @@ def list_kb_tree(
             ).where(Document.is_image.is_(True), Document.image_path.is_not(None))
         ).all()
     }
-    return KbTree(sources=build_kb_tree(names, doc_rows, summaries, images))
+    return names, doc_rows, summaries, images
+
+
+def _revalidated_json(request: Request, model: BaseModel) -> Response:
+    """Serialize once, hash the EXACT served bytes, answer 304 when the
+    client already holds them (phase 167, task 03, D3/A4 — the
+    :mod:`app.core.http_cache` pattern in ONE place for the three
+    catalogue reads).
+
+    The client is ``fetch``: a matched conditional request is served
+    from its cache transparently (fetch never surfaces the 304), and
+    ``must-revalidate`` guarantees a sync or an admin edit is seen on
+    the next load — the tag changes because the body changes (the A4
+    proof: a ``sources_meta.version``-derived tag would keep serving
+    stale trees after the folder-summary / document-date PATCHes, which
+    never bump it). ``model_dump_json`` runs exactly ONCE — the same
+    string feeds the tag and the body, and the returned ``Response``
+    short-circuits FastAPI's own serialization.
+    """
+    body = model.model_dump_json()
+    etag = weak_etag(body)
+    if etag_matches(request.headers.get("if-none-match"), etag):
+        return Response(status_code=304, headers=revalidate_headers(etag))
+    return Response(
+        content=body, media_type="application/json", headers=revalidate_headers(etag)
+    )
+
+
+@router.get("/docs/tree", response_model=KbTree)
+def list_kb_tree(
+    request: Request,
+    depth: int = Query(1, ge=1, le=8),
+    db: Session = Depends(get_db),  # noqa: B008
+    _admin: None = Depends(require_admin),  # noqa: B008
+) -> Response:
+    """The KB tree, DEPTH-LIMITED by default (phase 97, task 02;
+    phase 167, task 02, D2/A2 — the TODO L5 2 MB payload fix).
+
+    ``depth`` bounds the levels built under each source:
+    ``1`` (default) = sources + ONE level — the RAG view's first paint;
+    ``8`` = the practical KB ceiling (the whole tree, the pre-phase
+    shape plus the additive ``totals`` key). Every node whose children
+    were NOT built carries ``truncated: true`` (with its recursive
+    count, recursive ``updated_at``, ``summary`` and
+    ``summary_pending`` intact) so the view can drill level by level;
+    ``totals`` (documents / chunks / ``last_indexed``) is computed here
+    from the full catalogue rows, so the stat cards stay honest at any
+    depth.
+
+    Admin-only, like ``GET /api/docs`` — anonymous callers get 403
+    ``admin only`` (the RAG view's anonymous gate never fetches the
+    tree). The phase-97 "one fetch, drill client-side" contract is
+    deliberately superseded (LOCKED A3): one initial load plus one
+    fetch per drilled level (task 03's node route).
+
+    Composition: :func:`_kb_catalogue` (the registry source names, the
+    SAME outerjoin/grouped catalogue query ``GET /api/docs`` runs, ALL
+    stored ``folder_summaries`` rows, and the phase-122 image-docs map)
+    through the pure :func:`build_kb_tree` + :func:`_kb_totals`.
+    ``GET /api/docs`` itself is untouched.
+
+    Phase 167 (task 03, D3/A4): the response carries the content-exact
+    weak ``ETag`` + ``Cache-Control: private, must-revalidate`` pair via
+    :func:`_revalidated_json` — a repeat load costs a 304 with no body
+    (the client is ``fetch``, which serves a matched conditional request
+    from its cache transparently). The decorator keeps
+    ``response_model=KbTree`` for the OpenAPI schema only — a returned
+    :class:`~fastapi.responses.Response` short-circuits FastAPI's
+    serialization, so the model is dumped exactly ONCE here.
+    """
+    names, doc_rows, summaries, images = _kb_catalogue(db)
+    model = KbTree(
+        sources=build_kb_tree(names, doc_rows, summaries, images, depth),
+        totals=_kb_totals(doc_rows),
+    )
+    return _revalidated_json(request, model)
+
+
+@router.get("/docs/tree/node", response_model=None)
+def get_kb_tree_node(
+    request: Request,
+    source: str = Query(...),
+    path: str = Query(""),
+    depth: int = Query(1, ge=1, le=8),
+    db: Session = Depends(get_db),  # noqa: B008
+    _admin: None = Depends(require_admin),  # noqa: B008
+) -> KbTreeSource | KbTreeFolder | Response:
+    """ONE tree node's direct children — the RAG view's per-level drill
+    fetch (phase 167, task 03, D3/A3: the deliberate cost of the small
+    first payload is one request per drilled level).
+
+    ``source`` is the registry name OR an indexed source (the superset
+    rule); ``path`` is SOURCE-RELATIVE, ``""`` (the default) = the
+    source root. The node answers in the shape the full tree produces
+    for the same node — a :class:`~app.schemas.KbTreeSource` for
+    ``path == ""``, else a :class:`~app.schemas.KbTreeFolder` carrying
+    the recursive ``documents`` count, the recursive ``updated_at``
+    (the phase-106 D9 max — the truncation-proof max-date map), its
+    ``summary`` and ``summary_pending`` (the unchanged phase-98 D3
+    rule). The node's DIRECT children are always built — the *depth*
+    budget only decides whether those children's subfolders recurse
+    (the same rule that never truncates a SOURCE node at ``depth ≥ 1``,
+    see :func:`_level_children`) — so the node itself is never
+    ``truncated``: what arrived is what the level renders, and deeper
+    levels ride the children's own flags.
+
+    Admin-gated — the same :func:`~app.core.auth.require_admin` gate as
+    ``/api/docs/tree`` (anonymous → 403 ``admin only``). 404
+    ``{"detail": "unknown node"}`` when the source is NEITHER
+    registered NOR indexed, or when a non-empty ``path`` fails the
+    phase-94 existence rule (a folder exists ⟺ some indexed path
+    starts with ``path + "/"`` — a document's OWN path is not a folder,
+    unless a deeper path wears it).
+
+    Row-only, like every route in this file: NOTHING here builds a
+    filesystem path from ``path`` — the node is derived from catalogue
+    ROWS alone, so a ``../``-style value is simply not a folder and 404s
+    with no filesystem surface (the phase-10 traversal argument, reused
+    by :func:`build_kb_tree`).
+
+    Composition: the SAME catalogue query ``list_kb_tree`` runs (via
+    :func:`_kb_catalogue` — ONE query per drill, no N+1; the tree's
+    counts and recursive dates are only honest over the whole
+    catalogue), then the pure helpers scoped to the one source. Carries
+    the same ``ETag`` + ``private, must-revalidate`` pair as the other
+    two catalogue reads (:func:`_revalidated_json`).
+    ``response_model=None`` on the decorator: the return is a UNION
+    annotated for the reader while the body is returned as a raw
+    ``Response`` — the serializer must not run twice (once for the
+    tag, once inside FastAPI).
+    """
+    names, doc_rows, summaries, images = _kb_catalogue(db)
+    rows: list[TreeFileRow] = [
+        (row_path, title, chunks, indexed_at, created_at)
+        for row_source, row_path, title, chunks, indexed_at, created_at in doc_rows
+        if row_source == source
+    ]
+    if source not in names and not rows:
+        raise HTTPException(status_code=404, detail="unknown node")
+    folders, counts, max_dates = _folder_counts(rows)
+    if path and path not in folders:
+        raise HTTPException(status_code=404, detail="unknown node")
+    node: KbTreeSource | KbTreeFolder
+    if path:
+        children = _level_children(
+            source, path, folders, counts, max_dates, rows, summaries, images, depth
+        )
+        node = KbTreeFolder(
+            path=path,
+            documents=counts[path],
+            updated_at=max_dates[path],
+            summary=summaries.get((source, path)),
+            summary_pending=counts[path] >= MIN_DOCS_PER_FOLDER
+            and (source, path) not in summaries,
+            children=children,
+        )
+    else:
+        # The source root — the very node the full tree builds.
+        node = _source_node(source, rows, summaries, images, depth)
+    return _revalidated_json(request, node)

app/config.py

@@ -46,6 +46,35 @@ class Settings(BaseSettings):
     log_level: str = "INFO"
     static_dir: str = "frontend"
 
+    # --- HTTP response compression (phase 167, TODO L5) ---
+    #: Byte floor for the selective gzip layer
+    #: (``app/core/compression.py``, ``BOR_COMPRESS_MIN_BYTES``): responses
+    #: of at least this many bytes are gzipped when the client accepts it
+    #: (Starlette's ``GZipMiddleware`` owns the negotiation, the
+    #: already-encoded skip and the ``Vary`` append). ``0`` = the feature
+    #: is OFF — the middleware is not added at all (the kill switch);
+    #: ``> 0`` = the Starlette minimum-size floor, default 860 — the
+    #: classic nginx default, below which compression can INFLATE a body
+    #: instead of shrinking it. A negative value is a typo — the validator
+    #: fails startup loudly (the ``agent_max_rounds`` pattern). The SSE
+    #: stream (``POST /api/chat``, A15) is never compressed at any value.
+    compress_min_bytes: int = 860
+
+    # --- Response byte budget (phase 167, TODO L5) ---
+    #: INFO/DEBUG cut for the per-response ``api method=… bytes=…`` line
+    #: (``app/core/response_budget.py``, ``BOR_LOG_LARGE_RESPONSE_BYTES``).
+    #: ``/api/*`` responses of at least this many bytes (the bytes the ROUTE
+    #: produced, before compression) log at INFO; below it the line is
+    #: DEBUG, i.e. invisible at the default ``BOR_LOG_LEVEL=INFO`` so the
+    #: log stays readable while "which endpoint is heavy, right now" is
+    #: still one ``grep ' :: api '`` away. Default 65536 (64 KB) — the
+    #: "fine" bar of the phase-167 audit; the audit's 256 KB budget is what
+    #: marks an offender (LOCKED A5: a log line and a script, never a 413).
+    #: ``0`` = log EVERY ``/api/*`` response at INFO (the loud mode, for a
+    #: one-off investigation); a negative value is a typo — the validator
+    #: fails startup loudly (the ``agent_max_rounds`` pattern).
+    log_large_response_bytes: int = 65536
+
     # --- UI customization (phase 62, TODO L3) ---
     # Defaults are the phase-61 neutral copy — UNSET => byte-identical UI.
     # (Phase 91, task 03: the retired CSS-file theme env var is gone —
@@ -768,6 +797,29 @@ class Settings(BaseSettings):
             raise ValueError("chat_max_concurrent must be >= 1")
         return v
 
+    @field_validator("compress_min_bytes")
+    @classmethod
+    def _compress_min_bytes_non_negative(cls, v: int) -> int:
+        """``0`` is the compression kill switch (the middleware is not
+        added at all) — a negative value is a typo (the
+        ``agent_max_rounds`` pattern, phase 167)."""
+        if v < 0:
+            raise ValueError("compress_min_bytes must be >= 0 (0 = compression disabled)")
+        return v
+
+    @field_validator("log_large_response_bytes")
+    @classmethod
+    def _log_large_response_bytes_non_negative(cls, v: int) -> int:
+        """``0`` is the loud mode (every ``/api/*`` response logs its byte
+        line at INFO) — a negative value is a typo that would make the
+        comparison constant, so it fails startup naming the field (the
+        ``agent_max_rounds`` pattern, phase 167)."""
+        if v < 0:
+            raise ValueError(
+                "log_large_response_bytes must be >= 0 (0 = log every /api/* response)"
+            )
+        return v
+
     @field_validator("docs_branch", "docs_base_branch")
     @classmethod
     def _docs_branch_tokens(cls, v: str, info: ValidationInfo) -> str:

app/core/compression.py

@@ -0,0 +1,179 @@
+"""Selective gzip for every HTTP response but the SSE chat stream (phase 167, D1, TODO L5).
+
+The defect (TODO L5): ``GET /api/docs/tree`` loads ~2 MB in production and
+the Sources page pays that on every sync and every shell boot. Nothing in
+this stack compressed — ``app/main.py`` added only Session / caching /
+security-headers, and no response ever carried ``Content-Encoding``. KB
+JSON is extremely repetitive (key names, paths, ISO timestamps), so
+gzipping it lands at roughly **8-12× smaller** on the wire: the same bytes
+the client already knows how to read, ~one order of magnitude faster.
+
+The SSE stream is EXCLUDED (A15, phase 167 LOCKED A1): ``POST /api/chat``
+is a ``text/event-stream`` whose whole value is that frames arrive *while
+the model is still talking*. Any middleware that buffers the response —
+``BaseHTTPMiddleware`` is the classic offender, and the reason this module
+is **pure ASGI** — would deliver the entire turn at the end, silently
+breaking the typing animation, the tool lines, and the cancel surface. So
+the skip is by EXACT path (never a prefix: ``/api/chat-images/…`` keeps
+its compression) and the skipped scope is handed to the app untouched.
+
+The compression math is deliberately NOT re-implemented here. Starlette's
+:class:`~starlette.middleware.gzip.GZipMiddleware` owns ``Accept-Encoding``
+negotiation (a client that does not speak gzip gets identity bytes), the
+``minimum_size`` floor, the skip when a response ALREADY sets
+``content-encoding``, the ``Vary: Accept-Encoding`` append, and the
+streaming ``compressobj`` + final flush. It is also what makes the stack
+cooperate with a fronting proxy: **nginx may also gzip, but an
+already-encoded body is never double-encoded** — Starlette preserves the
+upstream ``Content-Encoding`` verbatim, and so does nginx.
+
+One seam IS patched here, and it is framing, not math: the
+``CachingMiddleware`` below us is a ``BaseHTTPMiddleware``, and starlette
+1.x re-frames every response it emits upstream (``_StreamingResponse``
+ships each body chunk with ``more_body=True`` plus an empty terminator),
+so Starlette's buffered small-response exemption can never fire for
+anything that travelled through it — a 33-byte ``/api/health`` would
+stream into the compressor and come back INFLATED, exactly what the
+``minimum_size`` floor exists to prevent. :class:`_RebufferSmallResponses`
+sits under the responder and hands it back the single buffered body
+message the exemption was written for, bounded to the floor.
+"""
+from __future__ import annotations
+
+from starlette.datastructures import Headers
+from starlette.middleware.gzip import GZipMiddleware
+from starlette.types import ASGIApp, Message, Receive, Scope, Send
+
+#: Paths that are NEVER compressed, matched EXACTLY (never by prefix).
+#: ``POST /api/chat`` is the SSE stream (A15): frame-by-frame delivery is
+#: the entire point of the endpoint, and buffering the response to feed it
+#: through a compressor would deliver the whole turn at completion (and a
+#: never-ending stream never completes on a cancel). The exact match is
+#: load-bearing — ``/api/chat-images/…`` shares the prefix and must keep
+#: routing through the compression layer.
+DEFAULT_SKIP_PATHS: tuple[str, ...] = ("/api/chat",)
+
+
+class _RebufferSmallResponses:
+    """Hold a declared-small body so Starlette's floor exemption fires.
+
+    When ``http.response.start`` carries ``content-length`` below
+    ``minimum_size`` — byte-for-byte the condition Starlette's buffered
+    path checks — the messages are held (bounded by the floor) and
+    re-emitted as ONE ``http.response.body`` with ``more_body=False``,
+    undoing the ``BaseHTTPMiddleware`` re-framing so the responder's
+    small-response exemption sees what it was written to see and the
+    bytes stay identity (no ``Content-Encoding``, no ``Vary`` append).
+
+    A lying header can only cost the buffer, never correctness: if the
+    body outgrows the floor despite the header, everything held is
+    forwarded as it came and the rest streams through the normal
+    (compressing) path. Non-body messages while holding (e.g.
+    ``http.response.pathsend``) also flush the hold untouched.
+    """
+
+    def __init__(self, app: ASGIApp, *, minimum_size: int) -> None:
+        self.app = app
+        self.minimum_size = minimum_size
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        holding = False
+        armed = True  # the start decision happens at most once per response
+        held: list[Message] = []
+        total = 0
+
+        async def send_filter(message: Message) -> None:
+            nonlocal holding, armed, total
+            message_type = message["type"]
+
+            if message_type == "http.response.start" and armed:
+                armed = False
+                length = Headers(scope=message).get("content-length")
+                if (
+                    length is not None
+                    and length.isdigit()
+                    and int(length) < self.minimum_size
+                ):
+                    held.append(message)
+                    holding = True
+                    return
+                await send(message)
+                return
+
+            if holding and message_type == "http.response.body":
+                held.append(message)
+                total += len(message.get("body", b""))
+                if not message.get("more_body", False):
+                    # Whole body in hand, still under the floor: re-emit
+                    # it as the single buffered message the exemption wants.
+                    start = held[0]
+                    merged = b"".join(m.get("body", b"") for m in held[1:])
+                    holding = False
+                    held.clear()
+                    await send(start)
+                    await send({"type": "http.response.body", "body": merged, "more_body": False})
+                elif total >= self.minimum_size:
+                    # The content-length lied — stop holding, stream on.
+                    holding = False
+                    for held_message in held:
+                        await send(held_message)
+                    held.clear()
+                return
+
+            if holding:
+                # pathsend or anything unexpected mid-hold: flush as-is.
+                holding = False
+                for held_message in held:
+                    await send(held_message)
+                held.clear()
+
+            await send(message)
+
+        await self.app(scope, receive, send_filter)
+
+
+class SelectiveGZipMiddleware:
+    """Pure-ASGI selective gzip (phase 167, D1 — no ``BaseHTTPMiddleware``).
+
+    Holds the wrapped ``app`` and a composed
+    :class:`~starlette.middleware.gzip.GZipMiddleware` built over the SAME
+    app (via the :class:`_RebufferSmallResponses` seam, so the floor still
+    works under the caching layer's re-framing); ``__call__`` delegates to
+    the gzip path unless the scope is not HTTP (websocket / lifespan pass
+    straight through) or the request path is one of ``skip_paths`` — in
+    which case the app is called DIRECTLY, with the original
+    ``receive``/``send``, so a streaming response reaches the client frame
+    by frame, uncompressed.
+
+    ``minimum_size`` is the Starlette floor (bytes): responses smaller than
+    it are forwarded without compression because gzip can make them
+    bigger. It is supplied by the caller (``Settings.compress_min_bytes``;
+    ``0`` removes this middleware entirely at wiring time — see
+    ``app/main.py``). The SSE stream (``POST /api/chat``, A15) is never
+    compressed at any value.
+    """
+
+    def __init__(
+        self,
+        app: ASGIApp,
+        *,
+        minimum_size: int,
+        skip_paths: tuple[str, ...] = DEFAULT_SKIP_PATHS,
+    ) -> None:
+        self.app = app
+        self.minimum_size = minimum_size
+        self.skip_paths = tuple(skip_paths)
+        # The compression itself stays Starlette's: negotiation, floor,
+        # already-encoded skip, Vary append, streaming compressobj + flush.
+        self.gzip = GZipMiddleware(
+            _RebufferSmallResponses(app, minimum_size=minimum_size),
+            minimum_size=minimum_size,
+        )
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        if scope["type"] != "http" or scope["path"] in self.skip_paths:
+            # Non-HTTP scopes have no response to compress; the SSE stream
+            # must never see this layer (A15 — see DEFAULT_SKIP_PATHS).
+            await self.app(scope, receive, send)
+            return
+        await self.gzip(scope, receive, send)

app/core/http_cache.py

@@ -0,0 +1,112 @@
+"""HTTP revalidation validators for the read-only catalogue reads
+(phase 167, task 03, D3 — LOCKED A4).
+
+Three pure functions, no I/O, ONE place for the pattern (the routes
+compose them; the alternative was a copy per route):
+
+* :func:`weak_etag` — the validator: a weak ``ETag`` over the EXACT
+  bytes being served,
+* :func:`etag_matches` — the tolerant ``If-None-Match`` evaluator,
+* :func:`revalidate_headers` — the header pair a validated response
+  must carry.
+
+**Why a content hash, not ``sources_meta.version``** (LOCKED A4):
+bumping that generation marks every saved chat ``stale`` (phase 53) —
+the wrong tool for a payload revalidation — and the admin PATCHes
+(``/api/documents/summary``, ``/api/documents/date``,
+``/api/folders/summary``) mutate the tree WITHOUT bumping it, so a
+version-derived tag would serve stale trees. A hash of the serialized
+body is content-exact by construction: the tag changes ⟺ the body
+changes, it needs no migration, and the tree is built anyway (the hash
+costs microseconds next to the query). The tag is over the EXACT served
+bytes — the endpoint serializes once and hands those bytes to both the
+hash and the ``Response``.
+
+**Why a truncation to 32 hex chars is safe**: 32 hex chars of SHA-256
+is this file's truncation convention. Collisions are not a correctness
+risk for a revalidated cache: a false match only ever costs one stale
+repaint on a ~2 MB namespace (the next unconditional load corrects it),
+while 128 bits of digest make even that astronomically unlikely.
+
+A19 SCOPE (:func:`revalidate_headers`, LOCKED A4 as a recorded, scoped
+extension of A19): HTML pages stay ``no-cache`` with NO validators (the
+caching middleware strips inbound conditionals and outbound validators —
+the phase-54 contract), ``/assets/*`` stay ``immutable`` for a year
+(URL-versioned), and this module governs the READ-ONLY API arm only —
+today the two KB-tree routes and ``GET /api/docs``. The SSE stream and
+every mutating route are untouched.
+"""
+from __future__ import annotations
+
+import hashlib
+
+#: Hex chars of the SHA-256 digest the tag carries (see the module
+#: docstring's collision argument).
+_TAG_HEX_CHARS = 32
+
+#: The ``Cache-Control`` paired with every validator here: ``private``
+#: (no shared cache may store the admin-gated catalogue) +
+#: ``must-revalidate`` (a sync or an admin edit is seen on the NEXT
+#: load — the client may never serve these bytes without asking, which
+#: is what distinguishes this from the ``/assets/*`` posture).
+REVALIDATE_CACHE_CONTROL = "private, must-revalidate"
+
+
+def weak_etag(body: str | bytes) -> str:
+    """Weak validator over the exact served bytes: ``W/"<sha256 hex[:32]>"``.
+
+    Content-exact by construction (the endpoint hashes the SAME string
+    it returns — see the module docstring for the A4 argument and the
+    truncation convention). ``str`` bodies are hashed as UTF-8, so the
+    str and its ``bytes`` form produce the SAME tag. Weak (``W/``)
+    because the comparison is byte-equivalence under revalidation, not
+    bit-identity — a differently gzipped encoding of the same content
+    must still match (RFC 9110 §8.8.3).
+    """
+    raw = body.encode("utf-8") if isinstance(body, str) else body
+    return f'W/"{hashlib.sha256(raw).hexdigest()[:_TAG_HEX_CHARS]}"'
+
+
+def _strip_weakness(candidate: str) -> str:
+    """One comparable form: trimmed, with the ``W/`` weakness marker gone
+    (weak/strong equivalence — ``W/"x"`` matches ``"x"`` in BOTH
+    directions, RFC 9110 §8.8.3.2's weak comparison rule applied in both
+    directions on purpose: our tag is over the exact bytes, so a client
+    that stored either form HAS these bytes)."""
+    value = candidate.strip()
+    if value[:2].upper() == "W/":
+        value = value[2:].strip()
+    return value
+
+
+def etag_matches(header: str | None, tag: str) -> bool:
+    """Does an inbound ``If-None-Match`` header value match our ``tag``?
+
+    Tolerant the way real clients send it: ``*`` matches (RFC 9110
+    §13.3.3 — the client holds nothing it trusts), a comma-separated
+    candidate list matches on ANY entry, weak/strong forms are
+    equivalent in both directions, and surrounding whitespace is
+    ignored. ``None`` or an empty/whitespace-only header never matches
+    (a request with no conditional header is an unconditional GET → 200).
+    """
+    if not header or not header.strip():
+        return False
+    candidates = header.split(",")
+    if any(candidate.strip() == "*" for candidate in candidates):
+        return True
+    target = _strip_weakness(tag)
+    return any(_strip_weakness(candidate) == target for candidate in candidates)
+
+
+def revalidate_headers(etag: str) -> dict[str, str]:
+    """The ONE header pair every validated catalogue response carries —
+    on the 200 AND on the 304 (RFC 9110 §8.8.3: a 304 must repeat the
+    validator, or the client cannot extend the stored entry's life).
+
+    A19 scope (module docstring): HTML pages stay ``no-cache`` with no
+    validators, ``/assets/*`` stay ``immutable``; this pair is the
+    read-only API arm — ``private, must-revalidate`` so a sync or an
+    admin edit is seen on the next load, because the tag changes exactly
+    when the body changes.
+    """
+    return {"ETag": etag, "Cache-Control": REVALIDATE_CACHE_CONTROL}

app/core/response_budget.py

@@ -0,0 +1,211 @@
+"""The byte budget: ONE log line per ``/api/*`` response (phase 167, task 05, D5, LOCKED A5).
+
+TODO L5 asked "which endpoint is heavy, right now, in production?" — and
+before this phase the app could not answer it. The 2 MB
+``GET /api/docs/tree`` defect was found by a human eyeballing a browser
+network tab; every other candidate endpoint was a guess. This module makes
+the answer a ``grep`` away:
+
+.. code-block:: text
+
+    api method=GET path=/api/docs/tree status=200 bytes=204511 enc=gzip duration_ms=41
+
+* ``bytes`` is the size of the body the ROUTE produced — the sum of the
+  ``http.response.body`` chunk lengths as they pass through, uncompressed
+  even when the wire carried gzip (the compression layer above us encodes
+  AFTER we counted). ``enc`` reports what actually went on the wire
+  (``content-encoding``, else ``identity``). The two together answer "how
+  fat is it, and how fat did the client see it".
+* level: ``INFO`` when ``bytes >= settings.log_large_response_bytes``
+  (``BOR_LOG_LARGE_RESPONSE_BYTES``, default 65536; ``0`` = every ``/api/*``
+  response at INFO), ``DEBUG`` below it — so the default log stays quiet
+  and an offender is loud without a re-deploy.
+* a ``304`` logs ``bytes=0`` deliberately: the revalidation win this phase
+  bought is visible in the log, not hidden behind a missing line.
+* `` stream=1`` is appended for ``text/event-stream`` responses, where
+  ``bytes`` is the streamed total and ``duration_ms`` is the stream's LIFE.
+
+Why **pure ASGI** (the phase-167 rule for anything on this path): the
+middleware must never buffer. It only reads message lengths, so the SSE
+chat stream (A15) passes through byte-identical — a
+``BaseHTTPMiddleware`` here would deliver the whole turn at the end,
+which is exactly the failure mode phase 167 exists to avoid elsewhere.
+
+Logged on the ``http.response.body`` message with ``more_body=False`` —
+for a stream that is when the **stream ends**, so ``duration_ms`` is the
+stream's whole life. A stream the client walks away from (a cancelled SSE
+turn) writes exactly one line too, with the bytes that actually got out:
+the app unwinds without a completing body message and the ``finally`` path
+emits it. Half a response happened, so half a count is the honest number —
+and no line at all would make a chatty, abandoned poll look free.
+
+An app exception is handled the same way (one line, bytes as counted, status
+500 when the app never got to answer) and re-raises UNCHANGED; non-HTTP
+scopes and non-``/api/`` paths pass through silently (pages, assets and the
+static catch-all are not the catalogue surface this budget governs).
+"""
+from __future__ import annotations
+
+import logging
+import time
+
+from starlette.datastructures import Headers
+from starlette.types import ASGIApp, Message, Receive, Scope, Send
+
+from app.config import get_settings
+
+#: The logger behind every line — renders as ``… :: api method=…`` with the
+#: §9 format, so ``grep ' :: api '`` isolates the budget log.
+logger = logging.getLogger("app.response_budget")
+
+#: Only paths under this prefix are measured. The admin catalogue (and
+#: whatever new read route joins it) is the surface TODO L5's "any any
+#: other endpoints that could load a ton of data" is about; pages, assets
+#: and the static catch-all are not.
+API_PATH_PREFIX = "/api/"
+
+#: The media type whose line carries `` stream=1`` (A15 SSE): the frame
+#: stream is where ``bytes`` is a total over a life rather than one body.
+STREAM_MEDIA_TYPE = "text/event-stream"
+
+#: Status logged when the app died before it could send
+#: ``http.response.start`` — the exception is propagating and Starlette's
+#: ``ServerErrorMiddleware`` (outermost) answers exactly that.
+UNKNOWN_STATUS = 500
+
+
+class _Tally:
+    """What one measured response accumulates: status, the start message,
+    bytes.
+
+    ``start`` holds the ``http.response.start`` MESSAGE (the dict), not a
+    snapshot of its headers, and its headers are read at emit time — after
+    the final body chunk has been awaited. That is how ``enc`` reports the
+    wire encoding from a layer sitting UNDER the compressor (the position
+    that gives an uncompressed ``bytes=``): Starlette's gzip responder holds
+    that same start message while it decides what to do with the body and
+    decorates it in place (``MutableHeaders(raw=initial_message["headers"])``
+    shares the list), so by the time the body completes the
+    ``content-encoding`` the client sees is on it. Holding the message
+    rather than the list is not pedantry — ``Headers(scope=message)``
+    REBINDS ``message["headers"]`` to a fresh list (the compression
+    responder and this stack's ``_RebufferSmallResponses`` both call it
+    that way), so a captured list would be a stale one.
+
+    The dependency is documented and pinned, not assumed:
+    ``tests/integration/test_response_budget_api.py`` reads the real
+    middleware stack for both ``enc=gzip`` and ``enc=identity``, so a
+    future layer that copies the message instead of decorating it fails a
+    test rather than quietly mislabeling the log (the byte count stays
+    exact either way — it is the number the budget compares).
+    """
+
+    __slots__ = ("bytes", "logged", "start", "status")
+
+    def __init__(self) -> None:
+        self.bytes = 0
+        self.status = UNKNOWN_STATUS
+        self.start: Message | None = None
+        self.logged = False
+
+    def line(self, scope: Scope, started: float) -> str:
+        """The §9-style single line — the exact shape README documents.
+
+        ``duration_ms`` is rounded from the monotonic clock (never
+        wall-clock: an NTP step must not produce a negative duration).
+        """
+        headers = Headers(raw=list(self.start["headers"]) if self.start else [])
+        enc = (headers.get("content-encoding") or "identity").strip() or "identity"
+        media = (headers.get("content-type") or "").partition(";")[0].strip().lower()
+        duration_ms = int(round((time.monotonic() - started) * 1000))
+        line = (
+            f"api method={scope.get('method', '')} path={scope.get('path', '')} "
+            f"status={self.status} bytes={self.bytes} enc={enc} "
+            f"duration_ms={duration_ms}"
+        )
+        if media == STREAM_MEDIA_TYPE:
+            line += " stream=1"
+        return line
+
+    def emit(self, scope: Scope, started: float, budget: int) -> None:
+        """Write the line ONCE, at the level the budget decides.
+
+        Idempotent on purpose: the completion path and the ``finally``
+        path describe the same response and must never double-log it.
+        """
+        if self.logged:
+            return
+        self.logged = True
+        line = self.line(scope, started)
+        if self.bytes >= budget:
+            logger.info(line)
+        else:
+            logger.debug(line)
+
+
+class ResponseBudgetMiddleware:
+    """Pure-ASGI response-size logger — zero buffering, SSE-safe.
+
+    Wraps ``send`` only (never ``receive``): the timing runs from
+    ``time.monotonic()`` at entry to the ``more_body=False`` completion,
+    so ``duration_ms`` is the response's (or the stream's) whole life
+    without the middleware needing to see one inbound message. Every
+    ``http.response.body`` chunk's length adds to the count and nothing is
+    held, so a stream still flows frame by frame.
+
+    ``threshold`` overrides the INFO/DEBUG cut for this instance; the
+    default (``None``) reads ``Settings.log_large_response_bytes`` per
+    response, so ``app/main.py`` wires it with no arguments and
+    ``BOR_LOG_LARGE_RESPONSE_BYTES`` stays the operator's knob (``0`` = log
+    every ``/api/*`` response at INFO, negative refuses startup in the
+    validator). LOCKED A5: this is a log line and a script, never a 413 —
+    no endpoint starts rejecting large payloads here.
+    """
+
+    def __init__(self, app: ASGIApp, *, threshold: int | None = None) -> None:
+        self.app = app
+        self.threshold = threshold
+
+    def _budget(self) -> int:
+        if self.threshold is not None:
+            return self.threshold
+        return get_settings().log_large_response_bytes
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        if scope["type"] != "http" or not str(scope.get("path", "")).startswith(
+            API_PATH_PREFIX
+        ):
+            # Nothing to measure: not HTTP (websocket/lifespan) or not the
+            # API surface. Silent pass-through — the original send.
+            await self.app(scope, receive, send)
+            return
+
+        started = time.monotonic()
+        tally = _Tally()
+
+        async def send_counting(message: Message) -> None:
+            message_type = message["type"]
+            if message_type == "http.response.start":
+                tally.status = int(message["status"])
+                # The MESSAGE, not a header snapshot — see _Tally: the
+                # layers above decorate this same dict in place, so reading
+                # it after the body completes yields the wire encoding.
+                tally.start = message
+            elif message_type == "http.response.body":
+                tally.bytes += len(message.get("body", b""))
+                if not message.get("more_body", False):
+                    # Await FIRST: the layers above decorate the start
+                    # headers while handling this final chunk, so `enc`
+                    # reads what actually went on the wire.
+                    await send(message)
+                    tally.emit(scope, started, self._budget())
+                    return
+            await send(message)
+
+        try:
+            await self.app(scope, receive, send_counting)
+        finally:
+            # A normal response already emitted at its completion; this
+            # covers an app that raised (or returned) without finishing a
+            # body. The exception itself re-raises UNCHANGED.
+            tally.emit(scope, started, self._budget())

app/main.py

@@ -45,8 +45,10 @@ from app.api.ui_settings import router as ui_settings_router
 from app.config import get_settings
 from app.core.auth import ensure_admin_configured
 from app.core.caching import configure_caching
+from app.core.compression import SelectiveGZipMiddleware
 from app.core.debugging import configure_debugging
 from app.core.logging import configure_logging
+from app.core.response_budget import ResponseBudgetMiddleware
 from app.core.security_headers import SecurityHeadersMiddleware
 
 configure_logging()
@@ -145,6 +147,34 @@ def create_app() -> FastAPI:
     # stream in particular — passes through untouched.
     configure_caching(app)
 
+    # Phase 167 (task 05, TODO L5): the byte budget — ONE
+    # ``api method=… path=… status=… bytes=… enc=… duration_ms=…`` line per
+    # /api/* response (INFO at/over BOR_LOG_LARGE_RESPONSE_BYTES, DEBUG
+    # below), so "which endpoint is heavy, right now" is a
+    # ``grep ' :: api '`` away. Position: added BEFORE the gzip call below,
+    # i.e. INSIDE it (SecurityHeaders → gzip → response-budget → caching →
+    # session → routes), so ``bytes=`` is the bytes the ROUTE produced
+    # (uncompressed) while ``enc=`` reports what went on the wire — the two
+    # together answer "how fat is it, and how fat did the client see it".
+    # Pure ASGI (never BaseHTTPMiddleware): the SSE stream is counted as it
+    # passes and logged once, when the stream ends.
+    app.add_middleware(ResponseBudgetMiddleware)
+
+    # Phase 167 (TODO L5): selective gzip — every response except the
+    # POST /api/chat SSE stream (skipped by EXACT path, A15) compresses
+    # when it is >= compress_min_bytes (the 2 MB /api/docs/tree defect).
+    # 0 = the feature is OFF: no middleware is added at all (the
+    # kill-switch convention). Position: added AFTER configure_caching but
+    # BEFORE the SecurityHeaders line below, so SecurityHeaders stays
+    # OUTERMOST (A20, the last add_middleware is outermost) and gzip sits
+    # outside the caching layer — an already-rewritten HTML page compresses
+    # too, and /api/* still passes the caching layer byte-identically
+    # before compression.
+    if settings.compress_min_bytes > 0:
+        app.add_middleware(
+            SelectiveGZipMiddleware, minimum_size=settings.compress_min_bytes
+        )
+
     # Phase 82: security headers (CSP + no-framing + nosniff) — outermost on
     # purpose (last add_middleware): every response carries them, including
     # the static catch-all's 404s below (audit SEC-04).

app/schemas.py

@@ -522,6 +522,17 @@ class KbTreeFolder(BaseModel):
     A 0-document folder cannot exist (a folder is a catalogue prefix
     only), so every existing folder with no stored row is pending —
     single-file folders included.
+
+    ``truncated`` (phase 167, D2/A2): true iff the depth-limited build
+    STOPPED here — the node's own children were not sent, so the RAG
+    view drills via ``GET /api/docs/tree/node`` instead of reading
+    ``children``. A truncated node still carries every field the level
+    row renders: the recursive ``documents`` count, the recursive
+    ``updated_at`` (the phase-106 D9 max — kept correct without the
+    children), its ``summary``, and ``summary_pending``. Omitted from
+    the wire when ``False`` (the :func:`_drop_image_fields` omission
+    precedent) — an untruncated node's wire shape stays byte-identical
+    to pre-phase-167.
     """
 
     kind: Literal["folder"] = "folder"
@@ -534,6 +545,13 @@ class KbTreeFolder(BaseModel):
     summary: str | None = None
     summary_pending: bool = False
     children: list[KbTreeFolder | KbTreeFile] = Field(default_factory=list)
+    #: Phase 167 (task 02, D2/A2) — the children were not built at this
+    #: depth (see the class docstring); OMITTED when ``False``.
+    truncated: bool = False
+
+    @model_serializer(mode="wrap")
+    def _serialize(self, handler: SerializerFunctionWrapHandler) -> Any:
+        return _drop_truncated_when_false(self, handler)
 
 
 class KbTreeSource(BaseModel):
@@ -560,6 +578,14 @@ class KbTreeSource(BaseModel):
     A registered 0-document source is never pending (there is nothing
     to summarize); a single-document source IS pending while its root
     row is absent.
+
+    ``truncated`` (phase 167, D2/A2) exists on the shape for symmetry
+    with :class:`KbTreeFolder` (and the task-03 node route), but the
+    depth budget never marks a SOURCE node truncated at ``depth ≥ 1``:
+    the top level of the RAG view is source → level 1, so the source's
+    direct children are always built. Omitted from the wire when
+    ``False`` — the source node's wire shape stays byte-identical to
+    pre-phase-167.
     """
 
     name: str
@@ -571,14 +597,52 @@ class KbTreeSource(BaseModel):
     summary: str | None = None
     summary_pending: bool = False
     children: list[KbTreeFolder | KbTreeFile] = Field(default_factory=list)
+    #: Phase 167 (task 02, D2/A2) — never true at ``depth ≥ 1`` (see the
+    #: class docstring); OMITTED from the wire when ``False``.
+    truncated: bool = False
+
+    @model_serializer(mode="wrap")
+    def _serialize(self, handler: SerializerFunctionWrapHandler) -> Any:
+        return _drop_truncated_when_false(self, handler)
+
+
+class KbTreeTotals(BaseModel):
+    """Catalogue-wide totals the RAG view's three stat cards read
+    (phase 167, task 02, D2/A2).
+
+    A depth-limited tree CANNOT be summed client-side — the phase-97
+    ``treeStats()`` walk only sees the sent levels, so a truncated
+    tree would under-count every card. The endpoint therefore computes
+    the totals from the FULL catalogue rows it already fetched (no
+    extra query) and ships them beside the tree.
+    """
+
+    #: Every indexed document (the whole catalogue, not one level).
+    documents: int = Field(default=0, ge=0)
+    #: Sum of the per-document chunk counts (content + ``is_summary``
+    #: chunks — the same numbers ``GET /api/docs`` reports).
+    chunks: int = Field(default=0, ge=0)
+    #: The newest ``indexed_at`` stamp across the catalogue (ISO-8601),
+    #: ``None`` for an empty KB.
+    last_indexed: str | None = None
 
 
 class KbTree(BaseModel):
     """Response of ``GET /api/docs/tree`` (phase 97, task 02).
 
-    The FULL recursive tree in ONE fetch — the RAG view (admin) drills
-    client-side, zero per-level fetches (the ``00_phase.md`` "The tree
-    endpoint" contract).
+    Since phase 167 (task 02, LOCKED A2/A3 — supersedes the phase-97
+    "full recursive tree in one fetch, drill client-side" contract) the
+    endpoint is DEPTH-LIMITED by default (``depth=1``: the sources plus
+    ONE level); a node whose children were not built carries
+    ``truncated: true`` and the client drills level by level. An
+    explicit ``depth`` up to 8 still returns the whole tree, and the
+    pure builder's ``depth=None`` default keeps every pre-phase
+    response byte-identical.
+
+    ``totals`` (phase 167, additive top-level key) is the catalogue's
+    true documents/chunks/last-indexed — the stat cards stay honest at
+    every depth because the server, not the (partial) tree walk,
+    computes them.
 
     Every SOURCE and FOLDER node carries ``summary_pending`` (phase
     98, D3): true iff its recursive document count ≥
@@ -592,6 +656,9 @@ class KbTree(BaseModel):
     """
 
     sources: list[KbTreeSource]
+    #: Phase 167 (task 02): the server-computed catalogue totals beside
+    #: any depth of tree (see :class:`KbTreeTotals`).
+    totals: KbTreeTotals = Field(default_factory=KbTreeTotals)
 
 
 class DocContent(BaseModel):
@@ -1560,6 +1627,21 @@ def _drop_image_fields(model: KbTreeFile, handler: SerializerFunctionWrapHandler
     return data
 
 
+def _drop_truncated_when_false(
+    model: KbTreeFolder | KbTreeSource, handler: SerializerFunctionWrapHandler
+) -> Any:
+    """The phase-167 (task 02, D2) truncation-omission rule for the
+    :class:`KbTreeFolder` / :class:`KbTreeSource` nodes: only a node whose
+    children were NOT built adds the ``truncated: true`` key — an
+    untruncated node's wire shape stays byte-identical to pre-phase-167
+    (every existing wire pin holds), the deliberate echo of the
+    :func:`_drop_image_fields` omission idiom."""
+    data = handler(model)
+    if not model.truncated:
+        data.pop("truncated", None)
+    return data
+
+
 def _drop_absent_share_url(model: BaseModel, handler: SerializerFunctionWrapHandler) -> Any:
     """The ``share_url`` omission rule (phase 51, task 02): ``None`` →
     ABSENT from the JSON (not ``"share_url": null``) — an unshared chat

frontend/assets/sources.js

@@ -52,12 +52,14 @@
  *
  * Phase 97 (task 04) — the catalog becomes the DRILL-DOWN TREE the
  * agent's `ls` sees (the phase-94 concept, one end to end). The load
- * is now `loadTree()`: ONE fetch of GET /api/docs/tree returns the
- * FULL recursive tree (sources → folders → files, each folder carrying
+ * is `loadTree()` (sources → folders → files, each folder carrying
  * its STORED description — the rows the agent reads), and the view
- * drills CLIENT-SIDE from that single fetch — zero per-level fetches,
- * no URL change (the navigation is a re-render of the already-fetched
- * tree).
+ * drills level by level with no URL change (the navigation is a
+ * re-render). PHASE 167 SUPERSEDED the single-payload model here
+ * (TODO L5 — the 2 MB first load, LOCKED A3): ONE
+ * `GET /api/docs/tree?depth=1` plus ONE node fetch per not-yet-loaded
+ * level — see the Phase 167 (task 04) section; the drill UX itself
+ * (breadcrumb, scroll panel, editing) is unchanged.
  *
  *   • the drill state is module-scoped: `current` —
  *     { source: null, folder: null } = the top level (the rows ARE the
@@ -76,8 +78,11 @@
  *     the same-page modal; the no-JS href escape hatch intact), hidden
  *     when the level has none (at the top level it is ALWAYS hidden —
  *     files are seen per source, as with ls(source)).
- *   • the stat cards walk the WHOLE tree (document count, chunks sum,
- *     max indexed_at) — the values identical to the former flat walk.
+ *   • the stat cards show the KB-wide numbers (document count, chunks
+ *     sum, max indexed_at) — identical to the former flat walk. Since
+ *     phase 167 they read the server's `totals` (a depth-limited tree
+ *     cannot be summed client-side); the whole-tree walk stays only as
+ *     the defensive no-`totals` fallback.
  *   • the empty state (#sources-empty) is now the "zero SOURCES"
  *     semantic (nothing registered, nothing indexed) — the deliberate
  *     phase-97 change: a registered 0-document source renders its row
@@ -423,6 +428,35 @@
  * control's immediate disable keeps the never-stale contract). The
  * running label, the button title, and the button aria-label are
  * byte-identical to pre-phase; no copy changed.
+ *
+ * Phase 167 (task 04, D4/A3 — TODO L5) — the LAZY DRILL: the 2 MB
+ * first payload is gone. The catalog load is ONE
+ * `fetch("/api/docs/tree?depth=1")` (the sources plus one level plus
+ * the server's `totals`), and a folder the payload only NAMES
+ * (`node.truncated === true`, not yet in `levelCache`) fetches
+ * `GET /api/docs/tree/node?source=…&path=…` the FIRST time it opens —
+ * the merged children ride the node (Object.assign, so
+ * `currentLevelNode()`/`renderLevel()` keep their pre-phase semantics
+ * for every loaded level), the level is cached (key
+ * `source + "/" + folder`), and re-opening it is free (zero network).
+ * A NEW tree load clears the cache — the sync/upload/re-show refresh
+ * call sites keep working unchanged — and rehydrates the user's OWN
+ * level when the fresh payload truncates it (a refresh never paints a
+ * half-empty level). §7.4 never-stale, in full: while the node fetch
+ * runs the level the user is LEAVING stays rendered, the clicked link
+ * carries aria-busy + aria-disabled (drillPending — a second click
+ * cannot start a parallel fetch; adopted popstate navigations
+ * rehydrate their level the same way, via ensureCurrentLevel) and
+ * ONE role="status"
+ * "Loading <folder>…" row sits in the folders panel; a failure shows
+ * a role="alert" line with a real Retry <button> and keeps the
+ * current level rendered; the phase-79 loadSeq token EXTENDS to the
+ * drill (a response superseded by a newer load — or by a navigation —
+ * never paints). The node route's 404 IS the vanished-folder signal:
+ * the honest reset to the top level, nothing cached. The existence
+ * rules stop walking a full tree — folderKnownIn() only claims what
+ * the HELD levels prove — and treeStats() prefers kbTree.totals (the
+ * walk is the pre-phase-167 fallback only).
  */
 
 import { fetchIsAdmin } from "./header.js";
@@ -1622,11 +1656,30 @@ export async function mount(root) {
   let loadSeq = 0;
   let levelEditor = null; // phase 97 (task 05): the level editor handle
 
+  /* Phase 167 (task 04, D4/A3 — the TODO L5 2 MB fix): the client no
+     longer holds the whole tree. `levelCache` marks the folder levels
+     the drill has already fetched (key: source + "/" + the
+     source-relative folder path); the merged children live ON THE
+     NODES (Object.assign in fetchLevel), so currentLevelNode() and
+     renderLevel() keep their pre-phase semantics for every loaded
+     level. `drillPending` is the in-flight drill { source, folder } —
+     a second drill click is a no-op while a node fetch runs (§7.4:
+     every settle path re-enables the controls). `navSeq` bumps on
+     every applyTarget so a navigation invalidates an in-flight drill
+     exactly like a newer load does. A NEW tree load clears the cache
+     (the sync/refresh call sites keep working unchanged) and any
+     in-flight drill with it. */
+  const levelCache = new Map();
+  let drillPending = null;
+  let drillRowEl = null; // the ONE in-flight row (loading OR alert)
+  let navSeq = 0;
+
   async function loadTree() {
     const my = ++loadSeq;
+    drillPending = null; // any in-flight drill is invalidated by this load
     let r;
     try {
-      r = await fetch("/api/docs/tree");
+      r = await fetch("/api/docs/tree?depth=1");
     } catch {
       if (my === loadSeq) renderEmpty();
       return;
@@ -1638,14 +1691,25 @@ export async function mount(root) {
     const tree = await r.json();
     if (my !== loadSeq) return; // a newer load owns the DOM now
     kbTree = tree && Array.isArray(tree.sources) ? tree : { sources: [] };
+    levelCache.clear(); // a fresh tree invalidates every drilled level
     resetVanishedLocation();
+    // Phase 167: the user's own level may be one the fresh depth-1
+    // payload only NAMES (truncated, cache just cleared) — rehydrate
+    // it through the node route BEFORE it paints, so a refresh never
+    // shows a half-empty level.
+    if (needsLevelFetch(currentLevelNode(), current.source)) {
+      enterFolder({ source: current.source, folder: current.folder }, null, false);
+      return;
+    }
     renderLevel();
   }
 
   /* Never-stale (PLAN §7.4, phase 97): after a re-fetch, the drilled
    * location may no longer exist in the NEW tree (the source was
    * unregistered/pruned, the folder vanished) — reset to the top level
-   * BEFORE rendering: no stale breadcrumb, no stale block. */
+   * BEFORE rendering: no stale breadcrumb, no stale block. Since
+   * phase 167 the check is folderKnownIn (what the HELD levels prove)
+   * — the fetch path's own vanished-signal is the node route's 404. */
   function resetVanishedLocation() {
     if (current.source === null) return;
     const src = kbTree.sources.find((s) => s.name === current.source);
@@ -1653,39 +1717,48 @@ export async function mount(root) {
       current = { source: null, folder: null };
       return;
     }
-    if (current.folder !== null && !folderExistsIn(src, current.folder)) {
+    if (current.folder !== null && !folderKnownIn(src, current.folder)) {
       current = { source: null, folder: null };
     }
   }
 
-  /* The phase-94 existence rule, mirrored client-side over the tree's
-   * file paths: a folder exists under the source iff some of the
-   * source's indexed paths starts with `folder + "/"` (the source root
-   * — `""` — always exists). */
-  function folderExistsIn(src, folderPath) {
-    if (folderPath === "") return true;
-    const paths = [];
-    const collect = (node) => {
-      for (const child of node.children || []) {
-        if (child.kind === "file") paths.push(child.path);
-        else collect(child);
-      }
-    };
-    collect(src);
-    return paths.some((p) => p.startsWith(folderPath + "/"));
-  }
-
-  /* The node for `current`: the source node at the root, or the folder
-   * node found by walking the tree with the folder's cumulative
-   * source-relative path (the builder's folder paths are
-   * source-relative, so `one/two` resolves one level at a time). */
-  function currentLevelNode() {
-    const src = kbTree.sources.find((s) => s.name === current.source);
-    if (!src) return null;
-    if (current.folder === null || current.folder === "") return src;
+  /* Phase 167 (task 04, D4): the client no longer holds the whole
+   * tree, so the phase-97 full-tree existence walk becomes KNOWN-ness:
+   * a folder is known iff every node of its ancestor chain exists in
+   * what the client holds (the depth-1 tree + the drilled levels —
+   * merged children ride the nodes, so one walk over the held children
+   * answers). A broken chain means the folder is gone OR simply not
+   * loaded that deep — for the never-stale RESET both are honestly
+   * "back to the top level". The drill path does not guess: the node
+   * route's 404 IS the "the folder vanished" signal (fetchLevel). */
+  function folderKnownIn(src, folderPath) {
+    if (folderPath === "") return true; // the source root always exists
     let node = src;
     let acc = "";
-    for (const part of current.folder.split("/")) {
+    for (const part of folderPath.split("/")) {
+      acc = acc ? acc + "/" + part : part;
+      const next = (node.children || []).find(
+        (c) => c.kind === "folder" && c.path === acc
+      );
+      if (!next) return false;
+      node = next;
+    }
+    return true;
+  }
+
+  /* The node for a drill target (phase 97, phase 167-generalized):
+   * the source node at the root, or the folder node found by walking
+   * the HELD levels with the folder's cumulative source-relative path
+   * (the builder's folder paths are source-relative, so `one/two`
+   * resolves one level at a time; a truncated node simply has no
+   * children to walk into). */
+  function nodeForTarget(target) {
+    const src = kbTree.sources.find((s) => s.name === target.source);
+    if (!src) return null;
+    if (target.folder === null || target.folder === "") return src;
+    let node = src;
+    let acc = "";
+    for (const part of target.folder.split("/")) {
       acc = acc ? acc + "/" + part : part;
       const next = (node.children || []).find(
         (c) => c.kind === "folder" && c.path === acc
@@ -1696,11 +1769,27 @@ export async function mount(root) {
     return node;
   }
 
-  /* The KB-wide stat cards (phase 97): walk the WHOLE tree — document
-   * count, chunks sum, max indexed_at — the values identical to the
-   * former flat /api/docs walk (the same documents, one level deeper).
-   * fmtDate reuse for the max. */
+  function currentLevelNode() {
+    return nodeForTarget(current);
+  }
+
+  /* The KB-wide stat cards. Phase 167 (task 04, D2/D4): the server's
+   * `totals` (computed from the FULL catalogue rows beside any depth
+   * of tree) are the source of truth — a depth-limited tree CANNOT be
+   * summed client-side. The phase-97 recursive walk (document count,
+   * chunks sum, max indexed_at — the values identical to the former
+   * flat /api/docs walk) stays ONLY as the defensive fallback for a
+   * tree served without `totals` (a pre-phase-167 process) — the
+   * no-crash path. fmtDate reuse for the max (renderLevel). */
   function treeStats() {
+    const totals = kbTree.totals;
+    if (totals && typeof totals === "object") {
+      return {
+        docs: totals.documents,
+        totalChunks: totals.chunks,
+        last: totals.last_indexed || "",
+      };
+    }
     let docs = 0;
     let totalChunks = 0;
     let last = "";
@@ -1722,8 +1811,10 @@ export async function mount(root) {
   /* The breadcrumb (phase 97): hidden at the top level; when drilled
    * in, one link per ancestor — the top level (back to the sources
    * list), the source, then the folder chain — the LAST segment a
-   * span with aria-current="page". Client-side only: no fetch, no URL
-   * change (the navigation is a re-render of the fetched tree). */
+   * span with aria-current="page". No URL change (the navigation is a
+   * re-render); phase 167: an ancestor is already loaded (you cannot
+   * drill past it without holding it), so a breadcrumb step costs no
+   * fetch in practice — goTo stays the ONE drill code either way. */
   function renderCrumb() {
     if (!crumbEl) return;
     crumbEl.replaceChildren();
@@ -1766,7 +1857,7 @@ export async function mount(root) {
     a.textContent = label; // document-derived text — never innerHTML
     a.addEventListener("click", (e) => {
       e.preventDefault();
-      goTo(target);
+      goTo(target, a);
     });
     return a;
   }
@@ -1779,9 +1870,9 @@ export async function mount(root) {
     return span;
   }
 
-  /* The drill navigation (phase 99, task 02, D2 — client-side, no
-   * fetch; the drill state IS the history state). `applyTarget(target,
-   * push)`: when `push` is true the target DIFFERS from the current
+  /* The drill navigation (phase 99, task 02, D2 — the drill state IS
+   * the history state; since phase 167 a user drill may FIRST fetch
+   * the level — see enterFolder). `applyTarget(target, push)`: when `push` is true the target DIFFERS from the current
    * level (a re-click of the current row/segment pushes NO duplicate
    * entry), it FIRST records a STATE-ONLY entry — history.pushState(
    * { view: "rag", kb: target }, "") — the second arg "" + no third:
@@ -1795,14 +1886,166 @@ export async function mount(root) {
       history.pushState({ view: "rag", kb: target }, "");
     }
     current = { source: target.source, folder: target.folder };
+    navSeq++; // phase 167: any navigation invalidates an in-flight drill
     renderLevel();
   }
 
   /* Every user-initiated drill (source row, folder link, breadcrumb
    * segment) — push a history entry, then drill (D2, phase 99 task
-   * 02). */
-  function goTo(target) {
-    applyTarget(target, true);
+   * 02). Since phase 167 (task 04, A3) the drill may FIRST fetch the
+   * level (enterFolder); the push lands when the level is in hand, so
+   * a failed drill records nothing. `link` is the clicked link — it
+   * carries the in-flight aria-busy/aria-disabled state. */
+  function goTo(target, link) {
+    enterFolder(target, link, true);
+  }
+
+  /* Phase 167 (task 04, D4/A3) — the lazy drill. The depth-1 tree
+   * NAMES deeper folders (truncated: true, children not built); the
+   * FIRST open of one fetches its level through the node route, merges
+   * the children into the node, and caches the level — a re-open is
+   * free (zero network). A level the client already holds (or a node
+   * the payload fully built) drills with NO fetch, exactly as before:
+   * applyTarget + render, nothing else. One drill at a time —
+   * drillPending is the §7.4 double-click guard, and every settle
+   * path (success, failure, 404, superseded) releases it. */
+  function needsLevelFetch(node, source) {
+    return (
+      !!node &&
+      node.truncated === true &&
+      !levelCache.has(source + "/" + node.path)
+    );
+  }
+
+  async function enterFolder(target, link, push) {
+    if (drillPending) return; // one drill at a time (§7.4 double-click guard)
+    const node = nodeForTarget(target);
+    if (needsLevelFetch(node, target.source)) {
+      drillPending = { source: target.source, folder: target.folder };
+      const loaded = await fetchLevel(target, node, link);
+      if (!loaded) return; // failed / vanished / superseded — handled inside
+    }
+    applyTarget(target, push);
+  }
+
+  /* ONE node fetch + its full §7.4 lifecycle. The fetch URL is
+   * /api/docs/tree/node?source=…&path=… (path source-relative; the
+   * route is documented in app/api/docs.py — one catalogue query per
+   * drill, no N+1). The race convention EXTENDS from loadTree: the
+   * response may only paint while NO newer load (loadSeq, phase 79)
+   * and NO navigation (navSeq) have happened — a stale response never
+   * touches the DOM. Success: Object.assign merges the node (children,
+   * counts, dates, summary, pending flags — the wire shape the full
+   * tree produces) and levelCache marks the level; only a 200 caches.
+   * A 404 IS the vanished-folder signal — honest reset to the top
+   * level, nothing cached. Any other failure: the alert + Retry row,
+   * the current level staying rendered. */
+  async function fetchLevel(target, targetNode, link) {
+    const my = loadSeq;
+    const nav = navSeq;
+    if (link) {
+      link.setAttribute("aria-busy", "true");
+      link.setAttribute("aria-disabled", "true"); // a double click cannot refetch
+    }
+    showDrillLoading(target.folder.split("/").pop());
+    let response = null;
+    let vanished = false;
+    try {
+      response = await fetch("/api/docs/tree/node?source=" + encodeURIComponent(target.source) + "&path=" + encodeURIComponent(target.folder));
+      if (response.status === 404) vanished = true;
+    } catch {
+      response = null; // network failure — the alert path below
+    }
+    // Every settle path re-enables the controls (§7.4) — the link is
+    // only ours to clear while this drill is still THE pending one (a
+    // newer load owns the flags the moment it starts).
+    if (drillPending && drillPending.source === target.source && drillPending.folder === target.folder) {
+      drillPending = null;
+    }
+    if (link) {
+      link.removeAttribute("aria-busy");
+      link.removeAttribute("aria-disabled");
+    }
+    if (my !== loadSeq || nav !== navSeq) return false; // superseded — never paints
+    clearDrillRow();
+    if (vanished) {
+      // The folder vanished under us (pruned/unregistered since the
+      // level was listed) — the phase-97 never-stale contract: reset
+      // to the top level, no stale breadcrumb, nothing cached.
+      current = { source: null, folder: null };
+      renderLevel();
+      return false;
+    }
+    if (response === null || !response.ok) {
+      showDrillFailure(target, link); // the current level stays rendered
+      return false;
+    }
+    const merged = await response.json();
+    if (my !== loadSeq || nav !== navSeq) return false; // stale — never paints
+    Object.assign(targetNode, merged);
+    levelCache.set(target.source + "/" + target.folder, true);
+    return true;
+  }
+
+  /* The ONE in-flight row of the folders panel — appended to the
+   * level the user is LEAVING (renderLevel's tbody clear owns it: a
+   * re-render always drops the row). Tokens-only .kb-level-loading
+   * (styles.css); textContent only, the house rule. */
+  function clearDrillRow() {
+    if (drillRowEl) {
+      drillRowEl.remove();
+      drillRowEl = null;
+    }
+  }
+
+  function showDrillLoading(name) {
+    clearDrillRow();
+    const row = document.createElement("tr");
+    row.className = "kb-level-loading";
+    const td = document.createElement("td");
+    td.colSpan = 4;
+    const note = document.createElement("span");
+    note.setAttribute("role", "status");
+    note.textContent = "Loading " + name + "…"; // never innerHTML
+    td.appendChild(note);
+    row.appendChild(td);
+    drillRowEl = row;
+    if (foldersWrap) foldersWrap.hidden = false; // the row needs its panel
+    if (foldersTbody) foldersTbody.appendChild(row);
+  }
+
+  /* Phase 167: rehydrate the CURRENT level when the held tree only
+   * names it — used by the ADOPTED navigations (popstate: the popped
+   * entry can point at a level the client has not fetched since the
+   * last load; the bor:view-refresh alignment is covered by loadTree's
+   * own rehydration). A no-op for held levels and at the top. */
+  function ensureCurrentLevel() {
+    if (drillPending) return;
+    if (needsLevelFetch(currentLevelNode(), current.source)) {
+      enterFolder({ source: current.source, folder: current.folder }, null, false);
+    }
+  }
+
+  function showDrillFailure(target, link) {
+    clearDrillRow();
+    const row = document.createElement("tr");
+    row.className = "kb-level-loading";
+    const td = document.createElement("td");
+    td.colSpan = 4;
+    const note = document.createElement("span");
+    note.setAttribute("role", "alert");
+    note.textContent =
+      "Couldn't load " + target.folder.split("/").pop() + " — "; // never innerHTML
+    const retry = document.createElement("button");
+    retry.type = "button"; // a REAL button (the phase-49 .retry-btn pill)
+    retry.className = "retry-btn";
+    retry.textContent = "Retry";
+    retry.addEventListener("click", () => enterFolder(target, link, true));
+    td.append(note, retry);
+    row.appendChild(td);
+    drillRowEl = row;
+    if (foldersWrap) foldersWrap.hidden = false;
+    if (foldersTbody) foldersTbody.appendChild(row);
   }
 
   /* ONE table for every level (phase 97): at the TOP level the rows
@@ -1818,7 +2061,7 @@ export async function mount(root) {
     link.textContent = s.name; // document-derived text — never innerHTML
     link.addEventListener("click", (e) => {
       e.preventDefault();
-      goTo({ source: s.name, folder: "" });
+      goTo({ source: s.name, folder: "" }, link);
     });
     nameTd.appendChild(link);
     tr.appendChild(nameTd);
@@ -1853,7 +2096,7 @@ export async function mount(root) {
     link.textContent = f.path.split("/").pop(); // never innerHTML
     link.addEventListener("click", (e) => {
       e.preventDefault();
-      goTo({ source: current.source, folder: f.path });
+      goTo({ source: current.source, folder: f.path }, link);
     });
     nameTd.appendChild(link);
     tr.appendChild(nameTd);
@@ -2209,6 +2452,11 @@ export async function mount(root) {
       const kb = event.state && event.state.kb;
       if (kb) applyTarget(kb, false);
       else applyTarget({ source: null, folder: null }, false);
+      // Phase 167 (task 04): a forward/back can adopt an entry whose
+      // level the client has not fetched since — rehydrate it through
+      // the node route (a no-op for held levels; a truly vanished one
+      // takes the 404-reset / alert path inside).
+      ensureCurrentLevel();
     });
     loadTree();
   }

frontend/assets/styles.css

@@ -2997,6 +2997,14 @@ a.answer-image:hover { text-decoration: underline; } /* flat — the caption und
   background: var(--brand-soft);
   text-decoration: underline;
 }
+/* Phase 167 (task 04, D4): the lazy drill's ONE in-flight row — the
+   role="status" loading note and the role="alert" failure note (+ the
+   existing .retry-btn pill) shown in the folders panel of the level
+   the user is LEAVING: the level never blanks and the breadcrumb never
+   lies (PLAN §7.4). Tokens only — ink-soft on the wrap's --surface
+   (the .kb-level p pair, 5.1:1 AA); no animation beyond the house
+   idiom (the Retry button's :focus-visible is the global rule). */
+.kb-level-loading { color: var(--ink-soft); }
 
 /* Image-document rows (phase 122, task 04): the file table's Path cell
    carries a FIXED 48px thumbnail box before the path link (image rows

scripts/api_payload_audit.py

@@ -0,0 +1,378 @@
+"""Audit the payload of every endpoint that could load a ton of data
+(phase 167, task 05, D5, LOCKED A5).
+
+TODO L5 is one sentence: "Fix that **any any other endpoints** that could
+load a ton of data." ``GET /api/docs/tree`` was the reported offender; this
+script is the measured answer for the rest of the API, against the app the
+operator actually runs:
+
+    uv run python -m scripts.api_payload_audit --base-url http://localhost:8000
+    # (or against the deployed app: --base-url https://brain.reeseapps.com)
+
+It signs in through the real ``POST /api/login`` (the session cookie is the
+only credential — nothing is hardcoded: ``--password`` or the
+``BOR_ADMIN_PASSWORD`` the app itself reads from ``.env``), walks the
+candidate read routes, and prints one row per endpoint with the IDENTITY and
+the GZIP transfer size, the ratio, the wall time and a verdict against the
+budget (default 256 KB — the phase's "look at it" bar; LOCKED A5: this is a
+report, never a gate, so the exit code is 0 whenever the measurement
+succeeded).
+
+**Credentials are password-only on purpose.** The single-admin session is
+``POST /api/login {"password": …}`` (PLAN A10) — there is no username field
+to send, so there is no ``--username``; ``BOR_ADMIN_PASSWORD`` (or
+``--password``) is the whole credential.
+
+Both variants are measured on every endpoint because that is the pair the
+byte-budget log line reports (``bytes=`` / ``enc=``): ``Accept-Encoding:
+identity`` gives the payload the route produced, ``Accept-Encoding: gzip``
+gives what the client puts on the wire (``response.num_bytes_downloaded``,
+the raw bytes — ``response.content`` is already decompressed by httpx). The
+routes that publish a validator (the three catalogue reads, phase 167 task
+03) get one extra conditional request each: the expected answer is
+``304`` with an empty body, which is the row's verdict.
+
+Run it against the 1,000-document live-replica snapshot (the
+``restore-test-db`` skill) so the numbers are the production order of
+magnitude and not the fixture KB's — the corpus the run measured is printed
+with the table for exactly that reason.
+
+Exit codes: **0** the table printed (an over-budget row is a FINDING, shown
+as ``LOOK``, not a failure), **2** a precondition failed — the app
+unreachable, the login rejected, or no credential available.
+"""
+from __future__ import annotations
+
+import argparse
+import os
+import sys
+import time
+from dataclasses import dataclass
+from typing import Any
+
+import httpx
+from dotenv import load_dotenv
+
+load_dotenv()
+
+#: The phase's "look at it" bar (256 KB uncompressed — D5/A5). Rows over it
+#: are verdict ``LOOK`` and must be fixed in the phase or carry a named
+#: follow-up in ``AUDIT.md``; nothing here rejects a request.
+DEFAULT_BUDGET = 262_144
+
+#: Per-request wall-clock ceiling. A large catalogue read on the restored
+#: 1,000-doc replica is fast, but a cold first query can take seconds.
+DEFAULT_TIMEOUT = 60.0
+
+
+@dataclass
+class Row:
+    """One measured endpoint."""
+
+    label: str
+    path: str
+    status: int = 0
+    identity_bytes: int | None = None
+    identity_ms: int | None = None
+    gzip_bytes: int | None = None
+    gzip_ms: int | None = None
+    revalidated: bool | None = None
+    note: str = ""
+
+    @property
+    def ratio(self) -> str:
+        """identity / gzip — how much the compression layer bought (< 1 is
+        a win; ``—`` when the route did not compress)."""
+        if not self.gzip_bytes or not self.identity_bytes:
+            return "—"
+        return f"{self.gzip_bytes / self.identity_bytes:.3f}"
+
+    def verdict(self, budget: int) -> str:
+        if self.status == 0:
+            # Nothing was measured — the corpus has no such row (an empty KB,
+            # no saved chats). Honest gap, not a pass and not a finding.
+            return "n/a"
+        if self.status == 304:
+            return "304"
+        if self.status != 200:
+            return f"HTTP {self.status}"
+        if self.revalidated:
+            return "304"
+        if (self.identity_bytes or 0) > budget:
+            return f"LOOK (> {budget})"
+        return "ok"
+
+
+def build_parser() -> argparse.ArgumentParser:
+    p = argparse.ArgumentParser(
+        prog="python -m scripts.api_payload_audit",
+        description=(
+            "Measure the response size (identity AND gzip) of every "
+            "candidate large-payload endpoint, against a byte budget."
+        ),
+    )
+    p.add_argument(
+        "--base-url",
+        default="http://localhost:8000",
+        metavar="URL",
+        help="app to measure (default: http://localhost:8000)",
+    )
+    p.add_argument(
+        "--password",
+        default=None,
+        metavar="PASSWORD",
+        help=(
+            "admin password; falls back to $BOR_ADMIN_PASSWORD (the app's "
+            "own variable). The single-admin login has no username (A10)."
+        ),
+    )
+    p.add_argument(
+        "--budget",
+        type=int,
+        default=DEFAULT_BUDGET,
+        metavar="BYTES",
+        help=f"the LOOK threshold in bytes (default: {DEFAULT_BUDGET})",
+    )
+    p.add_argument(
+        "--timeout",
+        type=float,
+        default=DEFAULT_TIMEOUT,
+        metavar="SECONDS",
+        help=f"per-request timeout (default: {DEFAULT_TIMEOUT:g})",
+    )
+    return p
+
+
+def _fail(message: str) -> int:
+    print(f"api_payload_audit: {message}", file=sys.stderr)
+    return 2
+
+
+def _get(client: httpx.Client, path: str, accept: str) -> tuple[int, int, int, int]:
+    """GET ``path`` with an explicit ``Accept-Encoding`` →
+    ``(status, body bytes, wire bytes, ms)``.
+
+    ``body`` is the decoded payload (what the route produced); ``wire`` is
+    the raw transfer, which is what the browser pays. They differ exactly
+    when the response came back gzipped.
+    """
+    started = time.perf_counter()
+    response = client.get(path, headers={"Accept-Encoding": accept})
+    ms = int(round((time.perf_counter() - started) * 1000))
+    return response.status_code, len(response.content), response.num_bytes_downloaded, ms
+
+
+def measure(
+    client: httpx.Client, label: str, path: str, *, conditional: bool = False
+) -> Row:
+    """One endpoint, both encodings, plus the conditional re-request on the
+    routes that publish a validator (``conditional=True``)."""
+    row = Row(label=label, path=path)
+    row.status, row.identity_bytes, _, row.identity_ms = _get(client, path, "identity")
+    gzip_status, _, wire, ms = _get(client, path, "gzip")
+    row.gzip_bytes, row.gzip_ms = wire, ms
+    if gzip_status != row.status:  # pragma: no cover - defensive
+        row.status = gzip_status
+    if conditional:
+        # The ETag from a fresh unconditional GET, replayed (fetch does the
+        # same thing on the second page view).
+        etag = client.get(path, headers={"Accept-Encoding": "identity"}).headers.get("etag")
+        if etag is None:
+            row.revalidated = False
+        else:
+            again = client.get(
+                path, headers={"Accept-Encoding": "identity", "If-None-Match": etag}
+            )
+            row.revalidated = again.status_code == 304 and len(again.content) == 0
+    return row
+
+
+def _first_folder(tree: dict[str, Any]) -> tuple[str, str] | None:
+    """``(source, folder path)`` of the first folder node the depth-1 tree
+    reports — a representative drill target for the node route."""
+    for source in tree.get("sources", []):
+        for child in source.get("children", []):
+            if child.get("kind") == "folder":
+                return str(source.get("name", "")), str(child.get("path", ""))
+    return None
+
+
+def run(args: argparse.Namespace) -> tuple[int, list[Row], list[str]]:
+    """Do the whole audit; returns ``(exit code, rows, corpus lines)``."""
+    password = args.password or os.environ.get("BOR_ADMIN_PASSWORD", "")
+    if not password:
+        return (
+            _fail(
+                "no admin password — pass --password or set BOR_ADMIN_PASSWORD "
+                "(the app's own variable; it is never hardcoded here)"
+            ),
+            [],
+            [],
+        )
+
+    client = httpx.Client(
+        base_url=args.base_url, timeout=args.timeout, follow_redirects=False
+    )
+    try:
+        login = client.post("/api/login", json={"password": password})
+    except httpx.HTTPError as exc:
+        return (
+            _fail(
+                f"cannot reach the app at {args.base_url} ({exc.__class__.__name__}: {exc}) "
+                "— start it with `uv run uvicorn app.main:app`"
+            ),
+            [],
+            [],
+        )
+    if login.status_code != 204:
+        detail = ""
+        try:
+            detail = str(login.json().get("detail", ""))
+        except Exception:  # noqa: BLE001 - a non-JSON error body is still an error
+            detail = login.text[:120]
+        hint = {
+            401: "wrong password (BOR_ADMIN_PASSWORD of the app under test)",
+            429: "rate limited (10 failed attempts / 15 min) — wait for the window",
+        }.get(login.status_code, "unexpected answer from the real login route")
+        return (
+            _fail(
+                f"login to {args.base_url} failed: HTTP {login.status_code} — "
+                f"{hint} {detail}"
+            ),
+            [],
+            [],
+        )
+
+    rows: list[Row] = []
+    corpus: list[str] = []
+
+    # --- the corpus the numbers describe (printed with the table) ---------
+    docs_response = client.get("/api/docs", headers={"Accept-Encoding": "identity"})
+    documents: list[dict[str, Any]] = list(docs_response.json().get("documents", []))
+    by_chunks = sorted(documents, key=lambda d: int(d.get("chunks", 0)), reverse=True)
+    biggest_doc = by_chunks[0] if by_chunks else None
+    chunks_total = sum(int(d.get("chunks", 0)) for d in documents)
+    corpus.append(f"documents={len(documents)} chunks_total={chunks_total}")
+
+    chats: list[dict[str, Any]] = list(
+        client.get("/api/chats", headers={"Accept-Encoding": "identity"}).json().get("chats", [])
+    )
+    biggest_chat = (
+        sorted(chats, key=lambda c: int(c.get("message_count", 0)), reverse=True)[0]
+        if chats
+        else None
+    )
+
+    # --- the candidates ---------------------------------------------------
+    rows.append(
+        measure(client, "tree depth=1 (default)", "/api/docs/tree?depth=1", conditional=True)
+    )
+    rows.append(
+        measure(client, "tree depth=8 (full)", "/api/docs/tree?depth=8", conditional=True)
+    )
+
+    depth1 = client.get(
+        "/api/docs/tree?depth=1", headers={"Accept-Encoding": "identity"}
+    ).json()
+    folder = _first_folder(depth1)
+    if folder is not None and folder[0]:
+        node_path = f"/api/docs/tree/node?source={folder[0]}&path={folder[1]}&depth=1"
+        rows.append(
+            measure(client, f"tree node {folder[0]}/{folder[1]}", node_path, conditional=True)
+        )
+    else:  # the honest gap: an empty KB has no drill to measure
+        rows.append(
+            Row(label="tree node (drill)", path="/api/docs/tree/node", note="no folder in corpus")
+        )
+
+    rows.append(measure(client, "docs catalogue", "/api/docs", conditional=True))
+    if biggest_doc is not None:
+        content_path = (
+            f"/api/documents/content?source={biggest_doc['source']}&path={biggest_doc['path']}"
+        )
+        row = measure(client, f"document content (biggest: {biggest_doc['path']})", content_path)
+        body = client.get(content_path, headers={"Accept-Encoding": "identity"}).json()
+        row.note = f"content field {len(str(body.get('content', '')))} chars"
+        rows.append(row)
+    else:
+        rows.append(Row(label="document content", path="/api/documents/content", note="empty KB"))
+
+    rows.append(measure(client, "saved chats list", "/api/chats"))
+    if biggest_chat is not None:
+        rows.append(
+            measure(
+                client,
+                f"saved chat (biggest: {int(biggest_chat.get('message_count', 0))} msgs)",
+                f"/api/chats/{biggest_chat['id']}",
+            )
+        )
+    else:
+        rows.append(Row(label="saved chat (one)", path="/api/chats/{id}", note="no saved chats"))
+
+    rows.append(measure(client, "git sources", "/api/git-sources"))
+    rows.append(measure(client, "suggestions", "/api/suggestions"))
+    rows.append(measure(client, "sync status (polled)", "/api/sync/status"))
+    rows.append(measure(client, "tokens", "/api/tokens"))
+    rows.append(measure(client, "config", "/api/config"))
+    rows.append(measure(client, "whoami", "/api/whoami"))
+
+    corpus.append(f"sources={len(depth1.get('sources', []))}")
+    totals = depth1.get("totals") or {}
+    if totals:
+        corpus.append(
+            f"tree totals documents={totals.get('documents')} chunks={totals.get('chunks')}"
+        )
+    corpus.append(f"saved_chats={len(chats)}")
+    corpus.append(f"biggest_doc={biggest_doc['path'] if biggest_doc else '—'}")
+    return 0, rows, corpus
+
+
+def render(rows: list[Row], *, base_url: str, budget: int, corpus: list[str]) -> None:
+    """The table: endpoint, identity bytes, gzip bytes, ratio, ms, verdict."""
+    header: tuple[str, ...] = ("endpoint", "identity", "gzip", "ratio", "ms", "verdict")
+    table: list[tuple[str, ...]] = [header]
+    for row in rows:
+        table.append(
+            (
+                row.label,
+                f"{row.identity_bytes:,}" if row.identity_bytes is not None else "—",
+                f"{row.gzip_bytes:,}" if row.gzip_bytes is not None else "—",
+                row.ratio,
+                f"{row.gzip_ms}" if row.gzip_ms is not None else "—",
+                row.verdict(budget),
+            )
+        )
+    widths = [max(len(str(r[i])) for r in table) for i in range(len(header))]
+
+    def _line(values: tuple[Any, ...]) -> str:
+        return (
+            f"  {str(values[0]):<{widths[0]}}  {str(values[1]):>10}  "
+            f"{str(values[2]):>10}  {str(values[3]):>{widths[3]}}  "
+            f"{str(values[4]):>6}  {str(values[5])}"
+        )
+
+    print(f"api_payload_audit: base={base_url} budget={budget:,} bytes (LOOK threshold)")
+    print(f"api_payload_audit: corpus {' '.join(corpus)}")
+    print(_line(header))
+    print(f"  {'-' * (widths[0] + 44)}")
+    for row, source in zip(rows, table[1:], strict=True):
+        print(_line(source))
+        if row.note:
+            print(f"      note: {row.note}")
+    print(
+        "api_payload_audit: ms is the gzip (real-client) variant; identity is the "
+        "route's own bytes — the pair the ' :: api ' log line reports."
+    )
+
+
+def main(argv: list[str] | None = None) -> int:
+    args = build_parser().parse_args(argv)
+    code, rows, corpus = run(args)
+    if code != 0 or not rows:
+        return code
+    render(rows, base_url=args.base_url, budget=args.budget, corpus=corpus)
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())

tests/e2e/test_admin_auth.py

@@ -98,8 +98,9 @@ def _reset_db(mock_port: int, seed: bool) -> ImportSummary | None:
 
 def _drill(page: Page, *names: str) -> None:
     """Drill one level at a time: each name is the EXACT text of the
-    source/folder link at the current level (client-side — no fetch,
-    no URL change)."""
+    source/folder link at the current level (no URL change; since phase
+    167 the first open of a level the depth-1 payload only NAMES costs
+    one /api/docs/tree/node fetch, a level already held costs none)."""
     for name in names:
         page.click(f'#folders-tbody a.folder-link:text-is("{name}")')
 

tests/e2e/test_document_dates.py

@@ -301,7 +301,11 @@ def _docs_by_path(app_url: str, cookies: dict[str, str]) -> dict[str, dict[str,
 
 
 def _tree(app_url: str, cookies: dict[str, str]) -> dict[str, Any]:
-    r = httpx.get(f"{app_url}/api/docs/tree", timeout=10, cookies=cookies)
+    # Phase 167 (task 02): the callers below use ``_find_file``, which
+    # recurses (every fixture file sits under backups/, legacy/ or
+    # future/), so the WHOLE tree is requested — the endpoint's default
+    # (depth=1) is the RAG view's first paint only.
+    r = httpx.get(f"{app_url}/api/docs/tree?depth=8", timeout=10, cookies=cookies)
     assert r.status_code == 200, r.text
     return r.json()
 

tests/e2e/test_extensionless_import.py

@@ -122,9 +122,10 @@ def _delete_source_rows() -> None:
 
 
 def _drill(page: Page, *names: str) -> None:
-    """Drill one level at a time (phase 97 — client-side, no fetch,
-    no URL change): each name is the EXACT text of the source/folder
-    link at the current level."""
+    """Drill one level at a time (phase 97; phase 167: an UNCACHED level
+    costs exactly one /api/docs/tree/node fetch, a held level costs
+    nothing, and no click ever changes the URL): each name is the EXACT
+    text of the source/folder link at the current level."""
     for name in names:
         page.click(f'#folders-tbody a.folder-link:text-is("{name}")')
 

tests/e2e/test_folder_summary_voice.py

@@ -502,9 +502,11 @@ def _assert_change_terminal(body: dict[str, Any]) -> None:
 
 
 def _drill(page: Page, *names: str) -> None:
-    """Drill one level at a time (client-side — no fetch, no URL
-    change): each name is the EXACT text of the source/folder link at
-    the current level (the row builders' link text: the source name,
+    """Drill one level at a time (no URL change; phase 167: the first
+    open of a level the depth-1 payload only NAMES costs one
+    /api/docs/tree/node fetch, a level already held costs nothing):
+    each name is the EXACT text of the source/folder link at the
+    current level (the row builders' link text: the source name,
     or the folder's last path segment)."""
     for name in names:
         page.click(f'#folders-tbody a.folder-link:text-is("{name}")')
@@ -609,7 +611,7 @@ def test_rag_table_renders_the_stored_voice(
 
     login(page, app_url)  # → /sources.html (the RAG view)
     # The top level: the one source row (the ls() equivalence — the
-    # tree's single fetch settled) and its stored source-root voice in
+    # tree's ONE depth-1 fetch settled) and its stored source-root voice in
     # the Description cell (the exact canned line, text + title).
     page.wait_for_selector("#folders-tbody tr", timeout=30_000)
     expect(page.locator("#folders-tbody tr")).to_have_count(1)

tests/e2e/test_git_source_dates.py

@@ -416,7 +416,10 @@ def test_api_created_dates_are_true_per_file(
     # (old folder = the 2020 date alone, recent folder = the 2024
     # date alone, source = the whole subtree's 2024 max — D9, now over
     # TRUE dates).
-    t = page.request.get(f"{app_url}/api/docs/tree")
+    # Phase 167 (task 02): ``_find_file`` recurses by design (the files
+    # sit under ``old/`` and ``recent/``), so ask for the WHOLE tree —
+    # the endpoint's default (depth=1) is the RAG view's first paint.
+    t = page.request.get(f"{app_url}/api/docs/tree?depth=8")
     assert t.status == 200, t.text
     source = _source_node(t.json())
     assert source["documents"] == 2

tests/e2e/test_import_documents.py

@@ -204,9 +204,11 @@ SOURCE_NAME = "docs"  # the fixtures dir's basename (the indexed-only source)
 
 
 def _drill(page: Page, *names: str) -> None:
-    """Drill one level at a time (client-side — no fetch, no URL
-    change): each name is the EXACT text of the source/folder link at
-    the current level (the test_kb_tree house pattern)."""
+    """Drill one level at a time (no URL change; phase 167: the first
+    open of a level the depth-1 payload only NAMES costs one
+    /api/docs/tree/node fetch, a level already held costs nothing):
+    each name is the EXACT text of the source/folder link at the
+    current level (the test_kb_tree house pattern)."""
     for name in names:
         page.click(f'#folders-tbody a.folder-link:text-is("{name}")')
 
@@ -218,7 +220,7 @@ def _go_top(page: Page) -> None:
 
 
 def _wait_top_level(page: Page) -> None:
-    """The tree's single fetch settled: the source row is rendered
+    """The ONE ``?depth=1`` catalog fetch has settled: the source row is rendered
     (the catalog-rendered signal — the top-level file table is always
     hidden, so it is no longer a usable one)."""
     page.locator("#folders-tbody .folder-link").first.wait_for(state="visible")

tests/e2e/test_import_extensions_env.py

@@ -92,9 +92,10 @@ def _delete_source_rows() -> None:
 
 
 def _drill(page: Page, *names: str) -> None:
-    """Drill one level at a time (phase 97 — client-side, no fetch,
-    no URL change): each name is the EXACT text of the source/folder
-    link at the current level."""
+    """Drill one level at a time (phase 97; phase 167: an UNCACHED level
+    costs exactly one /api/docs/tree/node fetch, a held level costs
+    nothing, and no click ever changes the URL): each name is the EXACT
+    text of the source/folder link at the current level."""
     for name in names:
         page.click(f'#folders-tbody a.folder-link:text-is("{name}")')
 

tests/e2e/test_kb_tree.py

@@ -9,6 +9,24 @@ the level's files — and the owner edits (or clears) any directory's
 description with the phase-57 inline affordance. Everything is pinned
 against the real app + the deterministic mock.
 
+PHASE 167 (task 06, LOCKED A3) — THE CATALOG LOAD, MIGRATED NOT DELETED:
+the view no longer downloads the whole tree. The boot is ONE
+``GET /api/docs/tree?depth=1`` (the sources plus one level plus the
+server's ``totals``), and the FIRST time the owner opens a folder the
+payload only NAMES, the client fetches that one level through
+``GET /api/docs/tree/node?source=…&path=…`` (the phase-97 "ONE fetch of
+/api/docs/tree, the drill is client-side" contract, superseded by the
+TODO L5 2 MB fix — see
+``tests/e2e/test_kb_tree_lazy_load.py`` for the new load-shape suite).
+EVERY claim this suite owns stays and is re-pointed at the level the
+owner is ON: the breadcrumb text, the back navigation, the table
+columns, the dates, the summary-editor round trip, the pending markers,
+the empty state and the anonymous gate. A level the client already
+holds (the top level, a source root, a folder drilled into once — and
+the breadcrumb always walks back through a level it drilled) still
+renders with ZERO requests, so the pre-phase navigation claims below
+remain literally true at those levels.
+
 Run in isolation (DB must be up: ``podman compose up -d db``):
 
     uv run pytest tests/e2e/test_kb_tree.py -v --no-cov
@@ -498,18 +516,21 @@ def _run_sync(app_url: str) -> dict[str, Any]:
 
 def _wait_top_level(page: Page) -> None:
     """The admin boot has rendered the top level: the two source rows
-    are in the folders table (the tree's single fetch settled) and the
-    top-level file table is empty (files are per-source — always hidden
-    at the top)."""
+    are in the folders table (the ONE ``?depth=1`` catalog fetch has
+    settled — phase 167) and the top-level file table is empty (files
+    are per-source — always hidden at the top)."""
     expect(page.locator("#folders-tbody tr")).to_have_count(2, timeout=30_000)
     expect(page.locator("#docs-tbody tr")).to_have_count(0)
 
 
 def _drill(page: Page, *names: str) -> None:
-    """Drill one level at a time (client-side — no fetch, no URL
-    change): each name is the EXACT text of the source/folder link at
-    the current level (the row builders' link text: the source name,
-    or the folder's last path segment)."""
+    """Drill one level at a time (no URL change — the navigation is a
+    re-render; phase 167: a level the depth-1 payload already holds
+    costs ZERO requests, the first open of one it only NAMES costs
+    exactly one ``/api/docs/tree/node`` fetch): each name is the EXACT
+    text of the source/folder link at the current level (the row
+    builders' link text: the source name, or the folder's last path
+    segment)."""
     for name in names:
         page.click(f'#folders-tbody a.folder-link:text-is("{name}")')
 
@@ -668,7 +689,10 @@ def test_drill_into_folder(
     expect(page.locator("#folders-wrap")).to_be_hidden()
 
     # The breadcrumb link on the source goes back UP to the source
-    # level (client-side — no fetch, no URL change).
+    # level — a re-render with no fetch and no URL change, because
+    # reaching ``two`` means the client holds ``alpha`` (phase 167:
+    # the breadcrumb only ever walks back through a level it drilled,
+    # so this stays the zero-request path it was pre-phase).
     page.click(f'#kb-crumb a.kb-crumb-link:text-is("{ALPHA}")')
     expect(page.locator("#kb-crumb .kb-crumb-current")).to_have_text(ALPHA)
     expect(page.locator("#kb-crumb a.kb-crumb-link")).to_have_count(1)

tests/e2e/test_kb_tree_nav.py

@@ -446,12 +446,13 @@ def _wait_top_level(page: Page) -> None:
 
 
 def _drill(page: Page, *names: str) -> None:
-    """Drill one level at a time (client-side — no fetch, NO URL
-    change): each name is the EXACT text of the source/folder link at
-    the current level (the row builders' link text: the source name, or
-    the folder's last path segment). Phase 99 (D2): every drill click
-    is a STATE-ONLY history entry (``goTo`` → ``applyTarget(target,
-    true)``)."""
+    """Drill one level at a time (no URL change; phase 167: an UNCACHED
+    level costs exactly one /api/docs/tree/node fetch, a level already
+    held costs nothing): each name is the EXACT text of the
+    source/folder link at the current level (the row builders' link
+    text: the source name, or the folder's last path segment). Phase 99
+    (D2): every drill click is a STATE-ONLY history entry (``goTo`` →
+    ``applyTarget(target, true)``)."""
     for name in names:
         page.click(f'#folders-tbody a.folder-link:text-is("{name}")')
 

tests/e2e/test_navbar_refresh.py

@@ -110,9 +110,11 @@ TITLE_E = "Phase77 popstate E"
 DATA_VIEWS: tuple[tuple[str, str, str], ...] = (
     ("#nav-history", "/history.html", "/api/chats"),
     # Phase 97: the RAG view's list endpoint is the drill-down tree
-    # (GET /api/docs/tree — the flat /api/docs is the unchanged API
-    # surface, no longer the view's fetch).
-    ("#nav-sources", "/sources.html", "/api/docs/tree"),
+    # (the flat /api/docs is the unchanged API surface, no longer the
+    # view's fetch). Phase 167: the load carries depth=1 (the payload
+    # diet) — the per-level drill fetches /api/docs/tree/node, which
+    # these no-drill tests never trigger.
+    ("#nav-sources", "/sources.html", "/api/docs/tree?depth=1"),
     ("#nav-git-sources", "/git-sources.html", "/api/git-sources"),
     ("#nav-tuning", "/tuning.html", "/api/steering"),
 )

tests/e2e/test_quadlet_jinja_import.py

@@ -107,9 +107,10 @@ def _reset_db(mock_port: int, seed: bool) -> ImportSummary | None:
 
 
 def _drill(page: Page, *names: str) -> None:
-    """Drill one level at a time (phase 97 — client-side, no fetch,
-    no URL change): each name is the EXACT text of the source/folder
-    link at the current level."""
+    """Drill one level at a time (phase 97; phase 167: an UNCACHED level
+    costs exactly one /api/docs/tree/node fetch, a held level costs
+    nothing, and no click ever changes the URL): each name is the EXACT
+    text of the source/folder link at the current level."""
     for name in names:
         page.click(f'#folders-tbody a.folder-link:text-is("{name}")')
 

tests/e2e/test_sources_midstream_bug.py

@@ -119,9 +119,28 @@ def _run_in_thread(coro: Any) -> Any:
 
 
 def _reset_db(mock_port: int, seed: bool) -> ImportSummary | None:
-    """Truncate the KB (and query log), then optionally re-import fixtures."""
+    """Truncate the KB (and query log), then optionally re-import fixtures.
+
+    The truncate covers EVERY table that shapes the prompt, not just the
+    document rows: this suite's story asserts the EXACT long-answer text,
+    and the mock echoes a stored KB overview into the answer verbatim
+    (`` (kb: <first bullet>)``, phase 31) plus a stored steering note
+    (phase 15). Those rows survive a documents-only truncate — another
+    suite in the shared E2E Postgres ran a real sync and left its
+    overview behind — so the answer would arrive with a suffix this
+    suite never seeded and the byte-exact pin would fail on the OTHER
+    suite's corpus (``tests/e2e/test_kb_tree.py`` is the usual holder).
+    Same list as that suite's ``_truncate_all`` (the house isolation
+    pattern); no assertion changes — the corpus under test is exactly
+    the one this fixture seeds.
+    """
     with SessionLocal() as db:
-        db.execute(text("TRUNCATE chunks, documents, query_log"))
+        db.execute(
+            text(
+                "TRUNCATE chunks, documents, query_log, steering_notes, "
+                "kb_overview, git_sources, folder_summaries"
+            )
+        )
         db.commit()
     if not seed:
         return None

tests/e2e/test_sync_summary_visibility.py

@@ -593,9 +593,11 @@ def _wait_top_level(page: Page) -> None:
 
 
 def _drill(page: Page, *names: str) -> None:
-    """Drill one level at a time (client-side — no fetch, no URL
-    change): each name is the EXACT text of the source/folder link at
-    the current level (the row builders' link text: the source name,
+    """Drill one level at a time (no URL change; phase 167: the first
+    open of a level the depth-1 payload only NAMES costs one
+    /api/docs/tree/node fetch, a level already held costs nothing):
+    each name is the EXACT text of the source/folder link at the
+    current level (the row builders' link text: the source name,
     or the folder's last path segment — the phase-97 pattern)."""
     for name in names:
         page.click(f'#folders-tbody a.folder-link:text-is("{name}")')

tests/integration/test_chat_api.py

@@ -335,6 +335,39 @@ def test_chat_streams_deltas_then_done_with_sources(client, db, seeded_kb: FakeR
     assert "HONESTY GATE" in system["content"]
 
 
+def test_chat_turn_logs_one_budget_line_with_the_stream_flag(
+    client, db, seeded_kb: FakeRagLLM, caplog: pytest.LogCaptureFixture
+) -> None:
+    """Phase 167 (task 05, D5): the SSE turn is measured like every other
+    ``/api/*`` response but with `` stream=1`` — ONE line, written when the
+    stream ends, ``bytes=`` the whole streamed total (the frames, not one
+    frame) and ``duration_ms`` the stream's life. The stream itself stays
+    uncompressed (phase 167 A1, pinned above), so ``enc=identity`` here
+    documents the wire, and the chat turn never bloats the log with one
+    line per frame."""
+    fastapi_app.dependency_overrides[chat_api.get_llm] = lambda: seeded_kb
+    try:
+        with caplog.at_level(logging.DEBUG, logger="app.response_budget"):
+            _, _, frames = _stream_chat(client, QUESTION)
+    finally:
+        fastapi_app.dependency_overrides.clear()
+
+    assert frames[-1]["type"] == "done"  # the stream ran to completion
+    lines = [
+        record.getMessage()
+        for record in caplog.records
+        if record.name == "app.response_budget" and " path=/api/chat " in record.getMessage()
+    ]
+    assert len(lines) == 1, lines
+    line = lines[0]
+    assert line.endswith(" stream=1")
+    match = re.search(r"status=(\d+) bytes=(\d+) enc=(\S+)", line)
+    assert match is not None
+    assert match.group(1) == "200"
+    assert int(match.group(2)) > len(b"data: ")  # a real total, not a stub
+    assert match.group(3) == "identity"
+
+
 def test_chat_streams_thinking_before_deltas(client, db, seeded_kb: FakeRagLLM) -> None:
     """Phase 17: ``thinking`` frames precede every ``delta`` frame and
     reassemble to the model's reasoning; the ``done`` contract is

tests/integration/test_docs_api.py

@@ -15,12 +15,20 @@ qualifying row, ABSENT otherwise — never ``null``), plus the phase-163
 for a stamped strip, ABSENT for a pre-phase row and for every
 non-qualifying document).
 
+Also the phase-167 (task 03) per-level drill route
+``GET /api/docs/tree/node`` (the node shape, the 403/404 matrix, the
+pin that its children equal the same level of the unlimited
+``/api/docs/tree?depth=8`` response) and the ETag revalidation of the
+three catalogue reads (200 + validator, conditional 304 with no body,
+the admin-PATCH bust — the LOCKED A4 proof — coexisting with gzip).
+
 Uses the real compose Postgres (``db`` fixture) and FastAPI's TestClient.
 """
 from __future__ import annotations
 
 import asyncio
 import base64
+import gzip
 import hashlib
 import inspect
 import itertools
@@ -38,8 +46,9 @@ import app.api.docs as docs_api
 import app.rag.importer as rag_importer
 from app.config import Settings
 from app.core import tokens as token_service
+from app.core.security_headers import CSP
 from app.main import app as fastapi_app
-from app.models import Chunk, Document, FolderSummary, GitSource
+from app.models import Chunk, Document, FolderSummary, GitSource, SourcesMeta
 from app.rag import git_sources as rag_git_sources
 from app.rag.folder_summaries import missing_folder_summaries
 from app.rag.importer import import_sources
@@ -218,10 +227,19 @@ def test_docs_tree_populated_shape_order_counts_summaries(admin_client, db) -> N
     )
     db.commit()
 
-    r = admin_client.get("/api/docs/tree")
+    # Phase 167 (task 02): this shape pin walks the FULL tree, so it
+    # asks for the whole build explicitly — the endpoint's default is
+    # the view's first paint (depth=1), depth=8 is the entire tree.
+    r = admin_client.get("/api/docs/tree?depth=8")
     assert r.status_code == 200
     body = r.json()
-    assert set(body) == {"sources"}
+    # ``totals`` joins the top-level keys (phase 167, additive).
+    assert set(body) == {"sources", "totals"}
+    assert body["totals"] == {
+        "documents": 6,
+        "chunks": 11,
+        "last_indexed": base.isoformat(),
+    }
     sources = body["sources"]
     assert [s["name"] for s in sources] == ["Homelab", "Deployments"]
 
@@ -310,7 +328,12 @@ def test_docs_tree_empty_registry_and_catalog(
     _truncate_tree_tables(db)
     r = admin_client.get("/api/docs/tree")
     assert r.status_code == 200
-    assert r.json() == {"sources": []}
+    # Phase 167 (task 02): ``totals`` rides even the empty tree —
+    # 0 documents, 0 chunks, no last-indexed stamp.
+    assert r.json() == {
+        "sources": [],
+        "totals": {"documents": 0, "chunks": 0, "last_indexed": None},
+    }
 
 
 def test_docs_tree_indexed_only_source_after_registered(admin_client, db) -> None:
@@ -367,7 +390,10 @@ def test_docs_tree_summary_pending_on_source_and_folder_nodes(admin_client, db)
     )
     db.commit()
 
-    r = admin_client.get("/api/docs/tree")
+    # Phase 167 (task 02): the nested ``k8s/helm`` node needs the full
+    # build — depth=8 keeps this pin walking every level (the default
+    # is the view's first paint only).
+    r = admin_client.get("/api/docs/tree?depth=8")
     assert r.status_code == 200
     (homelab,) = r.json()["sources"]
     # Phase 106 (task 05): ``updated_at`` joins the source node keys.
@@ -458,7 +484,10 @@ def test_docs_tree_pending_set_equals_missing_folder_summaries(admin_client, db)
     )
     db.commit()
 
-    r = admin_client.get("/api/docs/tree")
+    # Phase 167 (task 02): the cross-check walks EVERY node, so it
+    # requests the full tree (the default depth ships only the view's
+    # first level — truncated nodes still carry their own markers).
+    r = admin_client.get("/api/docs/tree?depth=8")
     assert r.status_code == 200
     pending = _tree_pending_keys(r.json()["sources"])
     # THE cross-check: the marker set IS the gap-fill's candidate set.
@@ -828,8 +857,14 @@ def test_docs_tree_stat_walk_equivalence_with_flat_list(admin_client, db) -> Non
     _seed_doc(db, "Homelab", "solo.md", "Solo", 0, base)
     db.commit()
 
-    tree = admin_client.get("/api/docs/tree").json()
+    # Phase 167 (task 02): the walk needs EVERY file node, so it asks
+    # for the full tree; the server ``totals`` must equal the same walk
+    # over the flat list (the stat cards cannot drift either way).
+    tree = admin_client.get("/api/docs/tree?depth=8").json()
     flat = admin_client.get("/api/docs").json()
+    assert tree["totals"]["documents"] == len(flat["documents"])
+    assert tree["totals"]["chunks"] == sum(d["chunks"] for d in flat["documents"])
+    assert tree["totals"]["last_indexed"] == max(d["indexed_at"] for d in flat["documents"])
 
     files = _tree_file_nodes(tree["sources"])
     assert len(files) == len(flat["documents"])  # document count
@@ -1768,3 +1803,636 @@ def test_tree_with_no_image_rows_is_byte_identical(admin_client: TestClient, db)
         }
     finally:
         _truncate_tree_tables(db)
+
+
+# ---------------------------------------------------------------------------
+# Phase 167 (task 01): transport gzip on the real app — the /api/docs/tree
+# payload (the TODO L5 2 MB defect) compresses, small responses stay
+# identity, and the A20 security-header + A19 page contracts survive the
+# new layer sitting between SecurityHeaders (outermost) and Caching.
+# ---------------------------------------------------------------------------
+
+
+def _seed_gzip_corpus(db, count: int = 50) -> None:
+    """>= 50 indexed docs — the tree response lands well above the 860-byte
+    floor (the production corpus that motivates the phase is ~1 k docs)."""
+    _truncate_tree_tables(db)
+    base = datetime.now(UTC)
+    for i in range(count):
+        _seed_doc(db, "gzip-corpus", f"docs/sub-{i % 5}/file-{i:02d}.md", f"Doc {i:02d}", 1, base)
+    db.commit()  # the endpoint session reads committed data only
+
+
+def test_docs_tree_ships_gzip_and_decompresses_to_the_documented_json(
+    admin_client: TestClient, db
+) -> None:
+    """Accept-Encoding: gzip → Content-Encoding: gzip + Vary, and the raw
+    wire bytes decompress to EXACTLY the identity bytes (the documented
+    tree JSON, 50 file nodes) — ~an order of magnitude smaller."""
+    _seed_gzip_corpus(db)
+    # Phase 167 (task 02): the 50-file corpus must ride the wire for
+    # this byte pin, so the full tree is requested explicitly (the
+    # endpoint default ships one level, which would fall under the
+    # compression floor).
+    try:
+        identity = admin_client.get(
+            "/api/docs/tree?depth=8", headers={"Accept-Encoding": "identity"}
+        )
+        assert identity.status_code == 200
+        assert "content-encoding" not in identity.headers
+        assert len(identity.content) >= 860  # the seed is above the floor
+
+        r = admin_client.get(
+            "/api/docs/tree?depth=8", headers={"Accept-Encoding": "gzip"}
+        )
+        assert r.status_code == 200
+        assert r.headers["content-encoding"] == "gzip"
+        assert "accept-encoding" in r.headers.get("vary", "").lower()
+        # httpx decodes transparently: same documented JSON.
+        sources = r.json()["sources"]
+        assert len(_tree_file_nodes_all(sources)) == 50
+        # The RAW stream is a real gzip of the identity body.
+        with admin_client.stream(
+            "GET", "/api/docs/tree?depth=8", headers={"Accept-Encoding": "gzip"}
+        ) as streamed:
+            raw = b"".join(streamed.iter_raw())
+        assert gzip.decompress(raw) == identity.content
+        assert len(raw) < len(identity.content) // 2  # repetitive JSON: >> 2x
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_gzipped_response_still_carries_the_a20_headers(
+    admin_client: TestClient, db
+) -> None:
+    """The outer-layer invariant (A20): gzip sits INSIDE
+    SecurityHeadersMiddleware, so a gzipped response still carries the
+    CSP, nosniff and the no-framing headers."""
+    _seed_gzip_corpus(db, 60)
+    try:
+        # depth=8 (phase 167, task 02): the whole tree stays above the
+        # 860-byte floor — the point here is a GZIPPED response.
+        r = admin_client.get("/api/docs/tree?depth=8", headers={"Accept-Encoding": "gzip"})
+        assert r.headers["content-encoding"] == "gzip"
+        assert r.headers["content-security-policy"] == CSP
+        assert r.headers["x-content-type-options"] == "nosniff"
+        assert r.headers["x-frame-options"] == "DENY"
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_small_api_response_stays_identity(client: TestClient) -> None:
+    """A tiny /api/* response (under the 860-byte floor) is NOT compressed
+    even when the client offers gzip — the minimum-size exemption."""
+    r = client.get("/api/health", headers={"Accept-Encoding": "gzip"})
+    assert r.status_code == 200
+    assert "content-encoding" not in r.headers
+    assert r.json()["status"] == "ok"
+
+
+def test_html_page_contract_survives_gzip(client: TestClient) -> None:
+    """A19 through the new layer: the rewritten HTML page (>= the floor, so
+    it compresses) STILL serves no-cache with the ?v= asset rewrite and no
+    validators — and the contract holds AFTER decompression."""
+    r = client.get("/", headers={"Accept-Encoding": "gzip"})
+    assert r.status_code == 200
+    assert r.headers["content-encoding"] == "gzip"  # 95 KB page, far above the floor
+    assert r.headers["cache-control"] == "no-cache"
+    assert "etag" not in r.headers
+    assert "last-modified" not in r.headers
+    assert r.text.count("?v=") >= 1  # the caching rewrite ran, gzip preserved it
+
+
+# ---------------------------------------------------------------------------
+# Phase 167 (task 02): the depth-limited tree + server totals (D2/A2) — the
+# default depth is the RAG view's first paint (sources + ONE level), an
+# explicit depth builds deeper, out-of-range values are rejected, and an
+# untruncated node keeps its pre-phase-167 wire shape.
+# ---------------------------------------------------------------------------
+
+
+def _seed_depth_corpus(db) -> datetime:
+    """Four documents nested one level deeper each (root, ``a``, ``a/b``,
+    ``a/b/c``) with DISTINCT ``indexed_at`` stamps, plus one stored
+    summary on ``a`` — a fixture whose truncated nodes still carry a
+    recursive count, and whose level rows carry a description and a
+    pending marker."""
+    _truncate_tree_tables(db)
+    base = datetime.now(UTC)
+    db.add(GitSource(url="https://github.com/reese/Homelab.git", kind="git", added_at=base))
+    _seed_doc(db, "Homelab", "root.md", "Root", 1, base)
+    _seed_doc(db, "Homelab", "a/top.md", "Top", 2, base + timedelta(hours=1))
+    _seed_doc(db, "Homelab", "a/b/mid.md", "Mid", 3, base + timedelta(hours=2))
+    _seed_doc(db, "Homelab", "a/b/c/deep.md", "Deep", 4, base + timedelta(hours=3))
+    db.add(FolderSummary(source="Homelab", folder_path="a", summary="A desc."))
+    db.commit()  # the app's endpoint session reads committed data only
+    return base
+
+
+def test_docs_tree_default_depth_ships_one_level_with_totals(admin_client, db) -> None:
+    """``GET /api/docs/tree`` (no param) is the view's first paint:
+    sources + ONE level. The folder with unloaded children carries
+    ``truncated: true`` + ``children: []`` AND its recursive count;
+    ``totals`` matches ``GET /api/docs`` (documents, chunks, the newest
+    ``indexed_at``) — the stat cards stay honest on a partial tree."""
+    base = _seed_depth_corpus(db)
+    try:
+        r = admin_client.get("/api/docs/tree")
+        assert r.status_code == 200
+        body = r.json()
+        assert set(body) == {"sources", "totals"}
+        (homelab,) = body["sources"]
+        assert homelab["documents"] == 4  # the whole recursive source count
+        assert "truncated" not in homelab  # depth >= 1: never truncated
+        folders = [c for c in homelab["children"] if c["kind"] == "folder"]
+        files = [c for c in homelab["children"] if c["kind"] == "file"]
+        assert [f["path"] for f in files] == ["root.md"]  # direct files ship
+        (a,) = folders
+        assert a["path"] == "a"
+        assert a["truncated"] is True
+        assert a["children"] == []
+        assert a["documents"] == 3  # the recursive count, no children built
+        assert a["summary"] == "A desc."  # the level row shows the description
+        assert a["summary_pending"] is False  # the stored row covers it
+        # Nothing deeper is on the wire (the 2 MB payload stays home).
+        assert [f["path"] for f in _tree_file_nodes_all(body["sources"])] == ["root.md"]
+        # Totals from the SAME catalogue rows (no extra query).
+        assert body["totals"] == {
+            "documents": 4,
+            "chunks": 10,
+            "last_indexed": (base + timedelta(hours=3)).isoformat(),
+        }
+        flat = admin_client.get("/api/docs").json()
+        assert body["totals"]["documents"] == len(flat["documents"])
+        assert body["totals"]["chunks"] == sum(d["chunks"] for d in flat["documents"])
+        assert body["totals"]["last_indexed"] == max(
+            d["indexed_at"] for d in flat["documents"]
+        )
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_docs_tree_depth_three_builds_deeper_and_truncates_the_rest(
+    admin_client, db
+) -> None:
+    """``?depth=3``: ``a`` and ``a/b`` are built (no ``truncated`` key),
+    ``a/b``'s subfolder ``a/b/c`` is truncated — while ``a/b``'s OWN
+    direct files still ship: the budget limits DEPTH, never the row set
+    of the levels it does send. The phase-98 pending rule is unchanged
+    on truncated nodes (``a/b/c``: 1 doc, no stored row → pending)."""
+    _seed_depth_corpus(db)
+    try:
+        body = admin_client.get("/api/docs/tree?depth=3").json()
+        (homelab,) = body["sources"]
+        (a,) = [c for c in homelab["children"] if c["kind"] == "folder"]
+        assert "truncated" not in a  # expanded at this depth
+        a_children = {c["path"]: c for c in a["children"]}
+        assert set(a_children) == {"a/b", "a/top.md"}
+        a_b = a_children["a/b"]
+        assert "truncated" not in a_b
+        a_b_c, mid = a_b["children"]  # subfolder first, then the direct file
+        assert (a_b_c["kind"], a_b_c["path"], a_b_c["truncated"]) == (
+            "folder",
+            "a/b/c",
+            True,
+        )
+        assert (a_b_c["documents"], a_b_c["children"]) == (1, [])
+        assert a_b_c["summary"] is None
+        assert a_b_c["summary_pending"] is True  # 1 doc, no stored row
+        assert (mid["kind"], mid["path"]) == ("file", "a/b/mid.md")
+        assert {f["path"] for f in _tree_file_nodes_all(body["sources"])} == {
+            "root.md",
+            "a/top.md",
+            "a/b/mid.md",
+        }  # deep.md never reaches the wire at this depth
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_docs_tree_depth_validation_matrix(admin_client, db) -> None:
+    """``depth`` is bounded (1..8) and typed: ``0`` and ``9`` are out of
+    range and ``abc`` is not an int — all 422 before any build runs;
+    the bounds themselves answer 200."""
+    _seed_depth_corpus(db)
+    try:
+        for bad in ("0", "9", "abc"):
+            r = admin_client.get(f"/api/docs/tree?depth={bad}")
+            assert r.status_code == 422, bad
+        assert admin_client.get("/api/docs/tree?depth=1").status_code == 200
+        assert admin_client.get("/api/docs/tree?depth=8").status_code == 200
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_docs_tree_full_build_keeps_the_pre_phase_node_wire_shape(
+    admin_client, db
+) -> None:
+    """The wire-shape pin: on a FULL (``depth=8``) build nothing is
+    truncated, so NO node carries the ``truncated`` key — the source,
+    folder and file key sets are exactly the pre-phase-167 ones (the
+    omission serializer keeps every existing consumer byte-identical);
+    the additive top-level ``totals`` is the only body change."""
+    _seed_depth_corpus(db)
+    try:
+        body = admin_client.get("/api/docs/tree?depth=8").json()
+        assert set(body) == {"sources", "totals"}
+        (homelab,) = body["sources"]
+        assert set(homelab) == {
+            "name", "documents", "updated_at", "summary", "summary_pending", "children"
+        }
+        seen_folders = seen_files = 0
+        stack = list(homelab["children"])
+        while stack:
+            node = stack.pop()
+            if node["kind"] == "folder":
+                seen_folders += 1
+                assert set(node) == {
+                    "kind", "path", "documents", "updated_at", "summary",
+                    "summary_pending", "children",
+                }
+                stack.extend(node["children"])
+            else:
+                seen_files += 1
+                assert set(node) == {
+                    "kind", "path", "title", "chunks", "created_at", "indexed_at"
+                }
+        assert (seen_folders, seen_files) == (3, 4)
+    finally:
+        _truncate_tree_tables(db)
+
+
+# ---------------------------------------------------------------------------
+# Phase 167 (task 03): GET /api/docs/tree/node — the drill's per-level
+# fetch — and the ETag revalidation of the three catalogue reads
+# (D3, LOCKED A3/A4).
+# ---------------------------------------------------------------------------
+
+
+def _find_child(node: dict, path: str) -> dict:
+    """The child of *node* whose ``path`` equals *path* (the level walk
+    the drill does client-side, expressed once for the assertions)."""
+    return next(child for child in node["children"] if child["path"] == path)
+
+
+def test_tree_node_source_root_is_the_kb_tree_source_shape(
+    admin_client: TestClient, db
+) -> None:
+    """``?source=Homelab`` (``path`` omitted = ``""``; the explicit
+    ``path=`` answers the identical node): the SOURCE shape — the full
+    key set, never ``truncated``, the whole recursive source count —
+    with the root level only: the direct file plus the subfolder
+    flagged for the next drill."""
+    _seed_depth_corpus(db)
+    try:
+        r = admin_client.get("/api/docs/tree/node", params={"source": "Homelab"})
+        assert r.status_code == 200
+        node = r.json()
+        assert "truncated" not in node  # the node's OWN children are always built
+        assert set(node) == {
+            "name",
+            "documents",
+            "updated_at",
+            "summary",
+            "summary_pending",
+            "children",
+        }
+        assert node["name"] == "Homelab"
+        assert node["documents"] == 4  # the whole recursive source count
+        assert node["summary_pending"] is True  # no stored (Homelab, "") row
+        (a,) = [c for c in node["children"] if c["kind"] == "folder"]
+        (root,) = [c for c in node["children"] if c["kind"] == "file"]
+        assert (a["path"], a["truncated"], a["children"], a["documents"]) == (
+            "a",
+            True,
+            [],
+            3,
+        )
+        assert root["path"] == "root.md"
+        # The explicit empty path is the SAME node as the omitted one.
+        r2 = admin_client.get(
+            "/api/docs/tree/node", params={"source": "Homelab", "path": ""}
+        )
+        assert r2.status_code == 200
+        assert r2.json() == node
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_tree_node_nested_folder_fields_and_depth_one_children(
+    admin_client: TestClient, db
+) -> None:
+    """``?source=Homelab&path=a&depth=1``: the KbTreeFolder shape with
+    every field the level row renders — recursive count, recursive
+    ``updated_at``, ``summary``, ``summary_pending`` (phase 98 D3) — and
+    its DIRECT children only: the deeper subfolder flagged
+    ``truncated`` (the next drill's target), the direct file present."""
+    _seed_depth_corpus(db)
+    try:
+        r = admin_client.get(
+            "/api/docs/tree/node", params={"source": "Homelab", "path": "a"}
+        )
+        assert r.status_code == 200
+        node = r.json()
+        assert node["kind"] == "folder"
+        assert node["path"] == "a"
+        assert "truncated" not in node  # this node's children WERE built
+        assert node["documents"] == 3  # recursive — the phase-94 count rule
+        assert node["updated_at"] is not None  # recursive max (phase 106 D9)
+        assert node["summary"] == "A desc."
+        assert node["summary_pending"] is False  # the stored row covers it
+        a_b, top = node["children"]  # subfolder first, then the direct file
+        assert (a_b["kind"], a_b["path"], a_b["truncated"], a_b["children"]) == (
+            "folder",
+            "a/b",
+            True,
+            [],
+        )
+        assert a_b["documents"] == 2  # mid.md + deep.md, recursive
+        assert a_b["summary_pending"] is True  # docs but no stored row
+        assert (top["kind"], top["path"]) == ("file", "a/top.md")
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_tree_node_depth_two_builds_one_level_deeper(
+    admin_client: TestClient, db
+) -> None:
+    """``&depth=2`` spends one more level: ``a/b`` arrives BUILT (no
+    ``truncated`` key, its direct file present) and only its own
+    subfolder stays truncated — the same budget semantics the full tree
+    applies, answered for one node."""
+    _seed_depth_corpus(db)
+    try:
+        r = admin_client.get(
+            "/api/docs/tree/node",
+            params={"source": "Homelab", "path": "a", "depth": 2},
+        )
+        assert r.status_code == 200
+        a_b = _find_child(r.json(), "a/b")
+        assert "truncated" not in a_b  # expanded at this depth
+        a_b_c, mid = a_b["children"]
+        assert (a_b_c["path"], a_b_c["truncated"], a_b_c["documents"]) == (
+            "a/b/c",
+            True,
+            1,
+        )
+        assert (mid["kind"], mid["path"]) == ("file", "a/b/mid.md")
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_tree_node_404_unknown_source_folder_and_file_path(
+    admin_client: TestClient, db
+) -> None:
+    """404 ``{"detail": "unknown node"}`` for a source that is neither
+    registered nor indexed, a folder failing the phase-94 existence
+    rule, a FILE's path (a document's OWN path is not a folder), and
+    traversal strings — which have no filesystem surface to hit: the
+    node is derived from catalogue ROWS alone (the row-only rule, the
+    phase-10 traversal argument reused)."""
+    _seed_depth_corpus(db)
+    try:
+        for params in (
+            {"source": "nope"},  # neither registered nor indexed
+            {"source": "Homelab", "path": "nope"},  # no indexed path starts with it
+            {"source": "Homelab", "path": "a/top.md"},  # a FILE's own path
+            {"source": "Homelab", "path": "../Homelab"},  # traversal is not a folder
+            {"source": "..", "path": ""},  # nor is it a source
+        ):
+            r = admin_client.get("/api/docs/tree/node", params=params)
+            assert r.status_code == 404, params
+            assert r.json() == {"detail": "unknown node"}
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_tree_node_403_anonymous(client: TestClient, db) -> None:
+    """The same gate as ``/api/docs/tree`` — anonymous 403
+    ``{"detail": "admin only"}`` (the RAG view never fetches for guests,
+    the endpoint must not lean on that)."""
+    _seed_depth_corpus(db)
+    try:
+        r = client.get("/api/docs/tree/node", params={"source": "Homelab"})
+        assert r.status_code == 403
+        assert r.json() == {"detail": "admin only"}
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_tree_node_validation_matrix(admin_client: TestClient, db) -> None:
+    """``source`` is required and ``depth`` keeps the tree route's
+    bounds (``ge=1, le=8``): a missing source, ``depth=0``, ``depth=9``
+    and ``depth=abc`` all 422 before anything is built."""
+    _seed_depth_corpus(db)
+    try:
+        assert admin_client.get("/api/docs/tree/node").status_code == 422
+        for bad in ("0", "9", "abc"):
+            r = admin_client.get(
+                "/api/docs/tree/node", params={"source": "Homelab", "depth": bad}
+            )
+            assert r.status_code == 422, bad
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_tree_node_children_equal_the_full_tree_level(
+    admin_client: TestClient, db
+) -> None:
+    """ONE concept, two routes, pinned equal: at the same budget the
+    node route answers EXACTLY what ``GET /api/docs/tree`` carries at
+    the same level — the source root at the default budget AND the full
+    one, and the nested ``a`` folder on the full build — the JSON
+    compared whole, so the drill can never paint a node the tree would
+    not have produced."""
+    _seed_depth_corpus(db)
+    try:
+        for depth in (1, 8):
+            tree = admin_client.get(f"/api/docs/tree?depth={depth}").json()
+            (source_tree,) = tree["sources"]
+            node_root = admin_client.get(
+                "/api/docs/tree/node", params={"source": "Homelab", "depth": depth}
+            )
+            assert node_root.status_code == 200
+            assert node_root.json() == source_tree
+        full = admin_client.get("/api/docs/tree?depth=8").json()
+        a_tree = _find_child(full["sources"][0], "a")
+        node_a = admin_client.get(
+            "/api/docs/tree/node",
+            params={"source": "Homelab", "path": "a", "depth": 8},
+        )
+        assert node_a.status_code == 200
+        assert node_a.json() == a_tree
+    finally:
+        _truncate_tree_tables(db)
+
+
+_CATALOGUE_READ_URLS = (
+    "/api/docs",
+    "/api/docs/tree",
+    "/api/docs/tree/node?source=Homelab&path=a",
+)
+
+
+def test_catalogue_reads_carry_etag_and_answer_304(
+    admin_client: TestClient, db
+) -> None:
+    """The ETag matrix on all three catalogue reads (A4): the first GET
+    → 200 + weak ``ETag`` + ``Cache-Control: private, must-revalidate``;
+    the re-send with ``If-None-Match`` → 304 with an EMPTY body and
+    BOTH headers (RFC 9110 §8.8.3 — a 304 repeats the validator); the
+    strong form of the weak tag, a comma-separated list and ``*`` all
+    revalidate; a wrong tag is a plain 200 carrying the CURRENT tag."""
+    _seed_depth_corpus(db)
+    try:
+        for url in _CATALOGUE_READ_URLS:
+            first = admin_client.get(url)
+            assert first.status_code == 200, url
+            tag = first.headers["etag"]
+            assert tag.startswith('W/"'), url
+            assert first.headers["cache-control"] == "private, must-revalidate", url
+
+            hit = admin_client.get(url, headers={"If-None-Match": tag})
+            assert hit.status_code == 304, url
+            assert hit.content == b"", url
+            assert hit.headers["etag"] == tag, url
+            assert hit.headers["cache-control"] == "private, must-revalidate", url
+
+            strong = tag[2:]  # the same tag WITHOUT the weakness marker
+            assert admin_client.get(
+                url, headers={"If-None-Match": strong}
+            ).status_code == 304, url
+            assert admin_client.get(
+                url, headers={"If-None-Match": f'W/"stale-tag", {strong}'}
+            ).status_code == 304, url
+            assert admin_client.get(url, headers={"If-None-Match": "*"}).status_code == (
+                304
+            ), url
+
+            miss = admin_client.get(url, headers={"If-None-Match": 'W/"not-the-tag"'})
+            assert miss.status_code == 200, url
+            assert miss.headers["etag"] == tag, url
+            assert miss.json() == first.json(), url
+    finally:
+        _truncate_tree_tables(db)
+
+
+def _seed_bust_corpus(db) -> datetime:
+    """The depth corpus with DETERMINISTIC, distinct ``created_at``
+    stamps (the default seeding's subtree max is insert-time
+    microseconds): ``a/b/c/deep.md`` IS the max of ``a``, so editing its
+    date provably MOVES every answer that carries ``a``'s
+    ``updated_at`` — which is what the bust test below asserts."""
+    _truncate_tree_tables(db)
+    base = datetime.now(UTC)
+    dates = {
+        "root.md": base - timedelta(days=30),
+        "a/top.md": base - timedelta(days=20),
+        "a/b/mid.md": base - timedelta(days=10),
+        "a/b/c/deep.md": base - timedelta(days=1),  # the subtree max of "a"
+    }
+    db.add(GitSource(url="https://github.com/reese/Homelab.git", kind="git", added_at=base))
+    for i, (path, created) in enumerate(dates.items()):
+        doc = Document(
+            source="Homelab",
+            path=path,
+            full_path=f"/tmp/Homelab/{path}",
+            title=path,
+            content=f"# {path}\n\nBody.",
+            content_hash=f"{next(_counter):064d}",  # unique per row
+            indexed_at=base,
+            created_at=created,
+        )
+        db.add(doc)
+        db.flush()
+        db.add(
+            Chunk(document_id=doc.id, position=0, content=f"chunk {i}", embedding=[0.01] * 768)
+        )
+    db.add(FolderSummary(source="Homelab", folder_path="a", summary="A desc."))
+    db.commit()  # the app's endpoint session reads committed data only
+    return base
+
+
+def test_etag_busts_on_folder_summary_and_document_date(
+    admin_client: TestClient, db
+) -> None:
+    """The LOCKED A4 proof: the admin PATCHes mutate the tree WITHOUT
+    bumping ``sources_meta.version`` (asserted around the edits — a
+    version-derived tag would keep answering 304 here). After
+    ``PATCH /api/folders/summary`` AND after ``PATCH
+    /api/documents/date`` the tag CHANGES and the conditional GET with
+    the OLD tag answers a fresh 200 carrying the new data."""
+    _seed_bust_corpus(db)
+    try:
+        version_before = db.scalar(select(func.max(SourcesMeta.version)))
+        tree_url = "/api/docs/tree"
+        node_url = "/api/docs/tree/node?source=Homelab&path=a"
+        old_tree = admin_client.get(tree_url).headers["etag"]
+        old_node = admin_client.get(node_url).headers["etag"]
+
+        r = admin_client.patch(
+            "/api/folders/summary",
+            json={"source": "Homelab", "folder_path": "a", "summary": "Handwritten."},
+        )
+        assert r.status_code == 200
+        t = admin_client.get(tree_url, headers={"If-None-Match": old_tree})
+        assert t.status_code == 200 and t.headers["etag"] != old_tree
+        assert _find_child(t.json()["sources"][0], "a")["summary"] == "Handwritten."
+        n = admin_client.get(node_url, headers={"If-None-Match": old_node})
+        assert n.status_code == 200 and n.headers["etag"] != old_node
+        assert n.json()["summary"] == "Handwritten."
+
+        # The date edit moves the subtree max (deep.md is "a"'s max in
+        # this corpus) — the tree, the node AND /api/docs all change.
+        old_tree2 = admin_client.get(tree_url).headers["etag"]
+        old_docs = admin_client.get("/api/docs").headers["etag"]
+        deep_max_before = _find_child(
+            admin_client.get(tree_url).json()["sources"][0], "a"
+        )["updated_at"]
+        r = admin_client.patch(
+            "/api/documents/date",
+            json={"source": "Homelab", "path": "a/b/c/deep.md", "date": "2020-01-01"},
+        )
+        assert r.status_code == 200
+        t2 = admin_client.get(tree_url, headers={"If-None-Match": old_tree2})
+        assert t2.status_code == 200 and t2.headers["etag"] != old_tree2
+        deep_max_after = _find_child(t2.json()["sources"][0], "a")["updated_at"]
+        assert deep_max_after != deep_max_before  # the body really moved
+        d2 = admin_client.get("/api/docs", headers={"If-None-Match": old_docs})
+        assert d2.status_code == 200 and d2.headers["etag"] != old_docs
+        deep = next(d for d in d2.json()["documents"] if d["path"] == "a/b/c/deep.md")
+        assert deep["created_at"].startswith("2020-01-01")
+        # A4's other half: NEITHER PATCH bumped the KB generation (that
+        # would mark every saved chat stale — phase 53).
+        assert db.scalar(select(func.max(SourcesMeta.version))) == version_before
+    finally:
+        _truncate_tree_tables(db)
+
+
+def test_gzipped_catalogue_200_carries_etag_and_conditional_304(
+    admin_client: TestClient, db
+) -> None:
+    """The compression layer and the validator coexist: the gzipped 200
+    of the tree still carries the ``ETag`` (the header survives the
+    encoding), and the conditional request answers a body-less 304 —
+    no ``Content-Encoding`` on a response with no body."""
+    _seed_gzip_corpus(db)
+    try:
+        # depth=8: the whole 50-doc tree stays above the 860-byte floor
+        # (the default depth-1 body would ride under it, identity).
+        r = admin_client.get("/api/docs/tree?depth=8", headers={"Accept-Encoding": "gzip"})
+        assert r.status_code == 200
+        assert r.headers["content-encoding"] == "gzip"
+        tag = r.headers["etag"]
+        assert tag.startswith('W/"')
+        assert r.headers["cache-control"] == "private, must-revalidate"
+        hit = admin_client.get(
+            "/api/docs/tree?depth=8",
+            headers={"Accept-Encoding": "gzip", "If-None-Match": tag},
+        )
+        assert hit.status_code == 304
+        assert hit.content == b""
+        assert "content-encoding" not in hit.headers
+        assert hit.headers["etag"] == tag
+        assert hit.headers["cache-control"] == "private, must-revalidate"
+    finally:
+        _truncate_tree_tables(db)

tests/integration/test_docs_api_dates.py

@@ -154,7 +154,10 @@ def test_tree_file_dates_and_derived_subtree_max(admin_client, db) -> None:
     _seed_doc(db, "Homelab", "a/top.md", "Top", 1, base, C2)
     _seed_doc(db, "Homelab", "root.md", "Root", 0, base, C0)
     try:
-        r = admin_client.get("/api/docs/tree")
+        # Phase 167 (task 02): this D9 walk visits ``a`` → ``a/b`` →
+        # ``a/b/c``, so it asks for the WHOLE tree (depth=8); the
+        # endpoint's default (depth=1) is the RAG view's first paint.
+        r = admin_client.get("/api/docs/tree?depth=8")
         assert r.status_code == 200
         sources = r.json()["sources"]
         # Registry order leads; the 0-document registered source lists.
@@ -198,6 +201,49 @@ def test_tree_file_dates_and_derived_subtree_max(admin_client, db) -> None:
         _truncate(db)
 
 
+def test_tree_truncated_folder_keeps_the_recursive_date_at_default_depth(
+    admin_client, db
+) -> None:
+    """Phase 167 (task 02, D2/A2) — the truncation-proof counterpart of
+    the D9 walk above: at the DEFAULT depth (the view's first paint)
+    the deep levels are not sent, but the truncated folder node still
+    carries the phase-106 D9 contract — ``updated_at`` = the subtree's
+    recursive MAX (deep C3) and ``documents`` = the recursive count —
+    alongside ``truncated: true`` and ``children: []`` (the builder's
+    max-date map is what buys this: the payload diet never loses the
+    date column). The server ``totals`` ride the same response.
+    """
+    _truncate(db)
+    base = datetime.now(UTC)
+    db.add(GitSource(url="https://github.com/reese/Homelab.git", kind="git", added_at=base))
+    _seed_doc(db, "Homelab", "a/b/c/deep.md", "Deep", 1, base, C3)
+    _seed_doc(db, "Homelab", "a/b/shallow.md", "Shallow", 0, base, C1)
+    _seed_doc(db, "Homelab", "a/top.md", "Top", 1, base, C2)
+    _seed_doc(db, "Homelab", "root.md", "Root", 0, base, C0)
+    try:
+        r = admin_client.get("/api/docs/tree")  # the DEFAULT depth = 1
+        assert r.status_code == 200
+        body = r.json()
+        homelab = _tree_node(body["sources"], "Homelab")
+        # One level only: the folder ``a`` is present but NOT built out.
+        (a,) = [c for c in homelab["children"] if c["kind"] == "folder"]
+        assert (a["path"], a["truncated"], a["children"]) == ("a", True, [])
+        # The truncation-proof fields: recursive count + recursive date.
+        assert a["documents"] == 3
+        assert a["updated_at"] == C3.isoformat()  # the DEEP max, unbuilt children
+        # The source node's own max still spans the whole (unsent) subtree.
+        assert homelab["updated_at"] == C3.isoformat()
+        assert "truncated" not in homelab  # depth >= 1: never truncated, key omitted
+        # Server totals beside the depth-limited tree (the stat cards).
+        assert body["totals"] == {
+            "documents": 4,
+            "chunks": 2,
+            "last_indexed": base.isoformat(),
+        }
+    finally:
+        _truncate(db)
+
+
 # ---------------------------------------------------------------------------
 # PATCH /api/documents/date (D7) — the admin document-date editor.
 # ---------------------------------------------------------------------------

tests/integration/test_response_budget_api.py

@@ -0,0 +1,238 @@
+"""Integration: the byte-budget log line on the REAL app stack (phase 167, task 05).
+
+``tests/unit/test_response_budget.py`` pins the middleware on a bare ASGI
+app; this file pins what only the real stack can prove —
+
+* the line describes the REAL ``GET /api/docs/tree``: ``bytes=`` equals the
+  JSON the route produced, ``enc=identity`` for an identity client and
+  ``enc=gzip`` through the actual ``SelectiveGZipMiddleware`` (the
+  reading that works because the middleware sits INSIDE the compressor),
+* a conditional revalidation of that route logs ``status=304 bytes=0`` —
+  the phase-167 win visible in the log, and
+* the level follows ``log_large_response_bytes`` (DEBUG under it, INFO at
+  or over it, and ``/api/health`` under the 64 KB default), while
+  non-``/api/`` paths (the shell page through the caching + compression
+  layers) write nothing.
+
+Uses the real compose Postgres (``db`` fixture) and FastAPI's TestClient.
+"""
+from __future__ import annotations
+
+import itertools
+import logging
+import re
+from datetime import UTC, datetime
+from typing import Any
+
+import pytest
+from fastapi.testclient import TestClient
+from sqlalchemy import text
+
+import app.core.response_budget as budget_mod
+from app.config import Settings
+from app.main import app as fastapi_app
+from app.models import Chunk, Document
+
+_LINE_RE = re.compile(
+    r"^api method=(?P<method>[A-Z]+) path=(?P<path>\S+) status=(?P<status>\d+) "
+    r"bytes=(?P<bytes>\d+) enc=(?P<enc>\S+) duration_ms=(?P<ms>\d+)$"
+)
+
+_counter = itertools.count()
+
+
+def _clean(db) -> None:
+    db.execute(text("TRUNCATE chunks, documents"))
+    db.commit()
+
+
+def _seed_corpus(db, count: int = 50) -> None:
+    """``count`` indexed docs — the tree response lands well above the
+    860-byte gzip floor (the corpus that motivates the phase is ~1 k docs)."""
+    _clean(db)
+    base = datetime.now(UTC)
+    for i in range(count):
+        doc = Document(
+            source="budget-corpus",
+            path=f"docs/sub-{i % 5}/file-{i:02d}.md",
+            full_path=f"/tmp/budget-corpus/docs/sub-{i % 5}/file-{i:02d}.md",
+            title=f"Doc {i:02d}",
+            content=f"# Doc {i:02d}\n\nBody of document {i:02d}.",
+            content_hash=f"{next(_counter):064d}",
+            indexed_at=base,
+        )
+        db.add(doc)
+        db.flush()
+        db.add(
+            Chunk(
+                document_id=doc.id,
+                position=0,
+                content=f"chunk of doc {i:02d}",
+                embedding=[0.01] * 768,
+            )
+        )
+    db.commit()  # the endpoint session reads committed data only
+
+
+def _budget_lines(
+    caplog: pytest.LogCaptureFixture, path: str | None = None
+) -> list[tuple[int, str]]:
+    """``(levelno, message)`` for the budget logger, optionally narrowed to
+    one request path (the ``admin_client`` fixture's own login is a real
+    ``/api/*`` response too)."""
+    lines = [
+        (record.levelno, record.getMessage())
+        for record in caplog.records
+        if record.name == "app.response_budget"
+    ]
+    if path is None:
+        return lines
+    return [(level, message) for level, message in lines if f" path={path} " in message]
+
+
+def test_tree_response_logs_the_exact_line_for_the_route_bytes(
+    admin_client: TestClient, db, caplog: pytest.LogCaptureFixture
+) -> None:
+    """``bytes=`` is what the route produced — with an identity client that
+    is exactly the body the caller received."""
+    _seed_corpus(db)
+    try:
+        with caplog.at_level(logging.DEBUG, logger="app.response_budget"):
+            response = admin_client.get(
+                "/api/docs/tree?depth=8", headers={"Accept-Encoding": "identity"}
+            )
+        assert response.status_code == 200
+
+        lines = _budget_lines(caplog, "/api/docs/tree")
+        assert len(lines) == 1
+        match = _LINE_RE.match(lines[0][1])
+        assert match is not None, lines[0][1]
+        assert match.group("method") == "GET"
+        assert match.group("status") == "200"
+        assert int(match.group("bytes")) == len(response.content)
+        assert match.group("enc") == "identity"
+        assert int(match.group("ms")) >= 0
+    finally:
+        _clean(db)
+
+
+def test_gzipped_tree_logs_enc_gzip_with_the_uncompressed_bytes(
+    admin_client: TestClient, db, caplog: pytest.LogCaptureFixture
+) -> None:
+    """The D5 reading, end to end: the client received gzip and the log says
+    ``enc=gzip`` while ``bytes=`` stays the ROUTE's size — which is why the
+    middleware is wired inside the compression layer."""
+    _seed_corpus(db)
+    try:
+        with caplog.at_level(logging.DEBUG, logger="app.response_budget"):
+            identity = admin_client.get(
+                "/api/docs/tree?depth=8", headers={"Accept-Encoding": "identity"}
+            )
+            response = admin_client.get(
+                "/api/docs/tree?depth=8", headers={"Accept-Encoding": "gzip"}
+            )
+        assert response.status_code == 200
+        assert response.headers["content-encoding"] == "gzip"
+        assert response.content == identity.content
+
+        lines = _budget_lines(caplog, "/api/docs/tree")
+        assert len(lines) == 2
+        identity_line, gzip_line = (
+            _LINE_RE.match(lines[0][1]),
+            _LINE_RE.match(lines[1][1]),
+        )
+        assert identity_line is not None and identity_line.group("enc") == "identity"
+        assert gzip_line is not None and gzip_line.group("enc") == "gzip"
+        # Same route bytes in both lines…
+        assert identity_line.group("bytes") == gzip_line.group("bytes")
+        # …and that is the ROUTE's size, not the compressed transfer.
+        assert int(gzip_line.group("bytes")) == len(identity.content)
+    finally:
+        _clean(db)
+
+
+def test_conditional_revalidation_logs_zero_bytes(
+    admin_client: TestClient, db, caplog: pytest.LogCaptureFixture
+) -> None:
+    """The phase-167 win is visible, not hidden: the 304 costs one line with
+    ``bytes=0``."""
+    _seed_corpus(db)
+    try:
+        fresh = admin_client.get("/api/docs/tree?depth=1")
+        assert fresh.status_code == 200
+        etag = fresh.headers["etag"]
+
+        with caplog.at_level(logging.DEBUG, logger="app.response_budget"):
+            revalidated = admin_client.get(
+                "/api/docs/tree?depth=1", headers={"If-None-Match": etag}
+            )
+        assert revalidated.status_code == 304
+
+        lines = _budget_lines(caplog, "/api/docs/tree")
+        assert len(lines) == 1
+        match = _LINE_RE.match(lines[0][1])
+        assert match is not None
+        assert match.group("status") == "304"
+        assert match.group("bytes") == "0"
+    finally:
+        _clean(db)
+
+
+def test_level_follows_the_configured_budget(
+    admin_client: TestClient,
+    monkeypatch: pytest.MonkeyPatch,
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """``/api/health`` sits under the 65536 default → DEBUG; the same
+    response with ``log_large_response_bytes=0`` is INFO (the loud mode). The
+    middleware reads the setting per response, so this is the operator's
+    runtime knob, not a boot-time one."""
+    with caplog.at_level(logging.DEBUG, logger="app.response_budget"):
+        assert admin_client.get("/api/health").status_code == 200
+    assert [level for level, _ in _budget_lines(caplog, "/api/health")] == [logging.DEBUG]
+
+    monkeypatch.setattr(
+        budget_mod,
+        "get_settings",
+        lambda: Settings(_env_file=None, log_large_response_bytes=0),  # pyright: ignore[reportCallIssue]
+    )
+    caplog.clear()
+    with caplog.at_level(logging.DEBUG, logger="app.response_budget"):
+        assert admin_client.get("/api/health").status_code == 200
+    assert [level for level, _ in _budget_lines(caplog, "/api/health")] == [logging.INFO]
+
+
+def test_html_page_writes_no_budget_line(
+    admin_client: TestClient, caplog: pytest.LogCaptureFixture
+) -> None:
+    """The shell page rides the caching rewrite AND the compression layer
+    and still writes nothing — the budget measures ``/api/*`` only."""
+    with caplog.at_level(logging.DEBUG, logger="app.response_budget"):
+        response = admin_client.get("/sources.html", headers={"Accept-Encoding": "gzip"})
+        assert response.status_code == 200
+        assert response.headers["content-encoding"] == "gzip"
+
+    assert not any(" path=/sources.html " in message for _, message in _budget_lines(caplog))
+
+
+def test_anonymous_403_is_measured_like_any_other_response(
+    client: TestClient, caplog: pytest.LogCaptureFixture
+) -> None:
+    """The gate is a response too — an unauthenticated catalogue read logs
+    its status (and the byte line is what makes a chatty 403 visible)."""
+    with caplog.at_level(logging.DEBUG, logger="app.response_budget"):
+        assert client.get("/api/docs").status_code == 403
+
+    lines = _budget_lines(caplog, "/api/docs")
+    assert len(lines) == 1
+    match: Any = _LINE_RE.match(lines[0][1])
+    assert match is not None
+    assert match.group("status") == "403"
+
+
+def test_the_module_level_app_carries_the_middleware() -> None:
+    """Smoke: the wiring pin in ``tests/unit/test_response_budget.py`` covers
+    the ORDER on a fresh ``create_app()``; this proves the module-level ``app``
+    the whole suite (and the dev server) runs against has the layer too."""
+    names = [getattr(mw.cls, "__name__", "") for mw in fastapi_app.user_middleware]
+    assert "ResponseBudgetMiddleware" in names

tests/unit/test_compression.py

@@ -0,0 +1,497 @@
+"""Unit tests: ``app.core.compression.SelectiveGZipMiddleware`` (phase 167, D1, LOCKED A1).
+
+Pins the three contracts of the selective-gzip layer on a tiny ASGI app —
+
+* a ~50 KB JSON route compresses (``Content-Encoding: gzip``, the body
+  ``gzip.decompress``es to the original, ``Vary`` mentions
+  ``accept-encoding``), an ``Accept-Encoding: identity`` client gets
+  identity bytes, a ~50-byte route stays under the floor, and a response
+  that already set ``content-encoding`` is forwarded byte-identical (never
+  double-encoded — the fronting-nginx contract);
+
+* the EXACT-path skip: a route named ``/api/chat`` is NOT compressed and
+  streams frame-by-frame (the raw-ASGI interleaving proof: every frame
+  reaches the client's ``send`` BEFORE the app yields the next one — a
+  buffering middleware fails this deterministically — plus the
+  ``list(stream)`` client idiom the existing SSE tests use), while
+  ``/api/chat-images/x`` shares the prefix and MUST compress;
+
+* non-HTTP scopes (``lifespan``/``websocket``) pass straight through.
+
+Plus the config kill switch: ``compress_min_bytes=0`` → ``create_app``
+adds no gzip layer at all; ``-1`` → the validator refuses startup naming
+the field; the middleware sits INSIDE ``SecurityHeadersMiddleware`` (A20)
+and OUTSIDE ``CachingMiddleware``.
+"""
+from __future__ import annotations
+
+import asyncio
+import gzip
+import json
+from typing import Any
+
+import pytest
+from fastapi.testclient import TestClient
+from pydantic import ValidationError
+from starlette.applications import Starlette
+from starlette.middleware.gzip import GZipMiddleware
+from starlette.requests import Request
+from starlette.responses import Response, StreamingResponse
+from starlette.routing import Route
+from starlette.types import Message, Receive, Scope, Send
+
+import app.main as main_mod
+from app.config import Settings
+from app.core.compression import DEFAULT_SKIP_PATHS, SelectiveGZipMiddleware
+
+
+def _settings(**kwargs: Any) -> Settings:
+    """Build Settings without reading a .env file (the test_config pattern)."""
+    kwargs.setdefault("_env_file", None)
+    return Settings(**kwargs)  # pyright: ignore[reportCallIssue]  (kwarg exists at runtime)
+
+
+# ~50 KB of highly compressible JSON — the /api/docs/tree shape class.
+_BIG_BODY = json.dumps(
+    {
+        "items": [
+            {"n": i, "path": f"source/docs/file-{i}.md", "title": f"Doc {i}"}
+            for i in range(1000)
+        ]
+    }
+).encode()
+
+# ~50 bytes — comfortably under the 860-byte floor (compression would inflate).
+_SMALL_BODY = json.dumps({"ok": True, "note": "intentionally tiny body"}).encode("utf-8")
+
+# Five SSE frames (~2 KB total — ABOVE the floor, so a missing skip cannot
+# hide behind the minimum_size exemption).
+_SSE_FRAMES = [
+    f"event: thinking\ndata: {'x' * 400}\n\n".encode() for _ in range(5)
+]
+
+_PRE_ENCODED_BODY = gzip.compress(_BIG_BODY)
+
+
+def _inner_app() -> Starlette:
+    async def big(request: Request) -> Response:
+        return Response(_BIG_BODY, media_type="application/json")
+
+    async def small(request: Request) -> Response:
+        return Response(_SMALL_BODY, media_type="application/json")
+
+    async def chat(request: Request) -> StreamingResponse:
+        async def frames():
+            for frame in _SSE_FRAMES:
+                yield frame
+
+        return StreamingResponse(frames(), media_type="text/event-stream")
+
+    async def chat_images(request: Request) -> Response:
+        # Shares the /api/chat PREFIX but is a different endpoint — the
+        # skip must be EXACT, so this one compresses.
+        return Response(_BIG_BODY, media_type="application/json")
+
+    async def pre_encoded(request: Request) -> Response:
+        # A body that arrived already encoded (e.g. from a future CDN
+        # layer): the responder skips it, never double-encodes.
+        return Response(
+            _PRE_ENCODED_BODY,
+            media_type="application/json",
+            headers={"Content-Encoding": "gzip"},
+        )
+
+    return Starlette(
+        routes=[
+            Route("/big", big),
+            Route("/small", small),
+            Route("/api/chat", chat, methods=["POST"]),
+            Route("/api/chat-images/x", chat_images),
+            Route("/pre-encoded", pre_encoded),
+        ]
+    )
+
+
+def _client(minimum_size: int = 860) -> TestClient:
+    return TestClient(
+        SelectiveGZipMiddleware(_inner_app(), minimum_size=minimum_size)
+    )
+
+
+# ---------------------------------------------------------------------------
+# The gzip path (Accept-Encoding negotiation, the floor, the pre-encoded skip)
+# ---------------------------------------------------------------------------
+
+
+def test_large_json_compresses_decompresses_and_sets_vary() -> None:
+    client = _client()
+    response = client.get("/big", headers={"Accept-Encoding": "gzip"})
+    assert response.status_code == 200
+    assert response.headers["content-encoding"] == "gzip"
+    assert "accept-encoding" in response.headers.get("vary", "").lower()
+    # httpx transparently decodes: the documented JSON arrives whole.
+    assert response.content == _BIG_BODY
+    assert len(response.content) > 860  # the seed really is above the floor
+
+    # The RAW wire bytes are a real gzip stream of the original body.
+    with client.stream("GET", "/big", headers={"Accept-Encoding": "gzip"}) as streamed:
+        raw = b"".join(list(streamed.iter_raw()))
+    assert gzip.decompress(raw) == _BIG_BODY
+    assert len(raw) < len(_BIG_BODY)  # ~8-12x on this JSON; order of magnitude suffices
+
+
+def test_identity_client_is_not_compressed() -> None:
+    client = _client()
+    response = client.get("/big", headers={"Accept-Encoding": "identity"})
+    assert response.status_code == 200
+    assert "content-encoding" not in response.headers
+    assert response.content == _BIG_BODY
+
+
+def test_small_response_stays_under_the_floor() -> None:
+    client = _client()
+    response = client.get("/small", headers={"Accept-Encoding": "gzip"})
+    assert response.status_code == 200
+    assert "content-encoding" not in response.headers
+    assert response.content == _SMALL_BODY
+
+
+def test_pre_encoded_body_is_never_double_encoded() -> None:
+    client = _client()
+    with client.stream("GET", "/pre-encoded", headers={"Accept-Encoding": "gzip"}) as streamed:
+        assert streamed.headers["content-encoding"] == "gzip"
+        raw = b"".join(list(streamed.iter_raw()))
+    # Byte-identical passthrough of the already-encoded body (Starlette's
+    # "skip when content-encoding is set" rule — the nginx coexistence pin).
+    assert raw == _PRE_ENCODED_BODY
+    assert gzip.decompress(raw) == _BIG_BODY
+
+
+# ---------------------------------------------------------------------------
+# The EXACT-path skip (A15: the SSE stream is never compressed, never buffered)
+# ---------------------------------------------------------------------------
+
+
+def test_default_skip_paths_constant() -> None:
+    assert DEFAULT_SKIP_PATHS == ("/api/chat",)
+
+
+def test_chat_route_is_not_compressed_and_streams_whole() -> None:
+    """The client-level pin (the ``list(stream)`` idiom the SSE tests use):
+    a route named /api/chat carries NO Content-Encoding and its raw bytes
+    are exactly the SSE frames, concatenated, above the size floor."""
+    client = _client()
+    with client.stream(
+        "POST", "/api/chat", json={"message": "hi"}, headers={"Accept-Encoding": "gzip"}
+    ) as streamed:
+        assert streamed.status_code == 200
+        assert "content-encoding" not in streamed.headers
+        chunks = list(streamed.iter_raw())
+    assert b"".join(chunks) == b"".join(_SSE_FRAMES)
+    assert sum(len(c) for c in chunks) > 860  # the skip beats the floor, not size
+
+
+def test_skipped_stream_arrives_frame_by_frame_uncompressed() -> None:
+    """The frame-by-frame proof (deterministic, no timing): every frame
+    reaches the client's ``send`` BEFORE the app produces the next one.
+    A buffering middleware (BaseHTTPMiddleware, or a gzip pass) yields
+    all ``app:*`` events first — this strict interleaving fails then."""
+    events: list[str] = []  # shared: the app logs a yield, send logs delivery
+    sent: list[Message] = []
+
+    async def sse_app(scope: Scope, receive: Receive, send: Send) -> None:
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 200,
+                "headers": [(b"content-type", b"text/event-stream")],
+            }
+        )
+        for i, frame in enumerate(_SSE_FRAMES):
+            events.append(f"app:frame-{i}")
+            await send({"type": "http.response.body", "body": frame, "more_body": True})
+        events.append("app:final")
+        await send({"type": "http.response.body", "body": b"", "more_body": False})
+
+    async def recording_send(message: Message) -> None:
+        events.append(f"client:{message['type']}")
+        sent.append(message)
+
+    async def receive() -> Message:
+        return {"type": "http.disconnect"}
+
+    middleware = SelectiveGZipMiddleware(sse_app, minimum_size=860)
+    asyncio.run(
+        middleware(
+            {"type": "http", "path": "/api/chat", "headers": [(b"accept-encoding", b"gzip")]},
+            receive,
+            recording_send,
+        )
+    )
+
+    expected = ["client:http.response.start"]
+    for i in range(len(_SSE_FRAMES)):
+        expected += [f"app:frame-{i}", "client:http.response.body"]
+    expected += ["app:final", "client:http.response.body"]
+    assert events == expected
+
+    start = sent[0]
+    headers = {k.decode().lower() for k, _ in start["headers"]}  # pyright: ignore[reportGeneralTypeIssues]
+    assert "content-encoding" not in headers
+    body = b"".join(m.get("body", b"") for m in sent[1:])  # pyright: ignore[reportGeneralTypeIssues]
+    assert body == b"".join(_SSE_FRAMES)
+
+
+def _raw_harness(
+    *,
+    path: str,
+    app: Any,
+) -> list[Message]:
+    """Drive the middleware at the raw ASGI level; return the exact
+    messages that reached the client's ``send``."""
+    sent: list[Message] = []
+
+    async def recording_send(message: Message) -> None:
+        sent.append(message)
+
+    async def receive() -> Message:
+        return {"type": "http.disconnect"}
+
+    scope: Scope = {
+        "type": "http",
+        "path": path,
+        "headers": [(b"accept-encoding", b"gzip")],
+    }
+    middleware = SelectiveGZipMiddleware(app, minimum_size=860)
+    asyncio.run(middleware(scope, receive, recording_send))
+    return sent
+
+
+def test_same_stream_at_a_non_skipped_path_goes_through_gzip() -> None:
+    """The skip is PATH-driven: the identical streaming app mounted at a
+    non-skipped path is compressed by the composed GZipMiddleware."""
+
+    async def json_stream_app(scope: Scope, receive: Receive, send: Send) -> None:
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 200,
+                "headers": [(b"content-type", b"application/json")],
+            }
+        )
+        for i, frame in enumerate(_SSE_FRAMES):
+            await send(
+                {"type": "http.response.body", "body": frame, "more_body": i < len(_SSE_FRAMES) - 1}
+            )
+
+    sent = _raw_harness(path="/api/other-stream", app=json_stream_app)
+    start_headers = {
+        k.decode().lower(): v.decode() for k, v in sent[0]["headers"]  # pyright: ignore[reportGeneralTypeIssues]
+    }
+    assert start_headers.get("content-encoding") == "gzip"
+    raw = b"".join(m.get("body", b"") for m in sent[1:])  # pyright: ignore[reportGeneralTypeIssues]
+    assert gzip.decompress(raw) == b"".join(_SSE_FRAMES)
+
+
+def test_reframed_small_body_stays_identity_under_the_floor() -> None:
+    """The CachingMiddleware seam (starlette 1.x ``BaseHTTPMiddleware``
+    re-frames every response it passes up: chunk with ``more_body=True``
+    + empty terminator): a body whose ``content-length`` is below the
+    floor is re-buffered into ONE buffered message so Starlette's own
+    small-response exemption fires — identity bytes, no ``Vary`` append."""
+    payload = b'{"status":"ok","version":"0.1.0"}'  # 33 bytes — the /api/health class
+
+    async def reframed_app(scope: Scope, receive: Receive, send: Send) -> None:
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 200,
+                "headers": [
+                    (b"content-type", b"application/json"),
+                    (b"content-length", str(len(payload)).encode()),
+                ],
+            }
+        )
+        await send({"type": "http.response.body", "body": payload, "more_body": True})
+        await send({"type": "http.response.body", "body": b"", "more_body": False})
+
+    sent = _raw_harness(path="/api/health", app=reframed_app)
+    start_headers = {
+        k.decode().lower(): v.decode() for k, v in sent[0]["headers"]  # pyright: ignore[reportGeneralTypeIssues]
+    }
+    assert "content-encoding" not in start_headers
+    assert "vary" not in start_headers  # the exemption adds nothing
+    body = b"".join(m.get("body", b"") for m in sent[1:])  # pyright: ignore[reportGeneralTypeIssues]
+    assert body == payload
+
+
+def test_lying_content_length_that_outgrows_the_floor_compresses() -> None:
+    """A body that outgrows the floor despite a small ``content-length``
+    stops the hold and streams through the normal compressing path — the
+    buffer is bounded by the floor and correctness never depends on the
+    header being honest."""
+
+    async def lying_app(scope: Scope, receive: Receive, send: Send) -> None:
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 200,
+                "headers": [
+                    (b"content-type", b"application/json"),
+                    (b"content-length", b"5"),  # a lie
+                ],
+            }
+        )
+        await send({"type": "http.response.body", "body": _BIG_BODY, "more_body": True})
+        await send({"type": "http.response.body", "body": b"", "more_body": False})
+
+    sent = _raw_harness(path="/api/lias", app=lying_app)
+    start_headers = {
+        k.decode().lower(): v.decode() for k, v in sent[0]["headers"]  # pyright: ignore[reportGeneralTypeIssues]
+    }
+    assert start_headers.get("content-encoding") == "gzip"
+    raw = b"".join(m.get("body", b"") for m in sent[1:])  # pyright: ignore[reportGeneralTypeIssues]
+    assert gzip.decompress(raw) == _BIG_BODY
+
+
+def test_non_body_message_while_holding_flushes_untouched() -> None:
+    """``http.response.pathsend`` (or anything unexpected) arriving while
+    the hold is active: the held messages and the message itself are
+    forwarded byte-for-byte — the hold never swallows a frame."""
+
+    async def pathsend_app(scope: Scope, receive: Receive, send: Send) -> None:
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 200,
+                "headers": [
+                    (b"content-type", b"text/plain"),
+                    (b"content-length", b"4"),
+                ],
+            }
+        )
+        await send({"type": "http.response.pathsend", "path": "/tmp/file"})
+
+    sent = _raw_harness(path="/assets/file", app=pathsend_app)
+    assert [m["type"] for m in sent] == ["http.response.start", "http.response.pathsend"]
+    start_headers = {k.decode().lower() for k, _ in sent[0]["headers"]}  # pyright: ignore[reportGeneralTypeIssues]
+    assert "content-encoding" not in start_headers
+
+
+def test_chat_images_prefix_still_compresses() -> None:
+    """The exact-match pin: /api/chat-images/x shares the /api/chat
+    prefix and MUST go through the gzip path."""
+    client = _client()
+    response = client.get("/api/chat-images/x", headers={"Accept-Encoding": "gzip"})
+    assert response.status_code == 200
+    assert response.headers["content-encoding"] == "gzip"
+    assert response.content == _BIG_BODY
+
+
+# ---------------------------------------------------------------------------
+# Non-HTTP scopes pass straight through
+# ---------------------------------------------------------------------------
+
+
+def test_non_http_scopes_pass_through_untouched() -> None:
+    seen: list[str] = []
+
+    async def probe_app(scope: Scope, receive: Receive, send: Send) -> None:
+        seen.append(scope["type"])
+
+    async def receive() -> Message:
+        return {"type": "http.disconnect"}
+
+    async def send(message: Message) -> None:  # pragma: no cover - app sends nothing
+        raise AssertionError("no ASGI messages expected")
+
+    middleware = SelectiveGZipMiddleware(probe_app, minimum_size=860)
+    for scope_type in ("lifespan", "websocket"):
+        asyncio.run(
+            middleware({"type": scope_type, "path": "/api/anything"}, receive, send)
+        )
+    assert seen == ["lifespan", "websocket"]
+
+
+def test_composes_starlette_gzip_with_minimum_size() -> None:
+    """The middleware reuses Starlette's GZipMiddleware, it does not fork
+    the compression math (D1)."""
+
+    async def app(scope: Scope, receive: Receive, send: Send) -> None: ...
+
+    middleware = SelectiveGZipMiddleware(app, minimum_size=1234)
+    assert isinstance(middleware.gzip, GZipMiddleware)
+    assert middleware.gzip.minimum_size == 1234
+    assert middleware.skip_paths == DEFAULT_SKIP_PATHS
+
+
+# ---------------------------------------------------------------------------
+# Config: the BOR_COMPRESS_MIN_BYTES kill switch and the loud negative
+# ---------------------------------------------------------------------------
+
+
+def _middleware_names(app: Any) -> list[str]:
+    """Middleware class names, outermost first (``user_middleware`` order)."""
+    return [getattr(mw.cls, "__name__", "") for mw in app.user_middleware]
+
+
+def test_compress_min_bytes_defaults_to_the_nginx_floor(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.delenv("BOR_COMPRESS_MIN_BYTES", raising=False)
+    assert _settings().compress_min_bytes == 860
+
+
+def test_compress_min_bytes_env_override(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("BOR_COMPRESS_MIN_BYTES", "2048")
+    assert _settings().compress_min_bytes == 2048
+    monkeypatch.setenv("BOR_COMPRESS_MIN_BYTES", "0")  # the kill switch is legal
+    assert _settings().compress_min_bytes == 0
+
+
+def test_compress_min_bytes_negative_refuses_startup_naming_the_field() -> None:
+    with pytest.raises(ValidationError, match="compress_min_bytes must be >= 0"):
+        _settings(compress_min_bytes=-1)
+
+
+def test_create_app_wires_gzip_inside_security_headers(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """A20 holds: SecurityHeaders stays outermost (index 0 — starlette's
+    last add_middleware is the outermost layer), gzip sits between it and
+    the caching layer, so an already-rewritten HTML page compresses and
+    the /api/* caching pass-through happens before compression.
+
+    Phase 167 task 05 adds ``ResponseBudgetMiddleware`` between gzip and
+    caching — INSIDE the compressor on purpose, so its ``bytes=`` counts the
+    body the ROUTE produced while its ``enc=`` still reads the encoding the
+    compressor put on the wire (see tests/unit/test_response_budget.py)."""
+    monkeypatch.setattr(main_mod.settings, "compress_min_bytes", 860)
+    app2 = main_mod.create_app()
+    assert _middleware_names(app2) == [
+        "SecurityHeadersMiddleware",
+        "SelectiveGZipMiddleware",
+        "ResponseBudgetMiddleware",
+        "CachingMiddleware",
+        "SessionMiddleware",
+    ]
+    # The app still serves.
+    assert TestClient(app2).get("/api/health").status_code == 200
+
+
+def test_create_app_adds_no_gzip_layer_when_disabled(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """BOR_COMPRESS_MIN_BYTES=0 removes the middleware entirely (the kill
+    switch): the gzip class is absent from app.user_middleware."""
+    monkeypatch.setattr(main_mod.settings, "compress_min_bytes", 0)
+    app2 = main_mod.create_app()
+    names = _middleware_names(app2)
+    assert "SelectiveGZipMiddleware" not in names
+    assert names == [
+        "SecurityHeadersMiddleware",
+        # Phase 167 task 05: the byte-budget line is not optional.
+        "ResponseBudgetMiddleware",
+        "CachingMiddleware",
+        "SessionMiddleware",
+    ]

tests/unit/test_frontend_router.py

@@ -572,11 +572,13 @@ def test_rag_view_refetches_on_reshow() -> None:
     """Phase 77 task 02 (+ phase 97 task 04, + phase 99 task 02):
     the RAG (knowledge base) view re-fetches on a user-initiated
     re-show — sources.js listens and re-runs ``loadTree()`` (the
-    phase-97 catalog load: ONE fetch of ``GET /api/docs/tree``), now
-    AFTER the phase-99 history alignment (adopt the entry's kb / reset
-    to the top — a block-arrow listener, hence the ``});`` close in
-    the pin helper below). The load is race-tokened (phase 79) and
-    the RE-ENTRANT render — ``renderLevel`` clears BOTH row containers
+    catalog load — ONE fetch of ``GET /api/docs/tree?depth=1`` since
+    phase 167's payload diet; the view loads the catalogue from the
+    TREE endpoint, not the flat one), now AFTER the phase-99 history
+    alignment (adopt the entry's kb / reset to the top — a block-arrow
+    listener, hence the ``});`` close in the pin helper below). The load
+    is race-tokened (phase 79) and the RE-ENTRANT render —
+    ``renderLevel`` clears BOTH row containers
     at the TOP before filling them (the History pattern from task 01,
     extended to the folders table) — so a refresh from a populated
     level into a sparser (or empty) result replaces the rows instead
@@ -588,7 +590,9 @@ def test_rag_view_refetches_on_reshow() -> None:
     load = js.find("async function loadTree()")
     assert load != -1, "loadTree must exist"
     body = js[load : js.find("\n  }", load)]
-    assert 'fetch("/api/docs/tree")' in body, "the load must fetch the tree endpoint"
+    assert 'fetch("/api/docs/tree?depth=1")' in body, (
+        "the load must fetch the tree endpoint (depth-limited, phase 167)"
+    )
     assert "++loadSeq" in body, "the race token stays (phase 79)"
     render = js.find("function renderLevel()")
     assert render != -1, "renderLevel must exist"

tests/unit/test_http_cache.py

@@ -0,0 +1,110 @@
+"""Unit tests: ``app/core/http_cache.py`` (phase 167, task 03).
+
+The three pure helpers behind the catalogue reads' revalidation
+(LOCKED A4): the weak content-hash tag (shape, stability, the bytes-vs-
+str equality), the tolerant ``If-None-Match`` evaluator (``None`` /
+empty / ``*`` / single / comma-list / weak↔strong both directions /
+whitespace / the prefix-is-not-a-match pin), and the ONE header pair.
+"""
+from __future__ import annotations
+
+import re
+
+from app.core.http_cache import (
+    REVALIDATE_CACHE_CONTROL,
+    etag_matches,
+    revalidate_headers,
+    weak_etag,
+)
+
+_TAG_RE = re.compile(r'W/"[0-9a-f]{32}"')
+
+
+def test_weak_etag_shape_is_weak_32_hex() -> None:
+    """``W/"<sha256 hexdigest[:32]>"`` — the prefix, the 32-hex-char body,
+    the total 36-char length (the file's truncation convention)."""
+    tag = weak_etag("hello")
+    assert _TAG_RE.fullmatch(tag), tag
+    assert len(tag) == 36
+    assert tag.startswith('W/"') and tag.endswith('"')
+
+
+def test_weak_etag_is_deterministic_and_content_sensitive() -> None:
+    assert weak_etag("same body") == weak_etag("same body")
+    assert weak_etag("same body") != weak_etag("same body!")
+    assert weak_etag("") != weak_etag(" ")
+
+
+def test_weak_etag_bytes_and_str_forms_agree() -> None:
+    """The hash is over the UTF-8 bytes either way — the endpoint may
+    hash the ``str`` and serve its encoding without diverging."""
+    text = '{"tree": "héllo — ünïcode"}'
+    assert weak_etag(text) == weak_etag(text.encode("utf-8"))
+
+
+def test_weak_etag_truncation_matches_the_sha256_prefix() -> None:
+    import hashlib
+
+    body = "exact served bytes"
+    expected = hashlib.sha256(body.encode("utf-8")).hexdigest()[:32]
+    assert weak_etag(body) == f'W/"{expected}"'
+
+
+def test_etag_matches_none_and_empty_never_match() -> None:
+    """No conditional header (or an empty/whitespace-only one) is an
+    unconditional GET → the route must answer 200."""
+    assert etag_matches(None, 'W/"x"') is False
+    assert etag_matches("", 'W/"x"') is False
+    assert etag_matches("   ", 'W/"x"') is False
+
+
+def test_etag_matches_star() -> None:
+    """RFC 9110 §13.3.3: ``*`` matches anything the server holds."""
+    assert etag_matches("*", 'W/"x"') is True
+    assert etag_matches(" * ", 'W/"anything"') is True
+
+
+def test_etag_matches_single_candidate() -> None:
+    assert etag_matches('W/"x"', 'W/"x"') is True
+    assert etag_matches('W/"x"', 'W/"y"') is False
+
+
+def test_etag_matches_comma_separated_list() -> None:
+    header = 'W/"a", W/"b", W/"c"'
+    assert etag_matches(header, 'W/"a"') is True
+    assert etag_matches(header, 'W/"c"') is True
+    assert etag_matches(header, 'W/"d"') is False
+    # no space after the comma — a real client sends both shapes
+    assert etag_matches('W/"a",W/"b"', 'W/"b"') is True
+
+
+def test_etag_matches_weak_strong_equivalence_both_directions() -> None:
+    """``W/"x"`` matches ``"x"`` whichever side carries the weakness
+    marker (the stored form must not decide the outcome)."""
+    assert etag_matches('"x"', 'W/"x"') is True  # client strong, ours weak
+    assert etag_matches('W/"x"', '"x"') is True  # client weak, ours strong
+
+
+def test_etag_matches_is_whitespace_tolerant() -> None:
+    assert etag_matches('  W/"x"  ', 'W/"x"') is True
+    assert etag_matches('W/"a" ,  "x" ', 'W/"x"') is True
+    assert etag_matches('W/ "x"', 'W/"x"') is True
+
+
+def test_etag_matches_prefix_is_not_a_match() -> None:
+    """A tag that merely SHARES a prefix with ours is a mismatch — the
+    comparison is the full 32-hex-char digest, not ``startswith``."""
+    ours = "abcdef0123456789abcdef0123456789"
+    shorter = ours[:-1]
+    assert etag_matches(f'W/"{shorter}"', f'W/"{ours}"') is False
+    assert etag_matches(f'"{shorter}"', f'W/"{ours}"') is False
+
+
+def test_revalidate_headers_is_the_one_pair() -> None:
+    """The 200 AND the 304 carry exactly this pair (A19's read-only API
+    arm: private + must-revalidate beside the validator)."""
+    assert revalidate_headers('W/"x"') == {
+        "ETag": 'W/"x"',
+        "Cache-Control": "private, must-revalidate",
+    }
+    assert REVALIDATE_CACHE_CONTROL == "private, must-revalidate"

tests/unit/test_kb_tree_builder.py

@@ -11,8 +11,18 @@ file metadata, and the ``group_folder_listing`` cross-check property
 """
 from __future__ import annotations
 
-from app.api.docs import TreeDocRow, build_kb_tree
+import pytest
+from pydantic import ValidationError
+
+from app.api.docs import (
+    TreeDocRow,
+    _folder_counts,
+    _kb_totals,
+    _subtree_max,
+    build_kb_tree,
+)
 from app.rag.agent import group_folder_listing
+from app.schemas import KbTree, KbTreeFile, KbTreeTotals
 
 T0 = "2026-09-01T08:00:00+00:00"
 T1 = "2026-09-02T08:00:00+00:00"
@@ -633,3 +643,284 @@ def test_image_affordance_absent_without_the_map() -> None:
     assert set(file.model_dump()) == {
         "kind", "path", "title", "chunks", "created_at", "indexed_at"
     }
+
+
+# ---------------------------------------------------------------------------
+# Phase 167 (task 02, D2/A2) — the depth budget: a node whose children were
+# NOT built says so (``truncated``) while keeping every field the Sources
+# view renders (recursive count, recursive updated_at — the D9 max via the
+# _folder_counts max-date map, summary, summary_pending), the untruncated
+# wire shape stays byte-identical, and the catalogue totals ride the
+# response (``_kb_totals`` — the stat cards cannot be summed client-side
+# off a truncated tree).
+# ---------------------------------------------------------------------------
+
+#: The depth fixture: one chain four levels deep, distinct created dates
+#: (C0 < C1 < C2 < C3 — the DEEPEST file holds the max, the D9 trap) and
+#: distinct indexed stamps.
+DEPTH_ROWS: list[TreeDocRow] = [
+    ("S", "root.md", "Root", 1, T0, C0),
+    ("S", "a/top.md", "Top", 2, T1, C1),
+    ("S", "a/b/mid.md", "Mid", 3, T1, C2),
+    ("S", "a/b/c/deep.md", "Deep", 4, T2, C3),
+]
+
+DEPTH_SUMMARIES = {("S", "a"): "A desc."}  # a/b and a/b/c stay un-stored
+
+
+def _all_nodes(node):
+    """The node plus every descendant node (folders and files)."""
+    yield node
+    for child in getattr(node, "children", []):
+        if child.kind == "folder":
+            yield from _all_nodes(child)
+        else:
+            yield child
+
+
+def _all_folder_nodes(node):
+    """Every FOLDER descendant (the SOURCE node itself is excluded — it
+    carries ``name``, not ``path``, and has no ``kind``)."""
+    return [n for n in _all_nodes(node) if getattr(n, "kind", None) == "folder"]
+
+
+def _paths_under(node) -> set[str]:
+    """Paths of every descendant (folders and files); the source node
+    has no path of its own."""
+    out: set[str] = set()
+    for child in getattr(node, "children", []):
+        out.add(child.path)
+        if child.kind == "folder":
+            out |= _paths_under(child)
+    return out
+
+
+def test_depth_none_is_byte_identical_to_the_unlimited_build() -> None:
+    """``depth=None`` (the builder default) reproduces today's tree
+    byte-for-byte: the default call, the explicit None, and an
+    over-deep budget all serialize identically, and NO node carries the
+    truncated key (the omission rule)."""
+    default = [s.model_dump() for s in build_kb_tree(["S"], DEPTH_ROWS, DEPTH_SUMMARIES)]
+    explicit = [
+        s.model_dump() for s in build_kb_tree(["S"], DEPTH_ROWS, DEPTH_SUMMARIES, depth=None)
+    ]
+    over_deep = [
+        s.model_dump() for s in build_kb_tree(["S"], DEPTH_ROWS, DEPTH_SUMMARIES, depth=8)
+    ]
+    assert default == explicit == over_deep
+    (source,) = build_kb_tree(["S"], DEPTH_ROWS, DEPTH_SUMMARIES)
+    assert all("truncated" not in n.model_dump() for n in _all_nodes(source))
+    # The full chain IS built: every level of the fixture is present.
+    assert _paths_under(source) == {
+        "root.md",
+        "a",
+        "a/top.md",
+        "a/b",
+        "a/b/mid.md",
+        "a/b/c",
+        "a/b/c/deep.md",
+    }
+
+
+def test_depth_one_ships_sources_plus_one_level_with_truncated_flags() -> None:
+    """``depth=1``: the source's direct children are built (direct file
+    ``root.md`` included) and the subfolder with unloaded contents
+    carries the FULL truncation field set — ``truncated is True``,
+    ``children == []``, the RECURSIVE count, the RECURSIVE max date
+    (the deep C3 — D9 survives without the children), the stored
+    ``summary`` and the ``summary_pending`` flag (rule unchanged)."""
+    (source,) = build_kb_tree(["S"], DEPTH_ROWS, DEPTH_SUMMARIES, depth=1)
+    assert [f.path for f in _file_nodes(source)] == ["root.md"]
+    (a,) = _folder_nodes(source)
+    assert a.path == "a"
+    assert a.truncated is True
+    assert a.model_dump()["truncated"] is True
+    assert a.children == []
+    assert a.documents == 3  # recursive — the count rule is depth-proof
+    assert a.updated_at == C3  # the DEEP max, no built children to derive from
+    assert a.summary == "A desc."  # the level row shows a subfolder's description
+    assert a.summary_pending is False  # the stored row covers it (phase 98)
+    # Nothing deeper is built anywhere.
+    assert _paths_under(source) == {"root.md", "a"}
+
+
+def test_depth_two_builds_two_levels_and_truncates_the_deeper_one() -> None:
+    """``depth=2``: level 1 (``a``) is expanded UNTRUNCATED (its direct
+    file ``a/top.md`` ships), level 2 (``a/b``) is the truncated node —
+    the pending rule applies to it (2 docs, no stored row →
+    ``summary_pending`` true with ``summary None``), and neither its
+    own file (``a/b/mid.md``) nor the third level appear: the budget
+    removes the LEVELS below it, level by level."""
+    (source,) = build_kb_tree(["S"], DEPTH_ROWS, DEPTH_SUMMARIES, depth=2)
+    (a,) = _folder_nodes(source)
+    assert "truncated" not in a.model_dump()  # untruncated: key absent
+    assert [f.path for f in _file_nodes(a)] == ["a/top.md"]
+    (a_b,) = _folder_nodes(a)
+    assert a_b.truncated is True
+    assert a_b.children == []
+    assert (a_b.documents, a_b.updated_at) == (2, C3)  # mid + deep, max C3
+    assert a_b.summary is None
+    assert a_b.summary_pending is True  # the phase-98 rule rides truncated nodes
+    assert _paths_under(source) == {
+        "root.md",
+        "a",
+        "a/top.md",
+        "a/b",
+    }
+
+
+def test_source_node_is_never_truncated_at_any_depth() -> None:
+    """The view's top level is source → level 1, so the budget never
+    marks the SOURCE node truncated (at any depth, empty sources
+    included) — its wire shape never carries the key."""
+    (ref,) = build_kb_tree(["S"], DEPTH_ROWS, DEPTH_SUMMARIES)
+    for depth in (1, 2, 3, 8):
+        full, empty = build_kb_tree(
+            ["S", "Empty"], DEPTH_ROWS, DEPTH_SUMMARIES, depth=depth
+        )
+        assert full.truncated is False
+        assert "truncated" not in full.model_dump()
+        assert empty.truncated is False
+        assert "truncated" not in empty.model_dump()
+        assert empty.children == []
+        # Direct children always built: the source's level is complete.
+        assert [c.path for c in full.children] == [c.path for c in ref.children]
+
+
+def test_truncated_key_never_touches_the_file_node_shape() -> None:
+    """A FILE node has no children to truncate: the field does not even
+    exist on the class, and no dump ever carries it (the phase-122 text
+    wire shape stays byte-identical at every depth)."""
+    assert "truncated" not in KbTreeFile.model_fields
+    for depth in (1, 2, 8):
+        (source,) = build_kb_tree(["S"], DEPTH_ROWS, {}, depth=depth)
+        file_nodes = [n for n in _all_nodes(source) if getattr(n, "kind", None) == "file"]
+        assert file_nodes  # the fixture has files at every shipped level
+        for node in file_nodes:
+            assert "truncated" not in node.model_dump()
+
+
+def test_max_dates_map_agrees_with_subtree_max_at_every_depth() -> None:
+    """One concept, two derivations, cross-pinned: the
+    ``_folder_counts`` max-date map equals ``_subtree_max`` over the
+    FULLY expanded children for every folder (the expanded builder path
+    stays ``_subtree_max``, so a divergence fails loudly) — and every
+    node present in a depth-limited tree carries the SAME
+    ``updated_at`` as the full build, truncated or not (the phase-106
+    D9 column survives the payload diet)."""
+    file_rows = [
+        (path, title, chunks, indexed, created)
+        for _source, path, title, chunks, indexed, created in DEPTH_ROWS
+    ]
+    _folders, _counts, max_dates = _folder_counts(file_rows)
+    (full,) = build_kb_tree(["S"], DEPTH_ROWS, DEPTH_SUMMARIES)
+    full_dates = {n.path: n.updated_at for n in _all_folder_nodes(full)}
+    assert full_dates == max_dates  # the cross-pin at full expansion
+    for node in _all_folder_nodes(full):
+        # The expanded node's derivation equals the aggregate map.
+        assert _subtree_max(node.children) == max_dates[node.path]
+    for depth in (1, 2, 3):
+        (limited,) = build_kb_tree(["S"], DEPTH_ROWS, DEPTH_SUMMARIES, depth=depth)
+        for node in _all_folder_nodes(limited):
+            assert node.updated_at == max_dates[node.path] == full_dates[node.path]
+        # The source max also spans the UNSENT levels.
+        assert limited.updated_at == C3
+
+
+def test_truncation_keeps_the_phase_94_and_98_invariants_at_every_depth() -> None:
+    """Existence rule, ordering, counts, summaries, pending markers and
+    dates: every node a limited tree DOES send is field-for-field the
+    node the full tree sends (the depth budget removes depth only — it
+    never changes what a shipped level says), and the top level is
+    identical at every depth."""
+    (full,) = build_kb_tree(["S"], CROSS_ROWS, CROSS_SUMMARIES)
+    full_map = {
+        n.path: (n.documents, n.summary_pending, n.updated_at, n.summary)
+        for n in _all_folder_nodes(full)
+    }
+    for depth in (1, 2, 3):
+        (limited,) = build_kb_tree(["S"], CROSS_ROWS, CROSS_SUMMARIES, depth=depth)
+        # Same level-1 rows, same order, at every depth.
+        assert [c.path for c in limited.children] == [c.path for c in full.children]
+        for node in _all_folder_nodes(limited):
+            assert (
+                node.documents,
+                node.summary_pending,
+                node.updated_at,
+                node.summary,
+            ) == full_map[node.path]
+            # Existence rule: a node is present ⟺ a catalogue prefix.
+            assert node.path in full_map
+        # Every truncated node is exactly the truncation contract.
+        for node in _all_folder_nodes(limited):
+            if node.truncated:
+                assert node.children == []
+            else:
+                assert node.model_dump().get("truncated") is None
+
+
+def test_truncated_nodes_keep_the_image_affordance_at_their_level() -> None:
+    """Phase 122's affordance is row-driven, not depth-driven: a DIRECT
+    image file of the source root still carries the thumbnail keys at
+    ``depth=1``; the truncated folder below simply does not list its
+    own rows (they arrive with the drill)."""
+    rows = [
+        ("S", "pic.png", "pic", 1, T0, C0),
+        ("S", "d/x.png", "x", 1, T0, C0),
+    ]
+    images = {
+        ("S", "pic.png"): (DOC_ID, "A red square."),
+        ("S", "d/x.png"): (DOC_ID, "A blue circle."),
+    }
+    (source,) = build_kb_tree(["S"], rows, {}, images, depth=1)
+    (pic,) = _file_nodes(source)
+    assert pic.is_image is True
+    assert pic.model_dump()["image_url"] == f"/api/documents/{DOC_ID}/image"
+    assert pic.model_dump()["summary"] == "A red square."
+    (d,) = _folder_nodes(source)
+    assert d.truncated is True
+    assert d.children == []
+
+
+# ---------------------------------------------------------------------------
+# _kb_totals (phase 167, task 02) — the stat-card totals the endpoint ships
+# beside any depth of tree (no extra query, empty-KB honest).
+# ---------------------------------------------------------------------------
+
+
+def test_kb_totals_math_over_the_catalogue_rows() -> None:
+    """documents = the row count, chunks = the sum, last_indexed = the
+    MAX indexed stamp (lexicographic over one ISO shape = chronological
+    — the same rule as the D9 max)."""
+    totals = _kb_totals(DEPTH_ROWS)
+    assert totals.documents == 4
+    assert totals.chunks == 10  # 1 + 2 + 3 + 4
+    assert totals.last_indexed == T2  # the newest indexed_at
+
+
+def test_kb_totals_empty_kb_is_zero_and_none() -> None:
+    """The empty KB: 0 documents, 0 chunks, ``last_indexed is None`` —
+    the stat cards render the designed empty state, never a crash."""
+    totals = _kb_totals([])
+    assert totals == KbTreeTotals(documents=0, chunks=0, last_indexed=None)
+    assert totals.model_dump() == {
+        "documents": 0,
+        "chunks": 0,
+        "last_indexed": None,
+    }
+
+
+def test_kb_tree_totals_default_and_floor_validation() -> None:
+    """``KbTree.totals`` is an additive top-level key with an honest
+    empty default (an older call shape still serializes), and the
+    ge=0 bounds refuse negative totals loudly (the endpoint computes
+    them — a negative is a bug, never data)."""
+    assert KbTree(sources=[]).model_dump()["totals"] == {
+        "documents": 0,
+        "chunks": 0,
+        "last_indexed": None,
+    }
+    with pytest.raises(ValidationError):
+        KbTreeTotals(documents=-1, chunks=0)
+    with pytest.raises(ValidationError):
+        KbTreeTotals(documents=0, chunks=-1)

tests/unit/test_kb_tree_ui.py

@@ -5,10 +5,13 @@ The RAG (Knowledge base) view lists the catalog the way the agent's
 top, then per level the subfolders (recursive count + the STORED
 description) and the level's files (the ``#docs-table`` — phase 106,
 task 08, D8, added the Created column BEFORE Indexed, making it the
-6-column contract). The view's catalog load is now
-``loadTree()``: ONE fetch of ``GET /api/docs/tree`` (the full recursive
-tree in a single payload — task 02), then client-side drill navigation
-(no per-level fetch, no URL change).
+6-column contract). The view's catalog load is ``loadTree()``: ONE
+fetch of ``GET /api/docs/tree?depth=1`` (sources + one level + the
+server's ``totals``) plus ONE ``GET /api/docs/tree/node`` fetch per
+not-yet-loaded level — phase 167 (task 04, LOCKED A3 — the TODO L5
+2 MB payload fix) SUPERSEDED the phase-97 "one full-tree fetch, drill
+purely client-side" contract; the drill UX (breadcrumb, no URL change)
+is unchanged.
 
 This module pins the source-level contract (the house pattern of
 ``tests/unit/test_sync_button.py`` — read the frontend files as text,
@@ -24,8 +27,14 @@ suite, task 06):
   ``#stat-cards`` and BEFORE the file table (now the 6-column
   Source | Path | Title | Chunks | Created | Indexed head — phase 106
   (task 08, D8) added the Created column BEFORE Indexed);
-* the exact ``/api/docs/tree`` fetch (and the flat ``/api/docs`` fetch
-  is gone from the view module);
+* the exact ``/api/docs/tree?depth=1`` load fetch + the single
+  ``/api/docs/tree/node`` drill fetch (the flat ``/api/docs`` fetch is
+  gone from the view module), the level cache (declared once, cleared
+  by every load, consulted by ``needsLevelFetch``, written only after a
+  200), the §7.4 drill states (in-flight ``aria-busy``/``aria-disabled``
+  + one ``role="status"`` row, ``role="alert"`` failure with a real
+  Retry ``<button>``, the node 404 → top-level reset, the race token
+  extended to the drill), and the ``totals``-first stat cards;
 * the drill state (``current`` / ``kbTree``), the state transitions
   (source row click, folder row click, breadcrumb up, top reset), and
   the aria-current last segment;
@@ -239,18 +248,25 @@ def test_rag_view_ships_the_tree_skeleton_in_order() -> None:
         assert f">{column}</th>" in doc_head.group(1), f"#docs-table head must keep {column!r}"
 
 
-# ---------- the tree load: the ONE fetch + the race token ----------
+# ---------- the tree load: the depth-1 fetch, the node fetch, the token ----------
 
 
 def test_sources_js_fetches_the_tree_endpoint_only() -> None:
-    """The catalog load is `loadTree()`: ONE fetch of the tree endpoint.
-    The flat `GET /api/docs` fetch is GONE from the view module (the
-    endpoint itself stays — the API surface is untouched, task 02)."""
+    """Phase 167 (task 04, A3 — supersedes the phase-97 "ONE full-tree
+    fetch, zero per-level fetches" pin): the catalog load is ONE
+    `fetch("/api/docs/tree?depth=1")` and the drill has exactly ONE
+    node-fetch call site (`fetch("/api/docs/tree/node?…`, reached only
+    through fetchLevel). The flat `GET /api/docs` fetch stays GONE from
+    the view module (the endpoint itself stays — task 02)."""
     js = _js()
-    assert 'fetch("/api/docs/tree")' in js, "the load must fetch /api/docs/tree"
-    assert js.count('fetch("/api/docs/tree")') == 1, (
-        "the tree is the view's SINGLE catalog fetch (zero per-level fetches)"
+    code = _code(js)
+    assert code.count('fetch("/api/docs/tree?depth=1")') == 1, (
+        "the load is ONE depth=1 request (the 2 MB first-payload fix)"
     )
+    assert code.count('fetch("/api/docs/tree/node?') == 1, (
+        "exactly ONE per-level fetch call site — every drill routes through it"
+    )
+    assert 'fetch("/api/docs/tree")' not in js, "the depth-less load URL is gone"
     assert 'fetch("/api/docs")' not in js, "the flat /api/docs fetch must be gone"
 
 
@@ -301,8 +317,9 @@ def test_source_row_click_drills_into_the_source_root() -> None:
     body = js[js.find("function makeSourceRow(") : js.find("function makeFolderRow(")]
     assert 'link.className = "folder-link"' in body, "the source row uses the row drill link"
     assert "link.textContent = s.name" in body
-    assert "goTo({ source: s.name, folder: \"\" })" in body, (
-        "a source row drills to the source ROOT (folder \"\")"
+    assert "goTo({ source: s.name, folder: \"\" }, link)" in body, (
+        "a source row drills to the source ROOT (folder \"\") — the link "
+        "rides the phase-167 in-flight state"
     )
     assert "countTd.textContent = String(s.documents)" in body, "the recursive count"
     assert 'makeDescCell(s, s.name, "", s.name)' in body, (
@@ -326,8 +343,9 @@ def test_folder_row_click_drills_into_the_folder() -> None:
         "the label is the last segment (the breadcrumb language)"
     )
     assert "link.title = current.source + \"/\" + f.path" in body, "full path on hover"
-    assert "goTo({ source: current.source, folder: f.path })" in body, (
-        "a folder row drills to that folder"
+    assert "goTo({ source: current.source, folder: f.path }, link)" in body, (
+        "a folder row drills to that folder — the link rides the "
+        "phase-167 in-flight state"
     )
     assert "countTd.textContent = String(f.documents)" in body
     assert (
@@ -370,19 +388,24 @@ def test_breadcrumb_links_go_up_and_the_top_reset() -> None:
 
 def test_drill_navigation_pushes_state_only_entries() -> None:
     """Phase 99 (task 02, D2): every user-initiated drill is a history
-    entry — ``goTo`` is the push variant of ``applyTarget``: when the
-    target DIFFERS from the current level (a re-click of the current
-    row/segment pushes NO duplicate entry), it FIRST records a
-    STATE-ONLY entry — ``history.pushState({ view: "rag", kb: target },
-    "")`` — the second arg is ``""`` and there is no third (the URL
-    stays the shell's pathname: no new route, the phase-76 deep-link
-    surface is untouched) — then sets ``current`` + re-renders. The
-    drill still never fetches (the tree is in memory) and the module
-    never ``replaceState``s."""
+    entry — ``applyTarget(target, push)``: when the target DIFFERS
+    from the current level (a re-click of the current row/segment
+    pushes NO duplicate entry), it FIRST records a STATE-ONLY entry —
+    ``history.pushState({ view: "rag", kb: target }, "")`` — the second
+    arg is ``""`` and there is no third (the URL stays the shell's
+    pathname: no new route, the phase-76 deep-link surface is
+    untouched) — then sets ``current`` + re-renders. The module never
+    ``replaceState``s. Phase 167 (task 04, A3 — supersedes the "the
+    drill never fetches" pin): ``goTo`` hands the target to
+    ``enterFolder`` with ``push=true``, so the push lands only when the
+    level is IN HAND (a failed drill records nothing) — ``applyTarget``
+    and ``goTo`` themselves stay fetch-free; the drill's only fetch
+    lives in ``fetchLevel`` (pinned below)."""
     js = _js()
-    start = js.find("function applyTarget(")
-    assert start != -1, "applyTarget must exist (the two-step drill)"
-    body = js[start : js.find("function makeSourceRow(")]
+    assert js.find("function applyTarget(") != -1, (
+        "applyTarget must exist (the two-step drill)"
+    )
+    body = _fn(js, "applyTarget")
     gate_i = body.find(
         "if (push && (target.source !== current.source || target.folder !== current.folder)) {"
     )
@@ -394,14 +417,191 @@ def test_drill_navigation_pushes_state_only_entries() -> None:
     )
     # STATE-ONLY: exactly ONE pushState call site — the "" second arg,
     # no third (no URL change).
-    assert _code(body).count("pushState") == 1, (
+    assert _code(js).count("pushState") == 1, (
         "ONE pushState call site — the state-only drill entry"
     )
     assert "replaceState" not in _code(js), "no replaceState anywhere in the module"
     # goTo is the push variant — every drill call site keeps calling it.
-    go = body[body.find("function goTo(") :]
-    assert "applyTarget(target, true)" in go, "goTo pushes (the user-initiated drill)"
-    assert "fetch(" not in body, "the drill never fetches (client-side only)"
+    go = _fn(js, "goTo")
+    assert "enterFolder(target, link, true)" in go, (
+        "goTo pushes (the user-initiated drill, phase 167 via enterFolder)"
+    )
+    assert "fetch(" not in _code(body), "applyTarget stays a pure state transition"
+    assert "fetch(" not in _code(go), "goTo delegates the fetch to enterFolder only"
+
+
+# ---------- phase 167 (task 04): the lazy drill (cache, states, reset) ----------
+
+
+def test_level_cache_declared_cleared_and_consulted() -> None:
+    """The level cache (phase 167, D4): ONE module-scoped Map keyed
+    `source + "/" + folder`; EVERY successful load clears it (a refresh
+    invalidates every drilled level — the sync/upload/re-show call
+    sites keep working unchanged) BEFORE the never-stale reset (the
+    reset re-derives known-ness from the NEW tree); `needsLevelFetch`
+    consults it (the second visit to a folder is free — zero network);
+    the write sits behind the ok gate (only a 200 caches, never the
+    404/failure arms) and follows the merge (children ride the node
+    before the level is marked loaded)."""
+    js = _js()
+    assert js.count("const levelCache = new Map();") == 1, (
+        "ONE module-scoped level cache"
+    )
+    load = _fn(js, "loadTree")
+    clear_i = load.find("levelCache.clear();")
+    reset_i = load.find("resetVanishedLocation();")
+    assert 0 < clear_i < reset_i, "clear the cache BEFORE the reset (new tree)"
+    needs = _fn(js, "needsLevelFetch")
+    assert "node.truncated === true" in needs, "only a TRUNCATED node needs a fetch"
+    assert "levelCache.has(" in needs, "a cached level never refetches"
+    fetch = _fn(js, "fetchLevel")
+    merge_i = fetch.find("Object.assign(targetNode, merged);")
+    set_i = fetch.find('levelCache.set(target.source + "/" + target.folder, true);')
+    ok_gate = fetch.find("if (response === null || !response.ok) {")
+    vanish_i = fetch.find("if (vanished) {")
+    assert -1 < vanish_i < ok_gate < merge_i < set_i, (
+        "404 arm → ok gate → merge → cache: only a 200 caches, after the merge"
+    )
+
+
+def test_drill_race_token_extends_to_the_node_fetch() -> None:
+    """Phase 79 carried over (task 04): fetchLevel captures the
+    loadSeq token (plus a navSeq bumped by every applyTarget) and a
+    response may only paint while NEITHER a newer load NOR a newer
+    navigation happened — pinned twice (after the fetch, after the
+    json read). A new loadTree invalidates the in-flight drill the
+    moment it starts, and rehydrates the user's own truncated level
+    through the node route BEFORE it paints (a refresh never shows a
+    half-empty level)."""
+    js = _js()
+    fetch = _fn(js, "fetchLevel")
+    assert "const my = loadSeq;" in fetch, "the phase-79 token extends to the drill"
+    assert "const nav = navSeq;" in fetch, "a navigation invalidates the drill too"
+    assert fetch.count("if (my !== loadSeq || nav !== navSeq) return false;") == 2, (
+        "a stale drill response never paints (checked after the fetch and the json)"
+    )
+    load = _fn(js, "loadTree")
+    assert load.find("drillPending = null;") < load.find("await fetch"), (
+        "a new load invalidates the in-flight drill immediately"
+    )
+    assert "if (needsLevelFetch(currentLevelNode(), current.source)) {" in load, (
+        "the user's own truncated level is REHYDRATED through the node route"
+    )
+    assert "let navSeq = 0;" in js
+    # …and the ADOPTED navigation (popstate) rehydrates the same way:
+    # a forward/back entry can point at a level the client has not
+    # fetched since the last load (ensureCurrentLevel, after the adopt).
+    pop = js[js.find('window.addEventListener("popstate"') :]
+    pop = pop[: pop.find("});")]
+    adopt_i = pop.find("applyTarget(kb, false)")
+    ensure_i = pop.find("ensureCurrentLevel();")
+    assert -1 < adopt_i < ensure_i, "adopt the entry, THEN rehydrate its level"
+    ensure = _fn(js, "ensureCurrentLevel")
+    assert "if (drillPending) return;" in ensure
+    assert "if (needsLevelFetch(currentLevelNode(), current.source)) {" in ensure
+
+
+def test_drill_pending_guards_and_aria_inflight_state() -> None:
+    """§7.4 in-flight: enterFolder early-returns while a drill is
+    pending (a double click cannot start a second fetch), sets the
+    drillPending state BEFORE awaiting fetchLevel, and applies the
+    target only once the level is in hand. fetchLevel marks the
+    clicked link aria-busy + aria-disabled BEFORE the fetch and clears
+    both on every settle path (the control never stays disabled — the
+    failure path re-enables)."""
+    js = _js()
+    enter = _fn(js, "enterFolder")
+    guard_i = enter.find("if (drillPending) return;")
+    pending_i = enter.find(
+        "drillPending = { source: target.source, folder: target.folder };"
+    )
+    fetch_i = enter.find("await fetchLevel(target, node, link)")
+    apply_i = enter.find("applyTarget(target, push);")
+    assert -1 < guard_i < pending_i < fetch_i < apply_i, (
+        "guard → in-flight state → fetch → apply (only when the level is in hand)"
+    )
+    fetch = _fn(js, "fetchLevel")
+    busy_i = fetch.find('link.setAttribute("aria-busy", "true")')
+    disabled_i = fetch.find('link.setAttribute("aria-disabled", "true")')
+    call_i = fetch.find("response = await fetch(")
+    clear_busy = fetch.find('link.removeAttribute("aria-busy")')
+    clear_disabled = fetch.find('link.removeAttribute("aria-disabled")')
+    assert -1 < busy_i < call_i and -1 < disabled_i < call_i, (
+        "the busy state rides the row link BEFORE the fetch"
+    )
+    assert call_i < clear_busy and call_i < clear_disabled, (
+        "every settle path re-enables the control (§7.4)"
+    )
+
+
+def test_drill_inflight_status_and_failure_alert_rows() -> None:
+    """The ONE in-flight row of the folders panel (phase 167, D4): the
+    loading row is a role="status" "Loading <folder>…" line while the
+    level the user is LEAVING stays rendered; the failure row is a
+    role="alert" "Couldn't load <folder> — " line with a REAL
+    <button type="button">Retry</button> whose click re-runs the SAME
+    enterFolder fetch. Neither builder calls renderLevel (the current
+    level never blanks); both clear the previous drill row first (ONE
+    row at a time) and textContent only (the house rule)."""
+    js = _js()
+    loading = _fn(js, "showDrillLoading")
+    assert "clearDrillRow();" in loading
+    assert 'note.setAttribute("role", "status");' in loading
+    assert 'note.textContent = "Loading " + name + "…"' in loading
+    assert 'row.className = "kb-level-loading"' in loading
+    assert "foldersTbody.appendChild(row)" in loading, "the row lives in the folders panel"
+    assert "renderLevel" not in _code(loading), "the leaving level stays rendered"
+    fail = _fn(js, "showDrillFailure")
+    assert "clearDrillRow();" in fail
+    assert 'note.setAttribute("role", "alert");' in fail
+    assert '"Couldn\'t load " + target.folder.split("/").pop() + " — "' in fail
+    assert 'retry.type = "button";' in fail, "Retry is a REAL button"
+    assert 'retry.textContent = "Retry";' in fail
+    assert (
+        "retry.addEventListener(\"click\", () => enterFolder(target, link, true));" in fail
+    ), "Retry re-runs the SAME drill fetch"
+    assert "renderLevel" not in _code(fail), "the failure keeps the current level rendered"
+    clear = _fn(js, "clearDrillRow")
+    assert "drillRowEl.remove();" in clear and "drillRowEl = null;" in clear
+
+
+def test_vanished_node_resets_to_the_top_level() -> None:
+    """The node route's 404 IS the "the folder vanished" signal (task
+    04, D4): the drill resets `current` to the TOP level and re-renders
+    (no stale breadcrumb), caches NOTHING for the failed level, and
+    never applies the vanished target (enterFolder returns on false).
+    A network/non-OK failure takes the alert arm instead — the current
+    level stays rendered (pinned above)."""
+    js = _js()
+    fetch = _fn(js, "fetchLevel")
+    assert "response.status === 404" in fetch, "the 404 is detected"
+    vanish = fetch[fetch.find("if (vanished) {") : fetch.find(
+        "if (response === null || !response.ok) {"
+    )]
+    assert "current = { source: null, folder: null };" in vanish, (
+        "the 404 resets to the top level (the phase-97 never-stale contract)"
+    )
+    assert "renderLevel();" in vanish, "the reset renders — no stale breadcrumb"
+    assert "return false;" in vanish, "the vanished target is never applied"
+    assert "levelCache" not in vanish, "the failing level is NOT cached"
+    enter = _fn(js, "enterFolder")
+    assert "if (!loaded) return;" in enter, "enterFolder applies only on a real merge"
+
+
+def test_module_docstring_documents_the_lazy_drill() -> None:
+    """The house per-phase module-note convention: the Phase 167
+    (task 04) section records the depth-1 load, the per-level node
+    fetch + cache, the §7.4 drill states, the extended race token, the
+    404 reset and the totals-first stat cards — citing TODO L5."""
+    js = _js()
+    header = js[: js.find("import { fetchIsAdmin }")]
+    assert "Phase 167 (task 04, D4/A3" in header
+    assert "TODO L5" in header, "the 2 MB payload defect is documented"
+    assert "LAZY DRILL" in header
+    assert "/api/docs/tree?depth=1" in header
+    assert "levelCache" in header
+    assert "loadSeq token EXTENDS" in header, "the race-token narrative stays accurate"
+    assert "folderKnownIn" in header, "the existence-rule change is documented"
 
 
 # ---------- the level rendering ----------
@@ -507,12 +707,39 @@ def test_render_clears_both_row_containers_before_filling() -> None:
         assert clear_docs < body.find(append), f"{append} must come after the clears"
 
 
-# ---------- the stat cards: the whole-tree walk ----------
+# ---------- the stat cards: server totals, walk as the fallback ----------
+
+
+def test_stat_cards_prefer_the_server_totals() -> None:
+    """Phase 167 (task 04, D2/D4): a depth-limited tree CANNOT be
+    summed client-side, so treeStats() reads the server's `totals`
+    (documents / chunks / last_indexed) FIRST; the phase-97 recursive
+    walk stays only as the defensive no-`totals` fallback (a tree
+    served by a pre-phase process — the no-crash path). The stat-card
+    rendering (String(st.docs)/String(st.totalChunks)/fmtDate(st.last)
+    with the "–" null idiom) is UNCHANGED."""
+    js = _js()
+    body = _fn(js, "treeStats")
+    totals_i = body.find("const totals = kbTree.totals;")
+    guard_i = body.find('if (totals && typeof totals === "object") {')
+    docs_i = body.find("docs: totals.documents,")
+    chunks_i = body.find("totalChunks: totals.chunks,")
+    last_i = body.find('last: totals.last_indexed || "",')
+    walk_i = body.find("const walk = (node) => {")
+    assert -1 < totals_i < guard_i < docs_i < chunks_i < last_i < walk_i, (
+        "totals first, the recursive walk ONLY as the no-totals fallback"
+    )
+    render = js[js.find("function renderLevel(") : js.find("function renderEmpty(")]
+    assert "const st = treeStats();" in render
+    assert 'st.last ? fmtDate(st.last) : "–"' in render, (
+        "the exact fmtDate formatting call is unchanged at every level"
+    )
 
 
 def test_stat_cards_walk_the_whole_tree() -> None:
-    """The KB-wide stat cards walk the WHOLE tree (every file node at
-    any depth): document count, chunks sum, max indexed_at (fmtDate
+    """The no-`totals` fallback walk (phase 97, kept defensive since
+    phase 167): walk the WHOLE held tree (every file node at any
+    depth): document count, chunks sum, max indexed_at (fmtDate
     reuse) — the values identical to the former flat walk. A
     0-document registered source contributes nothing (and still lists
     its row — the ls invariant)."""
@@ -577,21 +804,30 @@ def test_module_docstring_documents_the_semantic_change() -> None:
 
 def test_reset_to_top_when_the_location_vanished() -> None:
     """PLAN §7.4: after a re-fetch, if the current source is no longer
-    in the tree (unregistered/pruned) OR the current folder no longer
-    exists under it, `current` RESETS to the top level BEFORE rendering
-    — no stale breadcrumb, no stale block. The folder existence check
-    mirrors the phase-94 rule: some indexed path starts with
-    `folder + "/"` (the source root — \"\" — always exists)."""
+    in the tree (unregistered/pruned) OR the current folder is no
+    longer KNOWN under it, `current` RESETS to the top level BEFORE
+    rendering — no stale breadcrumb, no stale block. Phase 167 (task
+    04, D4): the phase-94 full-tree path-prefix walk becomes
+    folderKnownIn — a folder is known iff its WHOLE ancestor chain
+    exists in the levels the client HOLDS (the depth-1 tree + the
+    drilled-and-merged levels; a broken chain — gone OR simply not
+    loaded that deep — resets honestly; the node route's 404 is the
+    drill path's own vanished-signal, pinned separately)."""
     js = _js()
-    body = js[js.find("function resetVanishedLocation()") : js.find("function folderExistsIn(")]
+    body = _fn(js, "resetVanishedLocation")
     assert "if (current.source === null) return;" in body, "the top level never vanishes"
     assert "kbTree.sources.find((s) => s.name === current.source)" in body
     assert body.count("current = { source: null, folder: null };") == 2, (
         "both vanishing arms reset to the top level"
     )
-    exists = js[js.find("function folderExistsIn(") : js.find("function currentLevelNode(")]
-    assert 'if (folderPath === "") return true;' in exists, "the source root always exists"
-    assert "p.startsWith(folderPath + \"/\")" in exists, "the phase-94 existence rule"
+    assert "folderKnownIn(src, current.folder)" in body, (
+        "the reset consults KNOWN-ness (the held levels), never a full-tree walk"
+    )
+    known = _fn(js, "folderKnownIn")
+    assert 'if (folderPath === "") return true;' in known, "the source root always exists"
+    assert "if (!next) return false;" in known, "a broken ancestor chain is not known"
+    assert "collect" not in _code(known), "the recursive path-collection walk is gone"
+    assert "folderExistsIn" not in js, "the old full-tree existence walker is gone"
     # The reset runs on the success path of the load, before the render
     # (pinned in the load-order test — here: it is defined and called).
     assert "resetVanishedLocation();" in js[js.find("async function loadTree()"):]
@@ -736,6 +972,7 @@ def test_styles_carry_the_tree_classes_with_no_new_hue() -> None:
         ".kb-crumb-current",
         ".kb-crumb-sep",
         ".kb-level",
+        ".kb-level-loading",
         ".kb-folders-table",
         ".folder-link",
     ):

tests/unit/test_response_budget.py

@@ -0,0 +1,702 @@
+"""Unit tests: ``app.core.response_budget`` — the byte-budget log line
+(phase 167, task 05, D5, LOCKED A5).
+
+Pins the contract of the line, the level cut, and the pass-through rules
+on a tiny ASGI app (the raw-ASGI interleaving harness of
+``test_compression.py`` where ordering matters, ``TestClient`` where the
+composed stack matters):
+
+* the EXACT shape ``api method=… path=… status=… bytes=… enc=…
+  duration_ms=…`` for a large 200 at INFO and a small 200 at DEBUG;
+* a 304 logs ``bytes=0`` (the revalidation win stays visible);
+* a non-``/api/`` path and a non-HTTP scope log NOTHING;
+* a stream logs ONE line with ``stream=1``, ``bytes`` = the sum of the
+  chunks, emitted when the stream COMPLETES (never mid-flight) — and the
+  frames are never buffered;
+* ``enc`` reads the WIRE encoding (``gzip`` through the real compression
+  layer, ``identity`` for a client that does not accept it) while
+  ``bytes`` stays the size the ROUTE produced;
+* an app exception is logged (finally-style, bytes as counted) and
+  re-raised UNCHANGED;
+* the config knob: ``0`` logs every ``/api/*`` response at INFO, a
+  negative value refuses startup naming the field, and the middleware sits
+  INSIDE ``SelectiveGZipMiddleware`` so ``bytes=`` is the route's size.
+"""
+from __future__ import annotations
+
+import asyncio
+import gzip
+import json
+import logging
+import re
+from typing import Any
+
+import pytest
+from fastapi.testclient import TestClient
+from pydantic import ValidationError
+from starlette.applications import Starlette
+from starlette.responses import Response
+from starlette.routing import Route
+from starlette.types import Message, Receive, Scope, Send
+
+import app.core.response_budget as budget_mod
+import app.main as main_mod
+from app.config import Settings
+from app.core.compression import SelectiveGZipMiddleware
+from app.core.response_budget import (
+    API_PATH_PREFIX,
+    UNKNOWN_STATUS,
+    ResponseBudgetMiddleware,
+)
+
+#: One line, fully anchored — the shape README documents and the
+#: ``grep ' :: api '`` workflow parses.
+LINE_RE = re.compile(
+    r"^api method=(?P<method>[A-Z]+) path=(?P<path>\S+) status=(?P<status>\d+) "
+    r"bytes=(?P<bytes>\d+) enc=(?P<enc>\S+) duration_ms=(?P<ms>\d+)$"
+)
+
+#: ~28 KB of tree-shaped JSON.
+_BODY = json.dumps(
+    {
+        "sources": [
+            {
+                "name": "src",
+                "documents": i,
+                "children": [{"path": f"d/{i}/{n}.md"} for n in range(20)],
+            }
+            for i in range(200)
+        ]
+    }
+).encode()
+
+_SMALL_BODY = b'{"ok":true}'
+
+#: Four SSE-class frames, each comfortably above the gzip floor.
+_FRAMES = [f"data: {'x' * 500}\n\n".encode() for _ in range(4)]
+
+_LOGGER_NAME = "app.response_budget"
+
+
+def _settings(**kwargs: Any) -> Settings:
+    """Build Settings without reading a .env file (the test_config pattern)."""
+    kwargs.setdefault("_env_file", None)
+    return Settings(**kwargs)  # pyright: ignore[reportCallIssue]  (kwarg exists at runtime)
+
+
+def _api_lines(caplog: pytest.LogCaptureFixture) -> list[tuple[int, str]]:
+    """``(levelno, message)`` for this module's logger only, in order."""
+    return [
+        (record.levelno, record.getMessage())
+        for record in caplog.records
+        if record.name == _LOGGER_NAME
+    ]
+
+
+class _Counter(logging.Handler):
+    """Counts emitted lines live, so an app can observe "has the line been
+    written yet?" from INSIDE the response — the ordering proof."""
+
+    def __init__(self) -> None:
+        super().__init__()
+        self.count = 0
+
+    def emit(self, record: logging.LogRecord) -> None:
+        self.count += 1
+
+
+def _http_scope(path: str, method: str = "GET") -> Scope:
+    return {"type": "http", "method": method, "path": path, "headers": []}
+
+
+async def _receive() -> Message:
+    return {"type": "http.disconnect"}
+
+
+async def _send(message: Message) -> None:
+    return None
+
+
+def _raw_json_app(body: bytes, *, status: int = 200, media_type: str = "application/json") -> Any:
+    """The smallest ASGI app that answers one buffered body — the raw-ASGI
+    level is where the WIRE bytes (post-compression) are observable."""
+
+    async def app(scope: Scope, receive: Receive, send: Send) -> None:
+        await send(
+            {
+                "type": "http.response.start",
+                "status": status,
+                "headers": [(b"content-type", media_type.encode())],
+            }
+        )
+        await send({"type": "http.response.body", "body": body, "more_body": False})
+
+    return app
+
+
+def _raw_harness(middleware_app: Any, *, path: str, accept: str) -> list[Message]:
+    """Drive a middleware stack at the raw ASGI level; return the exact
+    messages that reached the client's ``send``."""
+    sent: list[Message] = []
+
+    async def recording_send(message: Message) -> None:
+        sent.append(message)
+
+    asyncio.run(
+        middleware_app(
+            {
+                "type": "http",
+                "method": "GET",
+                "path": path,
+                "headers": [(b"accept-encoding", accept.encode())],
+            },
+            _receive,
+            recording_send,
+        )
+    )
+    return sent
+
+
+def _inner_app(
+    *,
+    body: bytes = _BODY,
+    status: int = 200,
+    path: str = "/api/docs/tree",
+    media_type: str = "application/json",
+) -> Starlette:
+    async def respond(request: Any) -> Response:
+        return Response(body, status_code=status, media_type=media_type)
+
+    async def boom(request: Any) -> Response:
+        raise RuntimeError("kaboom")
+
+    return Starlette(
+        routes=[
+            Route(path, respond, methods=["GET", "POST"]),
+            Route("/api/boom", boom, methods=["GET"]),
+            Route("/sources.html", respond, methods=["GET"]),
+        ]
+    )
+
+
+def _client(app: Any, *, threshold: int = 1024) -> TestClient:
+    return TestClient(ResponseBudgetMiddleware(app, threshold=threshold))
+
+
+# ---------------------------------------------------------------------------
+# The line's exact shape, and the level the budget decides
+# ---------------------------------------------------------------------------
+
+
+def test_large_response_logs_one_info_line_in_the_exact_shape(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        client = _client(_inner_app(), threshold=1024)
+        assert client.get("/api/docs/tree").status_code == 200
+
+    lines = _api_lines(caplog)
+    assert len(lines) == 1
+    level, message = lines[0]
+    assert level == logging.INFO
+    match = LINE_RE.match(message)
+    assert match is not None, message
+    assert match.group("method") == "GET"
+    assert match.group("path") == "/api/docs/tree"
+    assert match.group("status") == "200"
+    assert int(match.group("bytes")) == len(_BODY)
+    assert match.group("enc") == "identity"
+    assert int(match.group("ms")) >= 0
+
+
+def test_small_response_logs_the_same_line_at_debug(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        client = _client(_inner_app(body=_SMALL_BODY), threshold=1024)
+        assert client.get("/api/docs/tree").status_code == 200
+
+    lines = _api_lines(caplog)
+    assert len(lines) == 1
+    level, message = lines[0]
+    assert level == logging.DEBUG
+    assert f"bytes={len(_SMALL_BODY)}" in message
+
+
+def test_budget_is_the_inclusive_cut(caplog: pytest.LogCaptureFixture) -> None:
+    """``bytes >= budget`` is INFO, one byte less is DEBUG — the boundary the
+    README and ``.env.example`` state."""
+
+    async def exactly(request: Any) -> Response:
+        return Response(b"x" * 100)
+
+    async def one_less(request: Any) -> Response:
+        return Response(b"x" * 99)
+
+    app = Starlette(
+        routes=[
+            Route("/api/a", exactly, methods=["GET"]),
+            Route("/api/b", one_less, methods=["GET"]),
+        ]
+    )
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        client = _client(app, threshold=100)
+        client.get("/api/a")
+        client.get("/api/b")
+
+    assert [level for level, _ in _api_lines(caplog)] == [logging.INFO, logging.DEBUG]
+
+
+def test_threshold_defaults_to_the_settings_value(
+    monkeypatch: pytest.MonkeyPatch, caplog: pytest.LogCaptureFixture
+) -> None:
+    """No ``threshold`` argument — the way ``create_app`` wires it — so the
+    live ``log_large_response_bytes`` decides per response."""
+    monkeypatch.setattr(
+        budget_mod, "get_settings", lambda: _settings(log_large_response_bytes=5)
+    )
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        TestClient(ResponseBudgetMiddleware(_inner_app(body=_SMALL_BODY))).get(
+            "/api/docs/tree"
+        )
+
+    assert [level for level, _ in _api_lines(caplog)] == [logging.INFO]
+
+
+def test_zero_budget_logs_every_api_response_at_info(
+    monkeypatch: pytest.MonkeyPatch, caplog: pytest.LogCaptureFixture
+) -> None:
+    """``BOR_LOG_LARGE_RESPONSE_BYTES=0`` is the loud mode: even an 11-byte
+    body is INFO (the operator's one-off investigation setting)."""
+    monkeypatch.setattr(
+        budget_mod, "get_settings", lambda: _settings(log_large_response_bytes=0)
+    )
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        TestClient(ResponseBudgetMiddleware(_inner_app(body=_SMALL_BODY))).get(
+            "/api/docs/tree"
+        )
+
+    # 11 bytes, budget 0 -> INFO (the same body is DEBUG at any positive
+    # budget: see test_small_response_logs_the_same_line_at_debug).
+    assert [level for level, _ in _api_lines(caplog)] == [logging.INFO]
+
+
+# ---------------------------------------------------------------------------
+# Statuses: a 304 logs 0 bytes, an error is counted like any other response
+# ---------------------------------------------------------------------------
+
+
+def test_304_logs_zero_bytes(caplog: pytest.LogCaptureFixture) -> None:
+    """The revalidation win stays in the log — a bodiless 304 is a line with
+    ``bytes=0``, not a missing line."""
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        client = _client(_inner_app(body=b"", status=304), threshold=1024)
+        assert client.get("/api/docs/tree").status_code == 304
+
+    lines = _api_lines(caplog)
+    assert len(lines) == 1
+    match = LINE_RE.match(lines[0][1])
+    assert match is not None
+    assert match.group("status") == "304"
+    assert match.group("bytes") == "0"
+    assert lines[0][0] == logging.DEBUG
+
+
+def test_error_status_is_logged_like_any_other_response(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        client = _client(_inner_app(body=b'{"detail":"admin only"}', status=403))
+        assert client.get("/api/docs/tree").status_code == 403
+
+    match = LINE_RE.match(_api_lines(caplog)[0][1])
+    assert match is not None and match.group("status") == "403"
+
+
+# ---------------------------------------------------------------------------
+# What is NOT measured
+# ---------------------------------------------------------------------------
+
+
+def test_non_api_path_logs_nothing(caplog: pytest.LogCaptureFixture) -> None:
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        # budget 0: ANY /api line would show, so silence is the whole pin.
+        client = _client(_inner_app(), threshold=0)
+        assert client.get("/sources.html").status_code == 200
+
+    assert _api_lines(caplog) == []
+
+
+def test_non_http_scopes_pass_through_silently(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """``lifespan`` / ``websocket`` scopes reach the app untouched and write
+    no line — there is no response to measure."""
+    seen: list[str] = []
+
+    async def app(scope: Scope, receive: Receive, send: Send) -> None:
+        seen.append(scope["type"])
+
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        for scope_type in ("lifespan", "websocket"):
+            asyncio.run(
+                ResponseBudgetMiddleware(app, threshold=0)(
+                    {"type": scope_type, "path": "/api/x"}, _receive, _send
+                )
+            )
+
+    assert seen == ["lifespan", "websocket"]
+    assert _api_lines(caplog) == []
+
+
+# ---------------------------------------------------------------------------
+# The stream: one line, stream=1, counted total, emitted at completion
+# ---------------------------------------------------------------------------
+
+
+def test_stream_logs_once_at_completion_with_stream_flag_and_total_bytes(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """The SSE contract of the line: nothing while frames are in flight, one
+    ``stream=1`` line when the final chunk completes, ``bytes`` = the sum of
+    the frames (NOT one frame)."""
+    counter = _Counter()
+    logger = logging.getLogger(_LOGGER_NAME)
+    logger.addHandler(counter)
+    in_flight: list[int] = []
+
+    async def sse_app(scope: Scope, receive: Receive, send: Send) -> None:
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 200,
+                "headers": [(b"content-type", b"text/event-stream")],
+            }
+        )
+        for frame in _FRAMES:
+            await send(
+                {"type": "http.response.body", "body": frame, "more_body": True}
+            )
+            in_flight.append(counter.count)  # after each frame is delivered
+        await send({"type": "http.response.body", "body": b"", "more_body": False})
+        in_flight.append(counter.count)
+
+    try:
+        with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+            asyncio.run(
+                ResponseBudgetMiddleware(sse_app, threshold=1)(
+                    _http_scope("/api/chat", "POST"), _receive, _send
+                )
+            )
+    finally:
+        logger.removeHandler(counter)
+
+    # Every in-flight probe except the last saw zero lines; the completion
+    # is what emitted.
+    assert in_flight == [0] * len(_FRAMES) + [1]
+    lines = _api_lines(caplog)
+    assert len(lines) == 1
+    message = lines[0][1]
+    assert message.endswith(" stream=1")
+    match = LINE_RE.match(message.removesuffix(" stream=1"))
+    assert match is not None
+    assert match.group("method") == "POST"
+    assert match.group("path") == "/api/chat"
+    assert int(match.group("bytes")) == sum(len(frame) for frame in _FRAMES)
+
+
+def test_stream_frames_are_never_buffered() -> None:
+    """Zero-buffering proof (the interleaving idiom of
+    ``test_compression.py``): every frame reaches the client's ``send``
+    before the app produces the next one — a ``BaseHTTPMiddleware`` here
+    would emit all ``app:*`` events first."""
+    events: list[str] = []
+
+    async def sse_app(scope: Scope, receive: Receive, send: Send) -> None:
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 200,
+                "headers": [(b"content-type", b"text/event-stream")],
+            }
+        )
+        for i, frame in enumerate(_FRAMES):
+            events.append(f"app:frame-{i}")
+            await send(
+                {"type": "http.response.body", "body": frame, "more_body": True}
+            )
+        await send({"type": "http.response.body", "body": b"", "more_body": False})
+
+    async def recording_send(message: Message) -> None:
+        events.append(f"client:{message['type']}")
+
+    asyncio.run(
+        ResponseBudgetMiddleware(sse_app, threshold=0)(
+            _http_scope("/api/chat", "POST"), _receive, recording_send
+        )
+    )
+
+    expected = ["client:http.response.start"]
+    for i in range(len(_FRAMES)):
+        expected += [f"app:frame-{i}", "client:http.response.body"]
+    expected += ["client:http.response.body"]
+    assert events == expected
+
+
+# ---------------------------------------------------------------------------
+# enc= reads the wire, bytes= stays the route's size
+# ---------------------------------------------------------------------------
+
+
+def _stack(*, threshold: int = 1024) -> TestClient:
+    """response-budget INSIDE the selective-gzip layer — the order
+    ``create_app`` builds."""
+    return TestClient(
+        SelectiveGZipMiddleware(
+            ResponseBudgetMiddleware(_inner_app(), threshold=threshold),
+            minimum_size=860,
+        )
+    )
+
+
+def test_enc_gzip_is_reported_through_the_compression_layer(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """The two halves of the line, measured against the wire: the client
+    receives a gzipped body, the log says ``enc=gzip`` and still counts the
+    ROUTE's uncompressed bytes (the position inside the compressor is what
+    buys that)."""
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        sent = _raw_harness(
+            SelectiveGZipMiddleware(
+                ResponseBudgetMiddleware(_raw_json_app(_BODY), threshold=1024),
+                minimum_size=860,
+            ),
+            path="/api/docs/tree",
+            accept="gzip",
+        )
+
+    wire = b"".join(m.get("body", b"") for m in sent if m["type"] == "http.response.body")
+    assert gzip.decompress(wire) == _BODY  # the transfer really was gzip
+    match = LINE_RE.match(_api_lines(caplog)[-1][1])
+    assert match is not None
+    assert match.group("enc") == "gzip"
+    assert int(match.group("bytes")) == len(_BODY) > len(wire)
+
+
+def test_enc_identity_when_the_client_does_not_accept_gzip(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        response = _stack().get(
+            "/api/docs/tree", headers={"Accept-Encoding": "identity"}
+        )
+
+    assert "content-encoding" not in response.headers
+    match = LINE_RE.match(_api_lines(caplog)[-1][1])
+    assert match is not None and match.group("enc") == "identity"
+    assert int(match.group("bytes")) == len(_BODY)
+
+
+def test_enc_reports_a_pre_encoded_response(caplog: pytest.LogCaptureFixture) -> None:
+    """A route that encodes its own body (the fronting-proxy case) is
+    reported as such — the header on the wire is the answer."""
+
+    async def encoded(request: Any) -> Response:
+        return Response(
+            gzip.compress(_BODY),
+            media_type="application/json",
+            headers={"Content-Encoding": "gzip"},
+        )
+
+    app = Starlette(routes=[Route("/api/encoded", encoded, methods=["GET"])])
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        TestClient(ResponseBudgetMiddleware(app, threshold=1024)).get("/api/encoded")
+
+    match = LINE_RE.match(_api_lines(caplog)[-1][1])
+    assert match is not None and match.group("enc") == "gzip"
+
+
+# ---------------------------------------------------------------------------
+# The app raising: logged, and re-raised unchanged
+# ---------------------------------------------------------------------------
+
+
+def test_app_exception_logs_and_reraises_unchanged(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """An app that dies before it can answer still writes one line (nothing
+    was counted, status 500 — what the outer error layer will answer) and the
+    exception propagates UNCHANGED."""
+
+    async def dying_app(scope: Scope, receive: Receive, send: Send) -> None:
+        raise RuntimeError("kaboom")
+
+    with (
+        caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME),
+        pytest.raises(RuntimeError, match="kaboom"),
+    ):
+        asyncio.run(
+            ResponseBudgetMiddleware(dying_app, threshold=0)(
+                _http_scope("/api/docs/tree"), _receive, _send
+            )
+        )
+
+    lines = _api_lines(caplog)
+    assert len(lines) == 1
+    match = LINE_RE.match(lines[0][1])
+    assert match is not None
+    assert match.group("status") == str(UNKNOWN_STATUS)
+    assert match.group("bytes") == "0"
+
+
+def test_a_500_produced_by_the_framework_is_counted_like_a_response(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """Through a real app the unhandled exception becomes a 500 WITH a body
+    (Starlette's ``Internal Server Error`` text): the line then reports that
+    status and those bytes — one line either way, never zero."""
+    with (
+        caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME),
+        pytest.raises(RuntimeError, match="kaboom"),
+    ):
+        _client(_inner_app(), threshold=0).get("/api/boom")
+
+    lines = _api_lines(caplog)
+    assert len(lines) == 1
+    match = LINE_RE.match(lines[0][1])
+    assert match is not None
+    assert match.group("status") == "500"
+    assert int(match.group("bytes")) == len(b"Internal Server Error")
+
+
+def test_exception_mid_stream_logs_the_bytes_counted_so_far(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """A stream that dies after two frames still writes ONE line with the
+    bytes it actually got out (finally-style) — and the exception is not
+    swallowed."""
+
+    async def dying_stream_app(scope: Scope, receive: Receive, send: Send) -> None:
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 200,
+                "headers": [(b"content-type", b"text/event-stream")],
+            }
+        )
+        await send({"type": "http.response.body", "body": _FRAMES[0], "more_body": True})
+        await send({"type": "http.response.body", "body": _FRAMES[1], "more_body": True})
+        raise RuntimeError("stream died")
+
+    with (
+        caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME),
+        pytest.raises(RuntimeError, match="stream died"),
+    ):
+        asyncio.run(
+            ResponseBudgetMiddleware(dying_stream_app, threshold=0)(
+                _http_scope("/api/chat", "POST"), _receive, _send
+            )
+        )
+
+    lines = _api_lines(caplog)
+    assert len(lines) == 1
+    message = lines[0][1]
+    assert message.endswith(" stream=1")
+    assert f"bytes={len(_FRAMES[0]) + len(_FRAMES[1])}" in message
+
+
+def test_a_cancelled_stream_still_logs_one_line_with_what_got_out(
+    caplog: pytest.LogCaptureFixture,
+) -> None:
+    """The client-walked-away path (the live audit hit it: an SSE turn cut
+    mid-answer): the app unwinds with ``CancelledError`` and no completing
+    body, the ``finally`` writes ONE line with the bytes that actually got
+    out, and the cancellation propagates (the middleware never swallows it)."""
+
+    async def cancelled_stream_app(scope: Scope, receive: Receive, send: Send) -> None:
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 200,
+                "headers": [(b"content-type", b"text/event-stream")],
+            }
+        )
+        await send({"type": "http.response.body", "body": _FRAMES[0], "more_body": True})
+        raise asyncio.CancelledError  # what the server does on a client disconnect
+
+    with (
+        caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME),
+        pytest.raises(asyncio.CancelledError),
+    ):
+        asyncio.run(
+            ResponseBudgetMiddleware(cancelled_stream_app, threshold=0)(
+                _http_scope("/api/chat", "POST"), _receive, _send
+            )
+        )
+
+    lines = _api_lines(caplog)
+    assert len(lines) == 1
+    assert lines[0][1].endswith(" stream=1")
+    assert f"bytes={len(_FRAMES[0])}" in lines[0][1]
+
+
+# ---------------------------------------------------------------------------
+# Config + wiring
+# ---------------------------------------------------------------------------
+
+
+def test_log_large_response_bytes_defaults_to_64_kb() -> None:
+    assert _settings().log_large_response_bytes == 65536
+
+
+def test_log_large_response_bytes_env_override(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setenv("BOR_LOG_LARGE_RESPONSE_BYTES", "4096")
+    assert _settings().log_large_response_bytes == 4096
+    monkeypatch.setenv("BOR_LOG_LARGE_RESPONSE_BYTES", "0")  # loud mode is legal
+    assert _settings().log_large_response_bytes == 0
+
+
+def test_negative_budget_refuses_startup_naming_the_field() -> None:
+    with pytest.raises(ValidationError, match="log_large_response_bytes must be >= 0"):
+        _settings(log_large_response_bytes=-1)
+
+
+def test_middleware_sits_inside_gzip_outside_caching(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """The documented stack (README + ``app/main.py``): SecurityHeaders
+    outermost → gzip → response-budget → caching → session. That position is
+    what makes ``bytes=`` the ROUTE's size while ``enc=`` still names the
+    wire encoding."""
+    monkeypatch.setattr(main_mod.settings, "compress_min_bytes", 860)
+    names = [
+        getattr(mw.cls, "__name__", "") for mw in main_mod.create_app().user_middleware
+    ]
+    assert names == [
+        "SecurityHeadersMiddleware",
+        "SelectiveGZipMiddleware",
+        "ResponseBudgetMiddleware",
+        "CachingMiddleware",
+        "SessionMiddleware",
+    ]
+
+
+def test_budget_is_wired_even_with_compression_off(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """``BOR_COMPRESS_MIN_BYTES=0`` removes gzip only — the budget line is
+    not optional (LOCKED A5: measure everything, reject nothing), so the
+    measurement survives the compression kill switch."""
+    monkeypatch.setattr(main_mod.settings, "compress_min_bytes", 0)
+    names = [
+        getattr(mw.cls, "__name__", "") for mw in main_mod.create_app().user_middleware
+    ]
+    assert "SelectiveGZipMiddleware" not in names
+    assert "ResponseBudgetMiddleware" in names
+
+
+def test_api_path_prefix_constant() -> None:
+    assert API_PATH_PREFIX == "/api/"