mirror/.agents/PLAN.md

# mirror — Master Plan

> **This file is the source of truth.** Anything marked `LOCKED` may not be
> changed without an explicit decision recorded here first. Phases implement
> this plan; they do not revise it.

## 1. Vision

Mirror a screen from any device that has a browser, and watch every screen that
anyone is mirroring, from any other device — with nothing to install and nothing
to sign in to.

Day one, one command:

```bash
uv run mirror serve
```

* `/mirror` — pick a window, screen, or tab using the **operating system's own**
  picker, and mirror it.
* `/watch` — see every live mirror as a wall of tiles; click one to take it over
  the whole screen.
* N sharers and M watchers at the same time, in any combination.

The single thing that must never regress: **a person on a Linux laptop, a person
on a phone, and a person on a work machine can all join by opening a URL.**

### Design principles

1. **The browser is the whole client.** No native helper, no extension, no
   WebAssembly to fetch, no build step for the frontend.
2. **Media never touches the server.** The server is a roster and a signaling
   relay. It handles JSON, always small, never frame data.
3. **Linux is the reference platform.** X11/Xorg is the guaranteed path; the
   Wayland portal path is supported where the compositor provides it. Every
   capture-related change is verified on Linux before anything else.
4. **Zero-configuration on a LAN.** STUN/TURN are opt-in, not defaults: peers on
   one network reach each other with host candidates, and shipping STUN by
   default would leak internal addresses to a third party.
5. **The prototype is the seed, not a spike.** Code in `src/mirror/` is
   production-shaped: typed, linted, covered, and gated by the same test suite
   future phases must keep green.

## 2. Architectural anchors

| # | Component | Decision | Rationale | Status |
|---|-----------|----------|-----------|--------|
| A1 | Language / runtime | Python **3.13+** (developed on 3.14), `uv` for everything | Matches the estate; `uv` is the only installer | **LOCKED** |
| A2 | HTTP + signaling | **FastAPI + uvicorn[standard]**, single process | Native WebSocket support, typed boundary, one dependency tree | **LOCKED** |
| A3 | Media transport | **WebRTC, mesh, one `RTCPeerConnection` per viewer** | Hardware codecs, sub-100ms LAN latency, no server media handling; server stays a roster | **LOCKED** |
| A4 | Who negotiates | The **sharer is always the offerer**; the viewer only answers | Removes renegotiation glare; the sharer owns the track set | **LOCKED** |
| A5 | Wire format | **JSON over one WebSocket** at `/ws`, camelCase keys, discriminated on `type` | One connection per peer, trivially debuggable with `websocat` | **LOCKED** |
| A6 | Validation | **Pydantic models with `alias_generator=to_camel`** and `extra="ignore"` | camelCase on the wire, snake_case in Python. Explicit aliases are mandatory: silent field-dropping is a bug we already shipped once | **LOCKED** |
| A7 | State | **In-memory roster only** (`mirror.registry.Registry`). No database, no ORM, no Alembic | A mirror that outlives the process is a lie: the peers are gone. Persistence would add a failure mode and remove none | **LOCKED** |
| A8 | Domain/transport split | `Registry` is pure and returns `list[Envelope]`; `Hub` owns sockets | The whole admission/teardown policy is unit-testable without a socket | **LOCKED** |
| A9 | Frontend | **Hand-written ES modules**, no framework, no bundler | Two pages and one WebSocket do not earn a build pipeline; `node --check` is the whole toolchain | **LOCKED** |
| A10 | Assets | **No external CDNs, fonts, or analytics.** Everything served from `/static` | Works offline on a LAN; nothing phones home; nothing to time out | **LOCKED** |
| A11 | Authentication | **None**, deliberately | Trusted-LAN tool, per the product brief. Threat model lives in `SECURITY.md` | **LOCKED** |
| A12 | Configuration | Environment variables (`MIRROR_*`) + optional `.env` | Twelve-factor, container-friendly, no config file format to invent | **LOCKED** |
| A13 | E2E testing | **Playwright + full Chromium** (`channel="chromium"`), headless, with a real frame-content assertion | See §5. The assertion is on decoded pixels, never on log lines | **LOCKED** |
| A14 | Linux-native capture test | Separate `realdisplay` marked suite: private **Xvfb** + headed Chromium pinned to `--ozone-platform=x11` | Proves the real capture backend without touching the developer's session | **LOCKED** |
| A15 | CLI | `argparse`, single `serve` subcommand, flags also accepted bare | One command; no dependency for a two-option CLI | **LOCKED** |
| A16 | Quality gates | `ruff check` + `ruff format --check` + `pyright` (0 errors) + `pytest` green, always | Non-negotiable before a phase is called complete | **LOCKED** |
| A17 | Scale-out beyond a mesh | *Proposed*: WebSocket/MSE relay or WebCodecs SFU when a sharer's uplink, not the CPU, is the limit | Revisit only with a measured problem. Status: **PROPOSED** | Proposed |
| A18 | TLS | *Proposed*: `mirror serve --https` with an auto-generated self-signed cert | Solves the secure-context problem (§6) for LAN IPs without a CA. Status: **PROPOSED** | Proposed |

## 3. Architecture

```
   sharer browser                                   watcher browser(s)
  ┌───────────────┐                              ┌───────────────────────┐
  │ /mirror       │                              │ /watch                │
  │ getDisplayMedia│                             │ tile per mirror       │
  │  ↓ track      │                              │  ↑ ontrack → <video>  │
  │ sendonly      │◄════════ WebRTC (SRTP) ═════►│ recvonly              │
  │ transceivers  │      LAN, direct, per-viewer  │ transceivers          │
  └───────┬───────┘                              └───────────┬───────────┘
          │ ws /ws (JSON only)                    ws /ws     │
          ▼                                                ▼
  ┌────────────────────────────────────────────────────────────────┐
  │ FastAPI  ── Hub (sockets)  ── Registry (pure roster)           │
  │            /api/streams · /api/config · /healthz · /static     │
  └────────────────────────────────────────────────────────────────┘
```

### Signaling sequence (the whole protocol)

```
sharer → hello{role:"sharer",name}      server → welcome{peerId,streams[]}
sharer → publish{label,width,height,displaySurface,audio}
                                        server → stream_added   (to every viewer)
viewer → subscribe{streamId}            server → peer_joined{viewerId}  (to sharer)
sharer → signal{to:viewer,sdp:"offer"}  server → signal{from:sharer}
viewer → signal{to:sharer,sdp:"answer"} server → signal{from:viewer}
          … ICE candidates both ways, same envelope …
sharer → unpublish                      server → stream_removed (to every viewer)
```

`streamId == sharer peerId`: one browser tab captures one surface, so a stream
and its publisher share a lifetime and an identity. Teardown therefore cannot
leak.

### Roster rules (enforced in `registry.py`, tested in `tests/test_registry.py`)

* A stream exists exactly as long as its sharer's socket does.
* A `signal` frame is relayed **only** between a viewer and the sharer of a
  mirror that viewer subscribed to — in either direction. Viewer↔viewer and
  unsolicited sharer→viewer traffic is refused (`not_subscribed`, `peer_to_peer`).
* Teardown is broadcast to **every** viewer, not merely the subscribers: a tile
  is drawn on `stream_added`, so a viewer that has not yet subscribed still has
  something on screen that must be cleared.
* Every exit path (explicit `unpublish`, clean disconnect, crash, replaced
  stream) emits the same notification set. No path may leak an
  `RTCPeerConnection`.
* A repeat `hello` renames the peer and refreshes the mirrors that display it.

## 4. Data model

No schema, no migration: the state is a live graph.

```
Peer   { id, role ∈ {sharer, viewer}, name, device, joined_at }
Stream { id == sharer_id, label, display_surface, width, height, audio,
         viewers: set[viewer_id], created_at }
```

Stream/viewer state transitions:

```
mirror:   (absent) ──publish──► advertised ──subscribe──► advertised+watching
                          ▲                                    │
                     republish(same)                     republish(changed)
                          │                                    │
                     unchanged: metadata only            re-offer to each viewer
        any of: unpublish · sharer disconnect · capacity eviction ──► (absent)

viewer leg: idle ──subscribe──► negotiating ──► connected ──► closed
                                    │ failed (×3, backoff) ──► unreachable
```

Roles are **exclusive per socket** in the prototype: a tab is either sharing or
watching. Making a peer able to do both is a phase, not an accident waiting to
happen.

## 5. Verification workflow

The gate, in order, always:

```bash
uv run ruff check . && uv run ruff format --check .
uv run pyright                       # 0 errors
uv run pytest                        # unit + API + browser E2E
MIRROR_REALDISPLAY=1 uv run pytest -m realdisplay   # Linux-native capture
```

### The E2E ladder — why each rung exists

| Rung | Command | What it can prove nothing else can |
|------|---------|------------------------------------|
| 1. Roster | `pytest tests/test_registry.py` | Admission, fan-out, authorization and teardown policy, without any I/O |
| 2. Wire | `pytest tests/test_protocol.py tests/test_app.py` | camelCase round-trip, malformed-frame handling, the WS lifecycle |
| 3. Media | `pytest -m e2e` | **Real capture → encode → SRTP → decode → painted pixels**, two Chromium processes |
| 4. Real desktop | `MIRROR_REALDISPLAY=1 pytest -m realdisplay` | The **actual X11 capture backend** on a private Xvfb display |

### Browser-harness findings (do not rediscover these)

* `navigator.mediaDevices` is **undefined on `about:blank` and on plain-HTTP
  non-localhost origins**. E2E must go through `http://127.0.0.1:<port>`, which
  *is* a secure context.
* Playwright's default `chromium` **headless shell has no media stack at all** —
  `navigator.mediaDevices` is absent. Use `channel="chromium"` (full binary,
  new headless).
* `--use-fake-ui-for-media-stream --use-fake-device-for-media-stream` gives a
  deterministic **1280×720 `displaySurface: monitor` stream with genuinely
  moving pixels** — no stubbing, and `distinct frames > 1` is a real assertion.
* For **real** capture: `--auto-select-desktop-capture-source="Entire screen"`
  plus `--ozone-platform=x11`. Passing `--use-fake-ui-for-media-stream` *instead*
  breaks the X11 capturer (instant `NotReadableError`), because it approves the
  request without ever selecting a source.
* Chromium scales a monitor capture to the capturing window, so real-capture
  tests assert **aspect ratio**, not absolute pixels (`1280×800 → 1200×750`).
* A headed Chromium that inherits `WAYLAND_DISPLAY` or
  `DBUS_SESSION_BUS_ADDRESS` will route capture through **xdg-desktop-portal**:
  it mirrors the developer's real desktop and raises a permission dialog on their
  actual screen. `browser_env()` strips both so that is structurally impossible.

### Rules for any new assertion about media

1. Assert on the **viewer's** `<video>` (`videoWidth`, advancing pixel hashes,
   `framesDecoded`), never on the sharer's preview — the preview proves nothing
   crossed the wire.
2. Prefer a content assertion (frames differ) over an event assertion
   (`ontrack` fired).
3. Where a test drives motion, drive it deliberately (mutate the captured page)
   rather than hoping a desktop animates on its own.

## 6. Known constraints (accepted, documented, not bugs)

| Constraint | Why | Handling |
|---|---|---|
| `getDisplayMedia` needs a secure context | Browser rule. `http://<lan-ip>` is not one | `/mirror` detects it and prints the three ways out (localhost, HTTPS, the per-origin Chrome flag). Real fix is A18 |
| Mesh upload = N × bitrate per sharer | WebRTC mesh | Per-leg `maxBitrate` cap (default 12 Mbit/s). A17 when it bites |
| Wayland capture depends on the portal | Compositor policy | Each share is confirmed by the user; X11 needs no such prompt. Documented in `SECURITY.md` |
| No reconnection of a *sharer* identity | Peer ids are per-socket | Viewer clears the tile and the sharer re-publishes on reconnect; the wall recovers by itself |
| Single process, in-memory | A11/A7 | One roster per process. Multi-process needs a shared roster + a way to pin a stream to its peer — a real phase, not a config flag |

## 7. Candidate phases (deliberately **not** yet written as phases)

Ordered by what a user would notice first. Each would become
`.agents/phases/todo/NN_name/{00_phase.md,NN_task.md}` via the `phase-authoring`
skill, with its own independently-runnable test suite.

1. **HTTPS mode** — self-signed cert + a "how to trust it" screen (A18). Unblocks
   capture from `http://<lan-ip>` on other devices, the single biggest papercut.
2. **Share-page resilience** — reconnect with roster resync, "source lost"
   states, `surfaceSwitching` UX, per-leg health on the sharer panel.
3. **Wall UX** — keyboard navigation (j/k/Enter/Esc), per-tile mute, layout
   memory, "auto-stage newest mirror", PiP.
4. **Capacity & fairness** — adaptive bitrate per leg by watcher count,
   `MIRROR_MAX_MIRRORS` surfaced in the UI, graceful degradation for a weak
   sharer uplink.
5. **Scale-out media path** — the relay/SFU evaluation in A17, decided with
   measurements from a 1-sharer/20-watcher LAN test.
6. **Roles beyond sharer/viewer** — one socket that shares and watches; a device
   naming/consent layer that keeps A11's threat model intact.
7. **Ops** — `/metrics` (Prometheus text), structured logging, `Containerfile`
   hardening, systemd unit for the always-on kiosk box.

## 8. Definition of done (per phase)

* All four gates in §5 green, including `-m e2e`.
* No new external asset, no CDN, no new dependency without a row in §2.
* Any new media behaviour has an assertion on the **viewer** side.
* `AGENTS.md` and this file updated if an anchor moved — with the reason.