chart/AGENTS.md

# Working on this repo

## Read current context first

Before planning work, read [`docs/plan.md`](docs/plan.md) for the decisions
and the reasoning behind them, and
[`docs/implementation.md`](docs/implementation.md) for the dated record of what
actually went wrong and how it was resolved — that one is the faster read when
debugging, because most entries describe something that looked like one bug and
turned out to be another. Then
[`docs/NEXT_STEPS.md`](docs/NEXT_STEPS.md) for current recommendations and known
deferred fixes. Mobile interaction work also has its own detailed plan in
[`docs/mobile_enhance.md`](docs/mobile_enhance.md). The CDN-to-Vite move is
[`docs/vite_build.md`](docs/vite_build.md). Light/dark theme constraints are
[`docs/plan_light_dark_themes.md`](docs/plan_light_dark_themes.md).
Daily MA alert toggles are [`docs/plan_dma_alerts.md`](docs/plan_dma_alerts.md).

## Tests earn their place by catching a real bug

When a bug is found, ask whether a unit test could reasonably have caught it. If
yes, write that test with the fix. If no — a rendering artefact, a browser
quirk, a data-source oddity — say so and don't add one.

The bar is "would this have failed before the fix, and would it fail again if
someone reintroduced it". Tests that restate the implementation, assert
constructor defaults, or exercise paths nothing depends on are noise; they make
the suite slow to run and expensive to change, which is how a suite stops being
trusted.

What has actually paid off here: bar aggregation and bucket boundaries, the
store's replace-vs-append rules, level and alert arithmetic, parsing real
market-data payloads (fixtures are trimmed real responses, not invented), and
the invariants that would otherwise be silent — a comment must never become a
level, a tick must never overwrite a settled bar, volume must be counted once.

Name the test after the failure, not the function: `test_a_tick_cannot_overwrite
_a_settled_bar` beats `test_put`.

## Where things run

**The agent works on a remote machine over SSH. The user's browser runs on a
different machine.** Consequences, all learned the hard way:

- You cannot see the user's screen, console, or cursor. Screenshots and pasted
  console output are the only window into it. Browser extensions that drive
  "your" Chrome do not help — they attach to the machine the browser is on.
- Headless Chromium here renders on server hardware: different screen, window
  size and device pixel ratio from the user's. "Works in my headless run" is not
  evidence that it works for them. When a UI bug will not reproduce, match their
  viewport and `deviceScaleFactor` explicitly before concluding anything.
- The dev stack is served to them over the network (e.g. `hera.local:8010`),
  which is the same app the headless browser reaches as `http://api:8000`.

When a visual bug resists reproduction, prefer putting the numbers **on screen**
in the app over asking for another console paste — one screenshot then carries
the whole diagnosis.

## Verify UI in a real browser

Chart bugs are invisible from the outside — the API, the socket and the
frontend source can each be correct while the screen is wrong. Drive the
Playwright container against the dev stack:

```
docker exec -i chart-playwright-1 node - <<'EOF'
const { chromium } = require('/usr/lib/node_modules/playwright');
// launch with args:['--lang=en-US'] — see below
EOF
```

**Always launch Chromium with `args: ['--lang=en-US']`.** The container has no
usable locale, so Chromium reports `en-US@posix`, `Intl` throws, and the chart
renders as a blank canvas that looks exactly like a broken app.

`window.__chart` is a deliberate debug handle. Querying it separates "the data
is missing" from "the data is off-screen" — which is how a viewport bug that
three passing API checks had missed was finally found.

## Diagnostic mode

Chart geometry bugs live in the browser, which is usually on a different machine
from whoever is debugging them. Rather than asking for console pastes:

```
open the chart with ?diag=1          # remembered until ?diag=0
docker compose logs api | grep SNAPDBG
```

With it on, every snap the trendline tool computes is posted to
`/api/debug/snap` and logged server-side — the cursor's time, price and x, the
snapped time and price, how many bars were held, the first and last bar, and the
chart's width. Throttled to about one a second. It reads the client's own
numbers, which is exactly what "works in my headless run" cannot tell you.

Extend it when the next geometry puzzle appears; the endpoint takes whatever
fields `SnapReport` declares.

`?diag=1` also exposes **Capture diagnostic**. The uploaded PNG URL at
`/api/debug/captures/{id}` is deliberately public: its 72-bit id is the
handoff from a browser to an agent on a different machine. Capture upload and
metadata remain authenticated. Inspect only a URL the user explicitly shares,
then immediately `DELETE /api/debug/captures/{id}`. The server also expires
captures after 24 hours and caps the directory at 50 files.

After the browser's required share picker closes, capture waits five seconds so
the user can restore a hover tooltip. `Alt+Shift+C` starts the same delayed flow
without clicking the status-bar button.

Diagnostic mode also shows a compact projection readout for visible manual
trendlines: historical/future canonical price changes, their screen slopes, and
whether the future canvas point exists. Include it in a capture when a line
looks kinked at the live edge; it separates bad geometry from a bad renderer.

## Future whitespace is a high-risk boundary

Drawing bugs repeatedly appear to the right of the last real candle. Treat any
change involving future slots, projection, drawing movement, or selection as a
geometry change that needs explicit browser verification.

- The displayed time scale, a drawing's source-timeframe bar space, and the
  server session calendar are different coordinate systems. Never substitute
  wall-clock seconds or the displayed grid for canonical source geometry.
- Daily future slots must skip non-session days. Intraday future slots cross the
  settlement break and weekend today; an endpoint that looks valid before the
  break can become unresolved when real history arrives.
- Do not clamp a future click to the last real candle. Do not let one null or
  unpriceable future sample disable an otherwise valid object's whole hit target.
- DOM/SVG overlays must use the chart's actual future coordinates. Extrapolating
  from the last two real candles is wrong when sparse-gap slots were inserted.
- Audit every drawing type, not just trendlines: Fibonacci hit testing and body
  targets, pinned comments/symbols, keyboard nudges, duplicate, cutoff, handles,
  selection glow, and drag persistence have separate paths.
- Tests must cover placement, selection, body drag, endpoint drag, duplicate,
  nudge, and cutoff beyond the live edge and across session boundaries. Several
  older future-interaction E2E tests remain quarantined against mutable live
  state; a skipped test is not protection.

Relevant history is concentrated near the trendline/future entries in
`docs/implementation.md`. Before fixing another symptom, measure timestamps,
logical/x coordinates, canonical prices, and painted pixels in the user's
viewport; self-consistent chart API numbers have missed real rendering bugs.

## Keep the two documents current

This is a running system under continual change, not a build being executed, so
both live documents decay unless updating them is part of finishing the work —
not a tidy-up afterwards.

- **`docs/plan.md`** — when a decision changes, change it here. A plan that
  contradicts the code is worse than no plan, because someone believes it. If
  you find a section describing behaviour that no longer exists, that is a bug
  in the document; fix it in the same commit that revealed it.
- **`docs/implementation.md`** — append when a fix was not obvious. The bar is
  "would this have saved me an hour": wrong theories that were measured and
  killed, the evidence that settled it, the thing that looked like one bug and
  was another. Not every fix. A log of trivia stops being read, and then the
  useful entries go unread too.

Rule of thumb: if you needed a measurement to be sure, write down what it was.
Git records what changed; these record why it was hard.

## Direction of travel

Two live planning documents, both written to be refactored toward rather than
implemented in one go:

- `docs/async_refactor.md` — nothing blocks the event loop. P0 and P1 are done;
  `/api/status` reports `loop_lag_ms`, and a rise there is the signal.
- `docs/multi_user.md` — separate people with their own drawings and alerts,
  authenticated by OIDC. Read it before adding state to `Runtime`: new state is
  either genuinely shared (market data) or belongs to a user, and knowing which
  now is much cheaper than untangling it later.
- `docs/vite_build.md` — pin and hash the frontend, stay on Coolify, do not
  split components on the way. A production Dockerfile first, then Vite;
  never a root `package.json` while nixpacks is still the builder.

Do not build local user accounts. The destination is OIDC, so password storage
would be written and then deleted.

## Running tests

```
docker exec chart-api-1 sh -c "cd /app && python -m pytest -q"
```

pytest + pytest-asyncio, declared in `requirements-dev.txt`. Tests live in
`tests/`, import from `app.*`, and use `tmp_path` for anything that persists.
Async paths are driven with `asyncio.run(...)` directly rather than async test
markers.

## Things that will cost you an hour

- **Never write scratch `.py` files into the repo root.** It is bind-mounted, so
  `--reload` restarts the app, and startup takes ~82 seconds. Pipe throwaway
  scripts over stdin instead: `docker exec -i chart-api-1 python - <<'EOF'`.
  Screenshots into `artifacts/` are safe; only `.py` triggers the reloader.
- **Dev and production keep separate drawing stores.** Dev writes
  `data/manual_lines.json`; production has its own Coolify volume. A fix that
  "didn't land" is often the other store.
- **Rebuild the image after touching `requirements.txt`.** The bind mount makes
  source edits look live while an added dependency is simply absent.
- **A deploy resets alert cooldowns**, so production may re-alert on whatever
  price is sitting on. There is no durable state yet.
- Times are epoch seconds, UTC, everywhere. Only the display is localised —
  never shift the stored values.
- **Do not hardcode how many bars a client gets.** A leftover `1000` on the
  snapshot made 1m look empty past ~1am while the store held 5,000. The
  store cap is `max_bars_per_tf`. The next step is a visible-window fetch,
  not another silent number.

## Stay cheap

Performance is a product feature, not a later cleanup. The app already
pays for a live stream, 5k bars, and a canvas. New work must not add
cost on the hot path unless the screen or an alert has to change.

- Nothing extra on the event loop each closed bar or tick. Watch
  `loop_lag_ms`. CPU stays in the threadpool or off the loop — see
  `docs/async_refactor.md`.
- Do not grow a payload because it is easier than asking what the
  client needs. Caps are named settings, not leftover literals.
- Crosshair move is for the cursor (OHLC, drawing tooltip). Do not
  rebuild overlays that only depend on the viewport.
- Overlay redraws go through `scheduleOverlays`. Force when data or
  size changed; let the range-key skip no-ops.
- `window` pointermove/up attach only while a tool is armed or a drag
  is live. Do not leave them on for the life of the page.
- Prefer one rAF over N DOM rebuilds. Read `offsetWidth` only if the
  layout actually changed.
- A feature that needs a per-frame or per-tick loop needs a reason,
  and an off switch.