diff --git a/.agents/phases/todo/82_security_headers/00_phase.md b/.agents/phases/complete/82_security_headers/00_phase.md
similarity index 100%
rename from .agents/phases/todo/82_security_headers/00_phase.md
rename to .agents/phases/complete/82_security_headers/00_phase.md
diff --git a/.agents/phases/todo/82_security_headers/01_headers_middleware.md b/.agents/phases/complete/82_security_headers/01_headers_middleware.md
similarity index 100%
rename from .agents/phases/todo/82_security_headers/01_headers_middleware.md
rename to .agents/phases/complete/82_security_headers/01_headers_middleware.md
diff --git a/.agents/phases/todo/82_security_headers/02_registration_and_integration.md b/.agents/phases/complete/82_security_headers/02_registration_and_integration.md
similarity index 100%
rename from .agents/phases/todo/82_security_headers/02_registration_and_integration.md
rename to .agents/phases/complete/82_security_headers/02_registration_and_integration.md
diff --git a/.agents/phases/todo/82_security_headers/03_e2e_and_commit.md b/.agents/phases/complete/82_security_headers/03_e2e_and_commit.md
similarity index 100%
rename from .agents/phases/todo/82_security_headers/03_e2e_and_commit.md
rename to .agents/phases/complete/82_security_headers/03_e2e_and_commit.md
diff --git a/.agents/reports/82_security_headers/82_security_headers__00_phase.a1.err b/.agents/reports/82_security_headers/82_security_headers__00_phase.a1.err
new file mode 100644
index 0000000..e69de29
diff --git a/.agents/reports/82_security_headers/82_security_headers__00_phase.a1.md b/.agents/reports/82_security_headers/82_security_headers__00_phase.a1.md
new file mode 100644
index 0000000..bfe16a5
--- /dev/null
+++ b/.agents/reports/82_security_headers/82_security_headers__00_phase.a1.md
@@ -0,0 +1,15 @@
+All completion criteria verified green — no defects found, nothing to fix. Final report:
+
+**Phase 82 (security headers) — final verification pass: all green**
+
+- Verified prior-run implementation: `app/core/security_headers.py` (pure-ASGI, header-only, exact A1 CSP), registration in `app/main.py` after `configure_caching` (outermost), unit/integration/E2E suites.
+- Deviation confirmed sound: `data:`-URI favicon (blocked by locked CSP) → static `frontend/assets/favicon.svg` in 5 templates + Containerfile `cp`; SVG element byte-identical to the old data-URI (verified programmatically); serves 200 with all three headers.
+- Curl check (server booted like e2e conftest, log: `/tmp/curl_security_headers_final.log`): `/`, `/api/health`, `/assets/styles.css`, `/nope` (404) → all three headers, CSP exactly `default-src 'self'; base-uri 'none'; frame-ancestors 'none'`.
+- `uv run pytest tests/unit/test_security_headers.py tests/integration/test_security_headers.py -v --no-cov` → 13 passed (incl. SSE byte-identity pin).
+- `uv run pytest tests/e2e/test_security_headers.py -v --no-cov` (isolated) → 2 passed (headers + zero CSP violations + painted page).
+- SSE tripwire `uv run pytest tests/e2e/test_chat_rag.py -v --no-cov` → 3 passed.
+- `uv run pytest --cov=app --cov-report=term-missing` → 1665 passed, app/ 99% (>90%); `uv run ruff check . && uv run pyright` → clean (0 errors).
+- `git diff --stat` limited to phase-82 files + the two documented deviations (favicon set, `tests/unit/__init__.py`); no `pyproject.toml`/`uv.lock`/JS diffs.
+- Commit + phase-dir move left to the harness per pipeline rules (not executed by me).
+
+Next pending phase: `83_chat_save_payload_limits`.
diff --git a/.agents/reports/82_security_headers/82_security_headers__00_phase.a1.validate b/.agents/reports/82_security_headers/82_security_headers__00_phase.a1.validate
new file mode 100644
index 0000000..8c8b417
--- /dev/null
+++ b/.agents/reports/82_security_headers/82_security_headers__00_phase.a1.validate
@@ -0,0 +1,84 @@
+........................................................................ [  4%]
+........................................................................ [  8%]
+........................................................................ [ 12%]
+........................................................................ [ 17%]
+........................................................................ [ 21%]
+........................................................................ [ 25%]
+........................................................................ [ 30%]
+........................................................................ [ 34%]
+........................................................................ [ 38%]
+........................................................................ [ 43%]
+........................................................................ [ 47%]
+........................................................................ [ 51%]
+........................................................................ [ 56%]
+........................................................................ [ 60%]
+........................................................................ [ 64%]
+........................................................................ [ 69%]
+........................................................................ [ 73%]
+........................................................................ [ 77%]
+........................................................................ [ 82%]
+........................................................................ [ 86%]
+........................................................................ [ 90%]
+........................................................................ [ 95%]
+........................................................................ [ 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                  178      0   100%
+app/api/chats.py                 110      0   100%
+app/api/config.py                  7      0   100%
+app/api/doc_drafts.py             93      0   100%
+app/api/docs.py                   50      0   100%
+app/api/git_sources.py           212      0   100%
+app/api/health.py                 10      0   100%
+app/api/steering.py               42      0   100%
+app/api/suggestions.py            29      0   100%
+app/api/sync.py                  101      0   100%
+app/api/tokens.py                 28      0   100%
+app/config.py                    141      0   100%
+app/core/__init__.py               0      0   100%
+app/core/auth.py                  45      0   100%
+app/core/caching.py              108      0   100%
+app/core/debugging.py             29      2    93%
+app/core/docs_push.py             39      0   100%
+app/core/logging.py               13      0   100%
+app/core/rate_limit.py            44      0   100%
+app/core/security_headers.py      19      0   100%
+app/core/tokens.py                33      0   100%
+app/db.py                         21      0   100%
+app/main.py                       64      0   100%
+app/models.py                     94      0   100%
+app/rag/__init__.py                0      0   100%
+app/rag/agent.py                 222      0   100%
+app/rag/archive_upload.py        128      0   100%
+app/rag/chunker.py               206      4    98%
+app/rag/git_sources.py            14      0   100%
+app/rag/importer.py              180      3    98%
+app/rag/llm.py                   216      0   100%
+app/rag/overview.py               71      0   100%
+app/rag/prompts.py                88      0   100%
+app/rag/retriever.py             150      3    98%
+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             24      0   100%
+app/schemas.py                   231      0   100%
+--------------------------------------------------
+TOTAL                           3232     12    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+validation OK
diff --git a/.agents/reports/82_security_headers/82_security_headers__01_headers_middleware.a1.err b/.agents/reports/82_security_headers/82_security_headers__01_headers_middleware.a1.err
new file mode 100644
index 0000000..e69de29
diff --git a/.agents/reports/82_security_headers/82_security_headers__01_headers_middleware.a1.md b/.agents/reports/82_security_headers/82_security_headers__01_headers_middleware.a1.md
new file mode 100644
index 0000000..81a9f57
--- /dev/null
+++ b/.agents/reports/82_security_headers/82_security_headers__01_headers_middleware.a1.md
@@ -0,0 +1,18 @@
+Task 01 complete — all gates green.
+
+**Implemented**
+- `app/core/security_headers.py` (new): `CSP` constant (exact A1 string `default-src 'self'; base-uri 'none'; frame-ancestors 'none'`) + `SecurityHeadersMiddleware` — pure-ASGI, wraps `send`, writes CSP / `X-Frame-Options: DENY` / `X-Content-Type-Options: nosniff` on `http.response.start` only, non-http fast path, never touches a body (SSE-safe); module docstring carries the SEC-04 audit basis, No-CDN verification, and pure-ASGI/SSE rationale.
+- `tests/unit/test_security_headers.py` (new, 8 tests): A1-string pin; plain 200 with exact header values; 404-shaped (empty header list); SSE `StreamingResponse` byte-identity pin (two frames in order, boundaries intact, headers present); message-level "only start is mutated" pin; http send-wrapped/receive-untouched identity; websocket + lifespan passthrough (original send/receive identity, zero injection).
+
+**Test / lint / coverage**
+- `uv run pytest tests/unit/test_security_headers.py -v --no-cov` → 8 passed
+- `uv run pytest --cov=app --cov-report=term-missing` → 1660 passed, TOTAL 99% (>90%); `app/core/security_headers.py` 100%
+- `uv run ruff check .` → All checks passed
+- `uv run pyright` → 0 errors
+- `git status` → only the two new files added (plus pre-existing harness artifacts under `.agents/`, untouched)
+
+**Notable decisions**
+- SSE pin accounts for starlette's own empty trailer frame + `; charset=utf-8` suffix (framework behavior, not the middleware).
+- Tests drive raw ASGI via a small `asyncio.run` helper (no pytest-asyncio in the repo; matches `test_caching.py` style) and also use a real `StreamingResponse` for the SSE test.
+
+**Next pending task:** `02_registration_and_integration.md` (`app/main.py` registration + integration tests on real app responses).
diff --git a/.agents/reports/82_security_headers/82_security_headers__01_headers_middleware.a1.validate b/.agents/reports/82_security_headers/82_security_headers__01_headers_middleware.a1.validate
new file mode 100644
index 0000000..3dfba88
--- /dev/null
+++ b/.agents/reports/82_security_headers/82_security_headers__01_headers_middleware.a1.validate
@@ -0,0 +1,84 @@
+........................................................................ [  4%]
+........................................................................ [  8%]
+........................................................................ [ 13%]
+........................................................................ [ 17%]
+........................................................................ [ 21%]
+........................................................................ [ 26%]
+........................................................................ [ 30%]
+........................................................................ [ 34%]
+........................................................................ [ 39%]
+........................................................................ [ 43%]
+........................................................................ [ 47%]
+........................................................................ [ 52%]
+........................................................................ [ 56%]
+........................................................................ [ 60%]
+........................................................................ [ 65%]
+........................................................................ [ 69%]
+........................................................................ [ 73%]
+........................................................................ [ 78%]
+........................................................................ [ 82%]
+........................................................................ [ 86%]
+........................................................................ [ 91%]
+........................................................................ [ 95%]
+........................................................................ [ 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                  178      0   100%
+app/api/chats.py                 110      0   100%
+app/api/config.py                  7      0   100%
+app/api/doc_drafts.py             93      0   100%
+app/api/docs.py                   50      0   100%
+app/api/git_sources.py           212      0   100%
+app/api/health.py                 10      0   100%
+app/api/steering.py               42      0   100%
+app/api/suggestions.py            29      0   100%
+app/api/sync.py                  101      0   100%
+app/api/tokens.py                 28      0   100%
+app/config.py                    141      0   100%
+app/core/__init__.py               0      0   100%
+app/core/auth.py                  45      0   100%
+app/core/caching.py              108      0   100%
+app/core/debugging.py             29      2    93%
+app/core/docs_push.py             39      0   100%
+app/core/logging.py               13      0   100%
+app/core/rate_limit.py            44      0   100%
+app/core/security_headers.py      19      0   100%
+app/core/tokens.py                33      0   100%
+app/db.py                         21      0   100%
+app/main.py                       62      0   100%
+app/models.py                     94      0   100%
+app/rag/__init__.py                0      0   100%
+app/rag/agent.py                 222      0   100%
+app/rag/archive_upload.py        128      0   100%
+app/rag/chunker.py               206      4    98%
+app/rag/git_sources.py            14      0   100%
+app/rag/importer.py              180      3    98%
+app/rag/llm.py                   216      0   100%
+app/rag/overview.py               71      0   100%
+app/rag/prompts.py                88      0   100%
+app/rag/retriever.py             150      3    98%
+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             24      0   100%
+app/schemas.py                   231      0   100%
+--------------------------------------------------
+TOTAL                           3230     12    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+validation OK
diff --git a/.agents/reports/82_security_headers/82_security_headers__02_registration_and_integration.a1.err b/.agents/reports/82_security_headers/82_security_headers__02_registration_and_integration.a1.err
new file mode 100644
index 0000000..e69de29
diff --git a/.agents/reports/82_security_headers/82_security_headers__02_registration_and_integration.a1.md b/.agents/reports/82_security_headers/82_security_headers__02_registration_and_integration.a1.md
new file mode 100644
index 0000000..0ed6983
--- /dev/null
+++ b/.agents/reports/82_security_headers/82_security_headers__02_registration_and_integration.a1.md
@@ -0,0 +1,19 @@
+All gates green. Final report:
+
+**Phase 82 · Task 02 — Registration + integration tests (complete)**
+
+Implemented:
+- `app/main.py`: `SecurityHeadersMiddleware` registered immediately after `configure_caching(app)` — outermost on purpose (headers on every response incl. static-catch-all 404s); nothing else reordered.
+- `tests/integration/test_security_headers.py` (new, 5 tests, shared `client` fixture — no new fixtures): `GET /`, `/api/health`, `/assets/styles.css`, `/definitely-not-a-page` (404) all carry the three headers with the exact A1 CSP; `?v=` rewrite coexistence pin (caching middleware intact under the header-only middleware).
+
+Results:
+- `uv run pytest tests/integration/test_security_headers.py tests/integration/test_caching_revalidation.py -v` → 10 passed (caching suite green untouched).
+- `uv run pytest --cov=app --cov-report=term-missing` → 1665 passed, **99%** coverage (`security_headers.py` 100%).
+- `uv run ruff check . && uv run pyright` → clean.
+- SSE tripwire: `uv run pytest tests/e2e/test_chat_rag.py -v --no-cov` → 3 passed (stream byte-identical through the new middleware).
+
+Notable decisions:
+- **Defect fixed:** full-suite collection crashed with pytest "import file mismatch" — `test_security_headers.py` now exists in both `tests/unit/` and `tests/integration/` (both paths mandated by the phase). Fix: added `tests/unit/__init__.py` (unit modules import as `unit.*`), mirroring the existing `tests/e2e/__init__.py` precedent; `tests/integration/` deliberately stays a plain dir because 3 files there use sibling imports (`from test_chat_api import …`). One extra new file beyond the task's list — unavoidable given the phase's own "full `uv run pytest` green" gate.
+- No `pyproject.toml`/template/JS diffs; no git add/commit (left in working tree for the harness).
+
+Next pending task: `.agents/phases/todo/82_security_headers/03_e2e_and_commit.md` (dedicated Playwright suite + full gate + atomic commit).
diff --git a/.agents/reports/82_security_headers/82_security_headers__02_registration_and_integration.a1.validate b/.agents/reports/82_security_headers/82_security_headers__02_registration_and_integration.a1.validate
new file mode 100644
index 0000000..8c8b417
--- /dev/null
+++ b/.agents/reports/82_security_headers/82_security_headers__02_registration_and_integration.a1.validate
@@ -0,0 +1,84 @@
+........................................................................ [  4%]
+........................................................................ [  8%]
+........................................................................ [ 12%]
+........................................................................ [ 17%]
+........................................................................ [ 21%]
+........................................................................ [ 25%]
+........................................................................ [ 30%]
+........................................................................ [ 34%]
+........................................................................ [ 38%]
+........................................................................ [ 43%]
+........................................................................ [ 47%]
+........................................................................ [ 51%]
+........................................................................ [ 56%]
+........................................................................ [ 60%]
+........................................................................ [ 64%]
+........................................................................ [ 69%]
+........................................................................ [ 73%]
+........................................................................ [ 77%]
+........................................................................ [ 82%]
+........................................................................ [ 86%]
+........................................................................ [ 90%]
+........................................................................ [ 95%]
+........................................................................ [ 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                  178      0   100%
+app/api/chats.py                 110      0   100%
+app/api/config.py                  7      0   100%
+app/api/doc_drafts.py             93      0   100%
+app/api/docs.py                   50      0   100%
+app/api/git_sources.py           212      0   100%
+app/api/health.py                 10      0   100%
+app/api/steering.py               42      0   100%
+app/api/suggestions.py            29      0   100%
+app/api/sync.py                  101      0   100%
+app/api/tokens.py                 28      0   100%
+app/config.py                    141      0   100%
+app/core/__init__.py               0      0   100%
+app/core/auth.py                  45      0   100%
+app/core/caching.py              108      0   100%
+app/core/debugging.py             29      2    93%
+app/core/docs_push.py             39      0   100%
+app/core/logging.py               13      0   100%
+app/core/rate_limit.py            44      0   100%
+app/core/security_headers.py      19      0   100%
+app/core/tokens.py                33      0   100%
+app/db.py                         21      0   100%
+app/main.py                       64      0   100%
+app/models.py                     94      0   100%
+app/rag/__init__.py                0      0   100%
+app/rag/agent.py                 222      0   100%
+app/rag/archive_upload.py        128      0   100%
+app/rag/chunker.py               206      4    98%
+app/rag/git_sources.py            14      0   100%
+app/rag/importer.py              180      3    98%
+app/rag/llm.py                   216      0   100%
+app/rag/overview.py               71      0   100%
+app/rag/prompts.py                88      0   100%
+app/rag/retriever.py             150      3    98%
+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             24      0   100%
+app/schemas.py                   231      0   100%
+--------------------------------------------------
+TOTAL                           3232     12    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+validation OK
diff --git a/.agents/reports/82_security_headers/82_security_headers__03_e2e_and_commit.a1.err b/.agents/reports/82_security_headers/82_security_headers__03_e2e_and_commit.a1.err
new file mode 100644
index 0000000..e69de29
diff --git a/.agents/reports/82_security_headers/82_security_headers__03_e2e_and_commit.a1.md b/.agents/reports/82_security_headers/82_security_headers__03_e2e_and_commit.a1.md
new file mode 100644
index 0000000..8f8c2cf
--- /dev/null
+++ b/.agents/reports/82_security_headers/82_security_headers__03_e2e_and_commit.a1.md
@@ -0,0 +1,7 @@
+**Phase 82 · Task 03 — E2E suite + full gate (complete; commit/move left to harness per pipeline rules)**
+
+- New `tests/e2e/test_security_headers.py` (2 tests, house conftest, no LLM interaction): real `page.goto` of `/` and `/sources.html` → all three headers, CSP exactly the A1 string; console listener (attached pre-nav) asserts zero `/Content Security Policy/i` messages; chat page painted (canvas bg `rgb(15,10,10)` + `#main` booted).
+- **Deviations (flagged):** (1) paint check pinned on the `<html>` canvas, not `body` — the phase-08 contract keeps `body` transparent by design, so the task's literal body check could never pass; (2) **audit miss fixed:** all 5 templates' `data:`-URI favicon is blocked by the locked `default-src 'self'` CSP (the mandated zero-violation gate was unmeetable). Fixed by serving `frontend/assets/favicon.svg` (byte-identical artwork) + `href="/assets/favicon.svg"` in the 5 templates + a Containerfile stage-1 `cp` (its asset-coverage pin required). The **locked A1 CSP string stays untouched** — no `img-src data:` weakening.
+- Curl check (server on final tree; full output in `/tmp/curl_security_headers.log`): `/`, `/api/health`, `/assets/styles.css`, `/nope` all carry the exact A1 CSP + `DENY` + `nosniff` (incl. 404s); note `HEAD /api/health` → 404 is pre-existing (FastAPI 0.141 doesn't auto-register HEAD; the static catch-all answers it — middleware-proven status-neutral); `GET /api/health` → 200 with all three.
+- Results: `uv run pytest tests/e2e/test_security_headers.py -v --no-cov` → 2 passed (isolated); `uv run pytest tests/e2e/test_chat_rag.py -v --no-cov` → 3 passed (SSE byte-identical); `uv run pytest --cov=app` → 1665 passed, **99%** (>90%); `uv run ruff check . && uv run pyright` → clean; template-touching E2E (`test_cache_busting`, `test_asset_cache_revalidation`, `test_dark_tech_theme`) → 15 passed.
+- Next pending task: `.agents/phases/todo/83_chat_save_payload_limits` (after the harness commits + moves phase 82).
diff --git a/.agents/reports/82_security_headers/82_security_headers__03_e2e_and_commit.a1.validate b/.agents/reports/82_security_headers/82_security_headers__03_e2e_and_commit.a1.validate
new file mode 100644
index 0000000..8c8b417
--- /dev/null
+++ b/.agents/reports/82_security_headers/82_security_headers__03_e2e_and_commit.a1.validate
@@ -0,0 +1,84 @@
+........................................................................ [  4%]
+........................................................................ [  8%]
+........................................................................ [ 12%]
+........................................................................ [ 17%]
+........................................................................ [ 21%]
+........................................................................ [ 25%]
+........................................................................ [ 30%]
+........................................................................ [ 34%]
+........................................................................ [ 38%]
+........................................................................ [ 43%]
+........................................................................ [ 47%]
+........................................................................ [ 51%]
+........................................................................ [ 56%]
+........................................................................ [ 60%]
+........................................................................ [ 64%]
+........................................................................ [ 69%]
+........................................................................ [ 73%]
+........................................................................ [ 77%]
+........................................................................ [ 82%]
+........................................................................ [ 86%]
+........................................................................ [ 90%]
+........................................................................ [ 95%]
+........................................................................ [ 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                  178      0   100%
+app/api/chats.py                 110      0   100%
+app/api/config.py                  7      0   100%
+app/api/doc_drafts.py             93      0   100%
+app/api/docs.py                   50      0   100%
+app/api/git_sources.py           212      0   100%
+app/api/health.py                 10      0   100%
+app/api/steering.py               42      0   100%
+app/api/suggestions.py            29      0   100%
+app/api/sync.py                  101      0   100%
+app/api/tokens.py                 28      0   100%
+app/config.py                    141      0   100%
+app/core/__init__.py               0      0   100%
+app/core/auth.py                  45      0   100%
+app/core/caching.py              108      0   100%
+app/core/debugging.py             29      2    93%
+app/core/docs_push.py             39      0   100%
+app/core/logging.py               13      0   100%
+app/core/rate_limit.py            44      0   100%
+app/core/security_headers.py      19      0   100%
+app/core/tokens.py                33      0   100%
+app/db.py                         21      0   100%
+app/main.py                       64      0   100%
+app/models.py                     94      0   100%
+app/rag/__init__.py                0      0   100%
+app/rag/agent.py                 222      0   100%
+app/rag/archive_upload.py        128      0   100%
+app/rag/chunker.py               206      4    98%
+app/rag/git_sources.py            14      0   100%
+app/rag/importer.py              180      3    98%
+app/rag/llm.py                   216      0   100%
+app/rag/overview.py               71      0   100%
+app/rag/prompts.py                88      0   100%
+app/rag/retriever.py             150      3    98%
+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             24      0   100%
+app/schemas.py                   231      0   100%
+--------------------------------------------------
+TOTAL                           3232     12    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+validation OK
diff --git a/Containerfile b/Containerfile
index 3754840..337d223 100644
--- a/Containerfile
+++ b/Containerfile
@@ -25,6 +25,7 @@ RUN mkdir -p /out/assets \
     && esbuild ./assets/markdown.js --minify --outfile=/out/assets/markdown.js \
     && esbuild ./assets/styles.css  --minify --outfile=/out/assets/styles.css \
     && cp -r ./assets/themes /out/assets/themes \
+    && cp ./assets/favicon.svg /out/assets/favicon.svg \
     && cp ./index.html ./document.html ./login.html ./shared.html ./doc-edit.html /out/
 
 # ---------- Stage 2: python dependencies ----------
diff --git a/app/core/security_headers.py b/app/core/security_headers.py
new file mode 100644
index 0000000..142f3c5
--- /dev/null
+++ b/app/core/security_headers.py
@@ -0,0 +1,81 @@
+"""Security headers on every response (phase 82 — security audit SEC-04).
+
+Audit basis (SEC-04, severity Medium): no response in the app carried
+``Content-Security-Policy``, ``X-Frame-Options``, or
+``X-Content-Type-Options``. Every page — including the admin sign-in and
+the admin-only views — could be embedded in a third-party page's
+iframe: a malicious LAN page could overlay the admin UI and trick the
+signed-in owner into clicking Sync / Revoke / Delete-source actions
+(audit PoC: ``<iframe src="http://<server>:8000/git-sources.html">``
+with a benign-looking overlay). ``frame-ancestors 'none'`` (in the
+CSP) plus ``X-Frame-Options: DENY`` (the legacy fallback for pre-CSP
+browsers) close that clickjacking vector. The CSP itself is the
+standard second line of defense against any future XSS regression —
+cheap here because the frontend is No-CDN and was verified for this
+phase to have **no** inline ``<style>`` blocks, **no** ``style="…"``
+attributes, **no** ``onclick``/``on*`` handlers, **no** ``javascript:``
+URLs, and **no** external hosts (inline ``<svg>`` elements are
+CSP-legal; the two ``el.style.…`` CSSOM writes in ``app.js`` are not
+CSP-blocked). If inline scripts/styles are ever introduced, re-verify
+that: the policy would break the affected page loudly, and that loud
+failure is the feature, not the bug.
+
+``X-Content-Type-Options: nosniff`` is the one-line belt-and-braces
+against MIME confusion on the static mount.
+
+Why pure ASGI: this is a *header-only* middleware. It wraps ``send``
+and writes the three headers on ``http.response.start`` only — it
+never reads or buffers a body, so the SSE chat stream and every other
+response pass through byte-identical. That is the explicit contrast
+with ``app/core/caching.py``'s ``CachingMiddleware``, which
+*does* buffer page bodies for the ``?v=`` rewrite. No HSTS (plain
+homelab HTTP — TLS is the documented non-goal), no ``Referrer-Policy``
+/ ``Permissions-Policy`` (owner decision A4: the three headers above
+are the scope).
+"""
+from __future__ import annotations
+
+from starlette.datastructures import MutableHeaders
+from starlette.types import ASGIApp, Message, Receive, Scope, Send
+
+#: The exact owner-approved policy (phase 82, decision A1).
+#: ``default-src 'self'`` is inherited by every sub-policy that has no
+#: explicit entry (``script-src``, ``style-src``, ``connect-src``, …),
+#: ``base-uri 'none'`` blocks base-tag hijacking, and
+#: ``frame-ancestors 'none'`` forbids any framing. No ``'unsafe-inline'``
+#: (verified unnecessary — see module docstring), no ``report-uri`` /
+#: ``report-to`` (no collector in the homelab — a report would just
+#: vanish).
+CSP = "default-src 'self'; base-uri 'none'; frame-ancestors 'none'"
+
+
+class SecurityHeadersMiddleware:
+    """Header-only, pure-ASGI — never reads or buffers a body (SSE-safe).
+
+    Adds exactly three headers to every HTTP response:
+
+    * ``Content-Security-Policy``: the strict same-origin policy above
+      (``frame-ancestors 'none'`` → clickjacking closed, SEC-04);
+    * ``X-Frame-Options: DENY`` — legacy no-framing fallback;
+    * ``X-Content-Type-Options: nosniff`` — MIME-confusion belt.
+
+    Non-HTTP scopes (websocket, lifespan) pass through untouched.
+    """
+
+    def __init__(self, app: ASGIApp) -> None:
+        self.app = app
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        if scope["type"] != "http":
+            await self.app(scope, receive, send)
+            return
+
+        async def send_wrapper(message: Message) -> None:
+            if message["type"] == "http.response.start":
+                headers = MutableHeaders(scope=message)
+                headers["Content-Security-Policy"] = CSP
+                headers["X-Frame-Options"] = "DENY"
+                headers["X-Content-Type-Options"] = "nosniff"
+            await send(message)
+
+        await self.app(scope, receive, send_wrapper)
diff --git a/app/main.py b/app/main.py
index 2fec545..40c1b04 100644
--- a/app/main.py
+++ b/app/main.py
@@ -45,6 +45,7 @@ from app.core.auth import ensure_admin_configured
 from app.core.caching import configure_caching
 from app.core.debugging import configure_debugging
 from app.core.logging import configure_logging
+from app.core.security_headers import SecurityHeadersMiddleware
 
 configure_logging()
 configure_debugging()
@@ -134,6 +135,11 @@ def create_app() -> FastAPI:
     # stream in particular — passes through untouched.
     configure_caching(app)
 
+    # 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.add_middleware(SecurityHeadersMiddleware)
+
     static_dir = Path(settings.static_dir).resolve()
     if static_dir.is_dir():
         # Phase 76: the folded navbar views serve the shell — the
diff --git a/frontend/assets/favicon.svg b/frontend/assets/favicon.svg
new file mode 100644
index 0000000..bcfba6d
--- /dev/null
+++ b/frontend/assets/favicon.svg
@@ -0,0 +1,7 @@
+<!-- Favicon — served statically (AGENTS.md rule 6) instead of a
+     data: URI: the phase-82 strict CSP (default-src 'self') blocks
+     data: images, and the audit's "no external hosts" verification
+     had missed this one. Byte-identical artwork, extracted from the
+     former <link rel="icon" href="data:image/svg+xml,…"> (2026-09-08,
+     phase 82 task 03 — the E2E CSP gate caught the violation). -->
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64"><path d="M32 4 55 18v28L32 60 9 46V18Z" fill="#1a0f0f" stroke="#f43f5e" stroke-width="4" stroke-linejoin="round"/><circle cx="32" cy="32" r="6.5" fill="#f43f5e"/><path d="M32 25.5V16M32 48v-9.5M25.5 32H16M48 32h-9.5" stroke="#fca5a5" stroke-width="3" stroke-linecap="round"/></svg>
diff --git a/frontend/doc-edit.html b/frontend/doc-edit.html
index f1446fd..4361218 100644
--- a/frontend/doc-edit.html
+++ b/frontend/doc-edit.html
@@ -5,7 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
   <meta name="description" content="Edit a saved chat answer before it is committed to the docs repository (admin-only).">
   <title>Edit doc · Brain of Reese</title>
-  <link rel="icon" href="data:image/svg+xml,%3Csvg%20xmlns=%22http://www.w3.org/2000/svg%22%20viewBox=%220%200%2064%2064%22%3E%3Cpath%20d=%22M32%204%2055%2018v28L32%2060%209%2046V18Z%22%20fill=%22%231a0f0f%22%20stroke=%22%23f43f5e%22%20stroke-width=%224%22%20stroke-linejoin=%22round%22/%3E%3Ccircle%20cx=%2232%22%20cy=%2232%22%20r=%226.5%22%20fill=%22%23f43f5e%22/%3E%3Cpath%20d=%22M32%2025.5V16M32%2048v-9.5M25.5%2032H16M48%2032h-9.5%22%20stroke=%22%23fca5a5%22%20stroke-width=%223%22%20stroke-linecap=%22round%22/%3E%3C/svg%3E">
+  <link rel="icon" type="image/svg+xml" href="/assets/favicon.svg">
   <link rel="stylesheet" href="/assets/styles.css">
 </head>
 <body>
diff --git a/frontend/document.html b/frontend/document.html
index ebd0445..86183a7 100644
--- a/frontend/document.html
+++ b/frontend/document.html
@@ -5,7 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
   <meta name="description" content="Read a document indexed in Brain of Reese.">
   <title>Document · Brain of Reese</title>
-  <link rel="icon" href="data:image/svg+xml,%3Csvg%20xmlns=%22http://www.w3.org/2000/svg%22%20viewBox=%220%200%2064%2064%22%3E%3Cpath%20d=%22M32%204%2055%2018v28L32%2060%209%2046V18Z%22%20fill=%22%231a0f0f%22%20stroke=%22%23f43f5e%22%20stroke-width=%224%22%20stroke-linejoin=%22round%22/%3E%3Ccircle%20cx=%2232%22%20cy=%2232%22%20r=%226.5%22%20fill=%22%23f43f5e%22/%3E%3Cpath%20d=%22M32%2025.5V16M32%2048v-9.5M25.5%2032H16M48%2032h-9.5%22%20stroke=%22%23fca5a5%22%20stroke-width=%223%22%20stroke-linecap=%22round%22/%3E%3C/svg%3E">
+  <link rel="icon" type="image/svg+xml" href="/assets/favicon.svg">
   <link rel="stylesheet" href="/assets/styles.css">
 </head>
 <body>
diff --git a/frontend/index.html b/frontend/index.html
index d1072af..43892fb 100644
--- a/frontend/index.html
+++ b/frontend/index.html
@@ -5,7 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
   <meta name="description" content="Ask anything about your indexed documents — every answer cites the exact doc.">
   <title>Brain of Reese</title>
-  <link rel="icon" href="data:image/svg+xml,%3Csvg%20xmlns=%22http://www.w3.org/2000/svg%22%20viewBox=%220%200%2064%2064%22%3E%3Cpath%20d=%22M32%204%2055%2018v28L32%2060%209%2046V18Z%22%20fill=%22%231a0f0f%22%20stroke=%22%23f43f5e%22%20stroke-width=%224%22%20stroke-linejoin=%22round%22/%3E%3Ccircle%20cx=%2232%22%20cy=%2232%22%20r=%226.5%22%20fill=%22%23f43f5e%22/%3E%3Cpath%20d=%22M32%2025.5V16M32%2048v-9.5M25.5%2032H16M48%2032h-9.5%22%20stroke=%22%23fca5a5%22%20stroke-width=%223%22%20stroke-linecap=%22round%22/%3E%3C/svg%3E">
+  <link rel="icon" type="image/svg+xml" href="/assets/favicon.svg">
   <link rel="stylesheet" href="/assets/styles.css">
 </head>
 <body>
diff --git a/frontend/login.html b/frontend/login.html
index 1ab751b..d3a4170 100644
--- a/frontend/login.html
+++ b/frontend/login.html
@@ -5,7 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
   <meta name="description" content="Admin sign-in for Brain of Reese — unlocks the full Sources catalog and answer tuning.">
   <title>Sign in · Brain of Reese</title>
-  <link rel="icon" href="data:image/svg+xml,%3Csvg%20xmlns=%22http://www.w3.org/2000/svg%22%20viewBox=%220%200%2064%2064%22%3E%3Cpath%20d=%22M32%204%2055%2018v28L32%2060%209%2046V18Z%22%20fill=%22%231a0f0f%22%20stroke=%22%23f43f5e%22%20stroke-width=%224%22%20stroke-linejoin=%22round%22/%3E%3Ccircle%20cx=%2232%22%20cy=%2232%22%20r=%226.5%22%20fill=%22%23f43f5e%22/%3E%3Cpath%20d=%22M32%2025.5V16M32%2048v-9.5M25.5%2032H16M48%2032h-9.5%22%20stroke=%22%23fca5a5%22%20stroke-width=%223%22%20stroke-linecap=%22round%22/%3E%3C/svg%3E">
+  <link rel="icon" type="image/svg+xml" href="/assets/favicon.svg">
   <link rel="stylesheet" href="/assets/styles.css">
 </head>
 <body>
diff --git a/frontend/shared.html b/frontend/shared.html
index d98701f..44937a1 100644
--- a/frontend/shared.html
+++ b/frontend/shared.html
@@ -5,7 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
   <meta name="description" content="A shared Brain of Reese conversation — read-only.">
   <title>Shared conversation · Brain of Reese</title>
-  <link rel="icon" href="data:image/svg+xml,%3Csvg%20xmlns=%22http://www.w3.org/2000/svg%22%20viewBox=%220%200%2064%2064%22%3E%3Cpath%20d=%22M32%204%2055%2018v28L32%2060%209%2046V18Z%22%20fill=%22%231a0f0f%22%20stroke=%22%23f43f5e%22%20stroke-width=%224%22%20stroke-linejoin=%22round%22/%3E%3Ccircle%20cx=%2232%22%20cy=%2232%22%20r=%226.5%22%20fill=%22%23f43f5e%22/%3E%3Cpath%20d=%22M32%2025.5V16M32%2048v-9.5M25.5%2032H16M48%2032h-9.5%22%20stroke=%22%23fca5a5%22%20stroke-width=%223%22%20stroke-linecap=%22round%22/%3E%3C/svg%3E">
+  <link rel="icon" type="image/svg+xml" href="/assets/favicon.svg">
   <link rel="stylesheet" href="/assets/styles.css">
 </head>
 <body>
diff --git a/tests/e2e/test_security_headers.py b/tests/e2e/test_security_headers.py
new file mode 100644
index 0000000..690a322
--- /dev/null
+++ b/tests/e2e/test_security_headers.py
@@ -0,0 +1,128 @@
+"""Phase 82 E2E (Playwright): the security headers reach a real browser and
+the strict CSP does not break the page.
+
+Story: n/a — security hardening, audit SEC-04 (``.agents/remediation_plan.md``).
+Run in isolation (DB must be up: ``podman compose up -d db``):
+
+    uv run pytest tests/e2e/test_security_headers.py -v --no-cov
+
+No LLM interaction: the page load IS the contract. The suite never touches
+the chat stream and never depends on the aipi endpoint — the shared
+``app_server`` fixture's deterministic LOCAL mock LLM is simply unused.
+
+Test → task mapping (phase 82, task 03):
+1. ``test_chat_page_navigation_headers_and_csp_clean`` — a real ``page.goto``
+   of the chat shell (``/``): the navigation response carries all three
+   headers (CSP exactly the A1 string), the page painted under the CSP
+   (the dark-tech canvas applied → the stylesheet loaded), the shell's
+   ``#main`` booted, and zero console CSP violations.
+2. ``test_sources_page_navigation_headers_and_csp_clean`` — the same header
+   + no-violation contract for the second page, ``/sources.html``.
+
+Paint-check note (vs. the task text): the task suggested asserting the
+``body`` background-color is not transparent, but the pinned design
+contract (phase 08, ``test_dark_tech_theme.py``) puts the visible canvas on
+``<html>`` (``html { background: var(--bg) }``) and keeps ``<body>``
+transparent on purpose (the z-index:-1 grid layer must show). The
+stylesheet-loaded-under-the-CSP contract is therefore pinned on the
+``<html>`` canvas color — the same value phase 08 pins.
+"""
+from __future__ import annotations
+
+import re
+
+from playwright.sync_api import ConsoleMessage, Page, expect
+
+#: The exact owner-approved policy (phase 82, decision A1).
+CSP = "default-src 'self'; base-uri 'none'; frame-ancestors 'none'"
+
+#: Chromium reports CSP denials to the console with this phrasing
+#: ("Refused to … because it violates the following Content Security
+#: Policy directive …" / "The Content Security Policy directive …
+#: prevents execution …").
+_CSP_CONSOLE = re.compile(r"Content Security Policy", re.IGNORECASE)
+
+#: The dark-tech canvas — ``frontend/assets/styles.css`` ``--bg: #0f0a0a``,
+#: pinned by phase 08 (``test_dark_tech_theme.py``).
+CANVAS_BG = "rgb(15, 10, 10)"
+
+
+def _attach_csp_watch(page: Page) -> list[str]:
+    """Record every console message that mentions a CSP denial.
+
+    Attached BEFORE navigation (the contract — denials of the main
+    document's resources are logged while it loads). Returns the live
+    list; assert it is empty after the load.
+    """
+    violations: list[str] = []
+
+    def _on_console(msg: ConsoleMessage) -> None:
+        if _CSP_CONSOLE.search(msg.text):
+            violations.append(msg.text)
+
+    page.on("console", _on_console)
+    return violations
+
+
+def _assert_navigation_headers(page: Page, url: str) -> None:
+    """A real navigation: the response carries all three headers, CSP exact."""
+    response = page.goto(url)
+    assert response is not None, f"no navigation response for {url}"
+    assert response.status == 200, f"{url} → {response.status}"
+    headers = response.headers  # lowercase keys
+    assert headers.get("content-security-policy") == CSP, (
+        f"{url}: CSP is {headers.get('content-security-policy')!r}, "
+        f"expected the exact A1 string {CSP!r}"
+    )
+    assert headers.get("x-frame-options") == "DENY", (
+        f"{url}: X-Frame-Options is {headers.get('x-frame-options')!r}"
+    )
+    assert headers.get("x-content-type-options") == "nosniff", (
+        f"{url}: X-Content-Type-Options is {headers.get('x-content-type-options')!r}"
+    )
+
+
+def _assert_chat_page_painted(page: Page) -> None:
+    """The page painted under the CSP — styles loaded, shell booted.
+
+    The visible canvas is ``<html>`` (see the module docstring): its
+    computed color can only be the dark-tech palette if ``/assets/*.css``
+    was fetched and applied under ``default-src 'self'`` — a CSP-broken
+    document would be a white/default canvas without ``#main`` styled.
+    """
+    bg = page.evaluate(
+        "() => getComputedStyle(document.documentElement).backgroundColor"
+    )
+    assert bg == CANVAS_BG, (
+        f"dark-tech canvas not applied under the CSP (stylesheet not loaded?): {bg}"
+    )
+    expect(page.locator("#main")).to_be_attached()
+    box = page.locator("#main").bounding_box()
+    assert box is not None and box["width"] > 0 and box["height"] > 0, (
+        "shell <main> is present but invisible/empty — the page did not boot"
+    )
+
+
+def test_chat_page_navigation_headers_and_csp_clean(
+    page: Page, app_url: str, db_ready: None
+) -> None:
+    """AC1: real navigation of the chat shell → three headers (CSP exact),
+    painted page, zero CSP violations on the console."""
+    violations = _attach_csp_watch(page)
+    _assert_navigation_headers(page, app_url + "/")
+    _assert_chat_page_painted(page)
+    # Console delivery is async — let pending messages settle before the
+    # zero-violation gate (the load event already fired).
+    page.wait_for_timeout(250)
+    assert violations == [], f"CSP violations on the chat page: {violations}"
+
+
+def test_sources_page_navigation_headers_and_csp_clean(
+    page: Page, app_url: str, db_ready: None
+) -> None:
+    """AC2: the second page (Sources) — same three headers, no CSP
+    violations."""
+    violations = _attach_csp_watch(page)
+    _assert_navigation_headers(page, app_url + "/sources.html")
+    page.wait_for_timeout(250)
+    assert violations == [], f"CSP violations on the sources page: {violations}"
diff --git a/tests/integration/test_security_headers.py b/tests/integration/test_security_headers.py
new file mode 100644
index 0000000..fb31df2
--- /dev/null
+++ b/tests/integration/test_security_headers.py
@@ -0,0 +1,104 @@
+"""Integration: the phase-82 security-header contract against the REAL app.
+
+Phase 82 (audit SEC-04) adds a header-only, pure-ASGI middleware
+(``app/core/security_headers.py``) registered OUTERMOST in
+``app/main.py`` (last ``add_middleware`` — after
+``configure_caching``), so every response this process serves — pages,
+API JSON, static assets, even the static catch-all's 404s — carries:
+
+* ``Content-Security-Policy`` — the exact A1 string (``default-src 'self';
+  base-uri 'none'; frame-ancestors 'none'`` → clickjacking closed, no
+  inline anything because the No-CDN frontend has none);
+* ``X-Frame-Options: DENY`` — the legacy no-framing fallback;
+* ``X-Content-Type-Options: nosniff`` — the MIME-confusion belt.
+
+This suite pins those three on the four response shapes the browser
+actually sees — a page (``/``), an API response (``/api/health``), a
+static asset (``/assets/styles.css``), and a 404
+(``/definitely-not-a-page`` — the outermost-position proof: the 404 is
+built deep inside the static catch-all, yet the headers are still on
+it) — against the real ``app.main:app`` with the real static mount on
+the real ``frontend/`` tree (no mocks, same client as
+``tests/integration/test_caching_revalidation.py`` via the shared
+``client`` fixture). It also pins that the two middlewares coexist: the
+CSP middleware is header-only, so the phase-33/54 ``?v=<token>``
+rewrite from ``CachingMiddleware`` still runs under it (the last
+regression below). The SSE chat stream's byte-identity is pinned by the
+unit streaming test and the chat E2E suites (the tripwires).
+"""
+from __future__ import annotations
+
+import httpx
+from fastapi.testclient import TestClient
+
+from app.core.security_headers import CSP
+
+#: The exact owner-approved A1 policy string (phase 82). The constant is
+#: the single source of truth; the unit suite additionally pins that the
+#: constant itself equals this literal.
+_EXPECTED = {
+    "content-security-policy": CSP,
+    "x-frame-options": "DENY",
+    "x-content-type-options": "nosniff",
+}
+
+
+def _assert_security_headers(response: httpx.Response) -> None:
+    """All three phase-82 headers present with the exact values (the
+    httpx header lookup is case-insensitive)."""
+    path = response.request.url.path
+    for name, value in _EXPECTED.items():
+        assert response.headers.get(name) == value, (
+            f"{path}: {name} must be {value!r}, "
+            f"got {response.headers.get(name)!r}"
+        )
+
+
+def test_page_carries_all_three_headers(client: TestClient) -> None:
+    """``GET /`` (the shell page) — 200 + all three headers, CSP exactly
+    the A1 string."""
+    response = client.get("/")
+    assert response.status_code == 200
+    _assert_security_headers(response)
+    assert response.headers["content-security-policy"] == (
+        "default-src 'self'; base-uri 'none'; frame-ancestors 'none'"
+    )
+
+
+def test_api_health_carries_all_three_headers(client: TestClient) -> None:
+    """``GET /api/health`` (an API JSON response) — 200 + all three
+    headers."""
+    response = client.get("/api/health")
+    assert response.status_code == 200
+    _assert_security_headers(response)
+
+
+def test_static_asset_carries_all_three_headers(client: TestClient) -> None:
+    """``GET /assets/styles.css`` (the static mount) — 200 + all three
+    headers (the nosniff belt guards exactly this surface)."""
+    response = client.get("/assets/styles.css")
+    assert response.status_code == 200
+    _assert_security_headers(response)
+
+
+def test_static_catchall_404_carries_all_three_headers(client: TestClient) -> None:
+    """``GET /definitely-not-a-page`` — 404 from the static catch-all,
+    built deep inside the app, yet all three headers ride on it: the
+    outermost-position proof (a response the middleware cannot see
+    unless it wraps everything)."""
+    response = client.get("/definitely-not-a-page")
+    assert response.status_code == 404
+    _assert_security_headers(response)
+
+
+def test_caching_rewrite_still_runs_under_headers_middleware(client: TestClient) -> None:
+    """Coexistence pin: the header middleware is header-only (it never
+    drains or rewrites a body), so the phase-33/54 ``CachingMiddleware``
+    ``?v=<token>`` asset rewrite on the same page must still run — the
+    served HTML still carries at least one ``?v=`` reference."""
+    response = client.get("/")
+    assert response.status_code == 200
+    assert response.text.count("?v=") >= 1, (
+        "the ?v=<token> asset rewrite no longer runs — the outermost "
+        "security-header middleware altered or swallowed the body"
+    )
diff --git a/tests/unit/__init__.py b/tests/unit/__init__.py
new file mode 100644
index 0000000..1782bc9
--- /dev/null
+++ b/tests/unit/__init__.py
@@ -0,0 +1,11 @@
+"""Unit test package.
+
+Like ``tests/e2e/``, this directory is a package so its test modules
+import as ``unit.<name>`` instead of a top-level ``<name>``: a
+same-basename file in two collected directories (phase 82 ships
+``test_security_headers.py`` in BOTH this directory and
+``tests/integration/``) makes pytest's default import mode raise an
+"import file mismatch" on the full suite. ``tests/integration/`` stays a
+plain directory on purpose — several of its files use sibling imports
+(``from test_chat_api import …``), which require top-level module names.
+"""
diff --git a/tests/unit/test_security_headers.py b/tests/unit/test_security_headers.py
new file mode 100644
index 0000000..aceee8c
--- /dev/null
+++ b/tests/unit/test_security_headers.py
@@ -0,0 +1,294 @@
+"""Unit tests: the pure-ASGI security-headers middleware (phase 82, task 01).
+
+Pins the middleware against the phase design:
+* the ``CSP`` constant is the exact owner-approved A1 string;
+* a plain 200 response carries all three headers with the exact values;
+* a 404-shaped response carries them too (the static catch-all's 404s
+  get the headers as soon as the middleware is registered in
+  ``app/main.py``, task 02);
+* a **streaming** SSE response passes through byte-identical — both
+  chunks, in order, chunk boundaries preserved (no buffering, no
+  coalescing) — AND carries the headers (the SSE passthrough pin);
+* the HTTP path wraps ``send`` (and leaves ``receive`` alone);
+* a non-HTTP scope (websocket / lifespan) passes through untouched —
+  the app is called with the ORIGINAL ``send``/``receive`` (no
+  wrapping, no header injection attempt, no crash).
+"""
+from __future__ import annotations
+
+import asyncio
+from collections.abc import AsyncIterator
+from typing import Any
+
+import pytest
+from starlette.responses import StreamingResponse
+from starlette.types import ASGIApp, Message, Scope
+
+from app.core.security_headers import CSP, SecurityHeadersMiddleware
+
+#: The exact expected header set (decision A1 for the CSP, A4 for the
+#: other two).
+EXPECTED_HEADERS = {
+    "content-security-policy": "default-src 'self'; base-uri 'none'; frame-ancestors 'none'",
+    "x-frame-options": "DENY",
+    "x-content-type-options": "nosniff",
+}
+
+
+# ---------------------------------------------------------------------------
+# ASGI drivers / test apps (the repo's unit suite has no pytest-asyncio —
+# ``asyncio.run`` matches the existing style, see test_caching.py)
+# ---------------------------------------------------------------------------
+
+
+def _drive(
+    app: ASGIApp, scope: Scope, receive_messages: list[Message] | None = None
+) -> list[Message]:
+    """Run ``app`` for one connection; return every ``send`` message in order."""
+    queued: list[Message] = list(
+        receive_messages
+        if receive_messages is not None
+        else [{"type": "http.request", "body": b"", "more_body": False}]
+    )
+    sent: list[Message] = []
+
+    async def receive() -> Message:
+        return queued.pop(0) if queued else {"type": "http.disconnect"}
+
+    async def send(message: Message) -> None:
+        sent.append(message)
+
+    async def _run() -> None:
+        await app(scope, receive, send)
+
+    asyncio.run(_run())
+    return sent
+
+
+def _http_scope() -> Scope:
+    return {
+        "type": "http",
+        "method": "GET",
+        "path": "/",
+        "query_string": b"",
+        "headers": [],
+        "raw_path": b"/",
+        "root_path": "",
+        "scheme": "http",
+        "server": ("testserver", 80),
+    }
+
+
+def _header(message: Message, name: str) -> str | None:
+    for key, value in message.get("headers", []):
+        if key.lower() == name.lower().encode("ascii"):
+            return value.decode("latin-1")
+    return None
+
+
+def _plain_app(status: int, body: bytes, headers: list | None = None) -> ASGIApp:
+    """A raw ASGI app answering with exactly one start + one body message."""
+
+    async def app(scope: Scope, receive: Any, send: Any) -> None:
+        await send(
+            {"type": "http.response.start", "status": status, "headers": list(headers or [])}
+        )
+        await send({"type": "http.response.body", "body": body, "more_body": False})
+
+    return app
+
+
+def _assert_security_headers(start: Message, expected_extra: dict[str, str] | None = None) -> None:
+    for name, value in EXPECTED_HEADERS.items():
+        assert _header(start, name) == value, f"expected header {name!r} == {value!r}"
+    for name, value in (expected_extra or {}).items():
+        assert _header(start, name) == value, f"expected pre-existing header {name!r} kept"
+
+
+# ---------------------------------------------------------------------------
+# The policy constant
+# ---------------------------------------------------------------------------
+
+
+def test_csp_constant_is_the_exact_a1_policy() -> None:
+    """The owner-approved A1 string, verbatim: same-origin default, no
+    base-tag hijack, no framing — no 'unsafe-inline', no report sink."""
+    assert CSP == "default-src 'self'; base-uri 'none'; frame-ancestors 'none'"
+    assert "unsafe-inline" not in CSP
+
+
+# ---------------------------------------------------------------------------
+# Plain responses
+# ---------------------------------------------------------------------------
+
+
+def test_plain_200_response_carries_all_three_headers_with_exact_values() -> None:
+    wrapped = SecurityHeadersMiddleware(_plain_app(200, b"hello",
+                                                   headers=[[b"content-type", b"text/plain"]]))
+    sent = _drive(wrapped, _http_scope())
+
+    assert [m["type"] for m in sent] == ["http.response.start", "http.response.body"]
+    start, body_msg = sent
+    assert start["status"] == 200
+    _assert_security_headers(start, expected_extra={"content-type": "text/plain"})
+    # The body passes through byte-identical.
+    assert body_msg["body"] == b"hello"
+    assert body_msg["more_body"] is False
+
+
+def test_404_shaped_response_carries_all_three_headers() -> None:
+    """A 404 (e.g. the static catch-all's "file not found" JSON) gets the
+    headers too — with an EMPTY downstream header list (no pre-existing
+    headers to preserve)."""
+    wrapped = SecurityHeadersMiddleware(_plain_app(404, b"not found"))
+    sent = _drive(wrapped, _http_scope())
+
+    start, body_msg = sent
+    assert start["status"] == 404
+    _assert_security_headers(start)
+    assert body_msg["body"] == b"not found"
+
+
+# ---------------------------------------------------------------------------
+# The SSE streaming passthrough pin
+# ---------------------------------------------------------------------------
+
+
+def test_sse_stream_passes_through_byte_identical_with_headers() -> None:
+    """A ``text/event-stream`` response driven through a REAL starlette
+    ``StreamingResponse`` (async generator yielding two SSE frames):
+    the body arrives as exactly the two chunks, in order, boundaries
+    intact — no buffering, no coalescing — and the three headers are on
+    the start message (the SSE passthrough pin; a body-touching
+    regression would break this and the chat E2E suites)."""
+
+    async def frames() -> AsyncIterator[bytes]:
+        yield b"data: a\n\n"
+        yield b"data: b\n\n"
+
+    response = StreamingResponse(frames(), media_type="text/event-stream")
+    sent = _drive(SecurityHeadersMiddleware(response), _http_scope())
+
+    start = sent[0]
+    assert start["type"] == "http.response.start"
+    assert start["status"] == 200
+    _assert_security_headers(start)
+    # starlette's own StreamingResponse appends ``; charset=utf-8`` — the
+    # media type itself must be SSE (the middleware never rewrites it).
+    content_type = _header(start, "content-type")
+    assert content_type is not None
+    assert content_type.split(";", 1)[0].strip() == "text/event-stream"
+
+    body_msgs = [m for m in sent[1:] if m["type"] == "http.response.body"]
+    # starlette's StreamingResponse closes the generator with a final
+    # EMPTY trailer frame (no bytes) after the last chunk — the two data
+    # frames must still arrive untouched, in order, boundaries intact.
+    assert [m["body"] for m in body_msgs] == [b"data: a\n\n", b"data: b\n\n", b""]
+    assert b"".join(m["body"] for m in body_msgs) == b"data: a\n\ndata: b\n\n"
+    assert [m["more_body"] for m in body_msgs] == [True, True, False]
+
+
+def test_sse_frames_are_forwarded_in_order_without_delayed_buffering() -> None:
+    """Message-level pin: every ``http.response.body`` message the app
+    sends is forwarded as-is (same bytes, same ``more_body``) — the
+    wrapper mutates ONLY the ``http.response.start`` message."""
+    original_frames: list[Message] = []
+
+    async def recording_app(scope: Scope, receive: Any, send: Any) -> None:
+        # Same response shape as the SSE route: start, then two frames.
+        await send(
+            {
+                "type": "http.response.start",
+                "status": 200,
+                "headers": [[b"content-type", b"text/event-stream"]],
+            }
+        )
+        for frame in (b"data: a\n\n", b"data: b\n\n"):
+            original_frames.append(
+                {"type": "http.response.body", "body": frame, "more_body": frame != b"data: b\n\n"}
+            )
+            await send(original_frames[-1])
+
+    sent = _drive(SecurityHeadersMiddleware(recording_app), _http_scope())
+
+    forwarded = [m for m in sent[1:] if m["type"] == "http.response.body"]
+    # Identical objects: forwarded byte-for-byte, in order, untouched.
+    assert forwarded == original_frames
+
+
+# ---------------------------------------------------------------------------
+# Scope handling: http wrapping vs non-http passthrough
+# ---------------------------------------------------------------------------
+
+
+def test_http_scope_wraps_send_but_not_receive() -> None:
+    seen: dict[str, Any] = {}
+
+    async def app(scope: Scope, receive: Any, send: Any) -> None:
+        seen["receive"] = receive
+        seen["send"] = send
+        await send({"type": "http.response.start", "status": 200, "headers": []})
+        await send({"type": "http.response.body", "body": b"", "more_body": False})
+
+    queued: list[Message] = [{"type": "http.request", "body": b"", "more_body": False}]
+    sent: list[Message] = []
+
+    async def receive() -> Message:
+        return queued.pop(0) if queued else {"type": "http.disconnect"}
+
+    async def send(message: Message) -> None:
+        sent.append(message)
+
+    asyncio.run(SecurityHeadersMiddleware(app)(_http_scope(), receive, send))
+
+    assert seen["send"] is not send  # the wrapped send did the header writes
+    assert seen["receive"] is receive  # receive is never touched
+    assert sent[0]["type"] == "http.response.start"
+    _assert_security_headers(sent[0])
+
+
+@pytest.mark.parametrize("scope_type", ["websocket", "lifespan"], ids=["websocket", "lifespan"])
+def test_non_http_scope_passes_through_untouched(scope_type: str) -> None:
+    """Non-HTTP connections (websocket, lifespan) reach the app with the
+    ORIGINAL ``send``/``receive`` — no wrapping, no header injection
+    attempt, no crash."""
+    seen: dict[str, Any] = {}
+
+    first, second = {
+        "websocket": (
+            {"type": "websocket.accept"},
+            {"type": "websocket.send", "text": "hi"},
+        ),
+        "lifespan": (
+            {"type": "lifespan.startup.complete"},
+            {"type": "lifespan.shutdown.complete"},
+        ),
+    }[scope_type]
+
+    async def app(scope: Scope, receive: Any, send: Any) -> None:
+        seen["scope"] = scope
+        seen["receive"] = receive
+        seen["send"] = send
+        await send(first)
+        await send(second)
+
+    queued: list[Message] = [
+        {"type": f"{scope_type}.connect"} if scope_type == "websocket"
+        else {"type": "lifespan.startup"}
+    ]
+    sent: list[Message] = []
+
+    async def receive() -> Message:
+        return queued.pop(0) if queued else {"type": "lifespan.shutdown"}
+
+    async def send(message: Message) -> None:
+        sent.append(message)
+
+    scope = {"type": scope_type, "path": "/", "raw_path": b"/"}
+    asyncio.run(SecurityHeadersMiddleware(app)(scope, receive, send))
+
+    assert seen["scope"] is scope
+    assert seen["send"] is send  # the ORIGINAL send — the fast path
+    assert seen["receive"] is receive
+    # Both messages pass through untouched — no headers injected anywhere.
+    assert sent == [first, second]