---
name: debug-ui
description: Drive and visually QA the dev-3.0 UI in a real browser (headless Chromium via agent-browser). Use when verifying a UI/UX change, reproducing a visual bug, taking screenshots of the running app, or self-QA before review. Triggers — "check the UI", "screenshot the app", "does this render", "QA this screen", "verify the UI change in a browser", "drive the app".
---

# debug-ui — QA the dev-3.0 UI in a real browser

See and drive the running dev-3.0 UI in headless Chromium — click, type, screenshot, read
console errors — instead of guessing whether a UI change works. No desktop/native dependency;
it works the same in a plain terminal session.

This is **dev-internal tooling for the dev-3.0 repo** — NOT one of the skills dev3 ships to
its users (those live in `src/bun/agent-skills.ts`).

## Prerequisite: `agent-browser` on PATH

Check with `agent-browser --version`. If it is missing, install it once per machine, not in the
project `setupScript` (that runs for every new worktree):

```bash
bun add -g agent-browser        # or: npm i -g agent-browser
agent-browser install           # downloads Chromium; on Linux add --with-deps for system libraries (needs sudo)
```

## One rule before the flow: the app starts through `dev3 dev-server`, nothing else

Every QA run in this skill boots the app with `dev3 dev-server start` and takes it down with
`dev3 dev-server stop`. **Never start the app any other way** — not a bare `bun run dev`, not
`bun run dev --qa`, and not `dev3 pane run "bun run dev …"`. A pane run looks like a shortcut
and costs you everything the dev-server owns: the port wait (`--wait`), `status` with its port
conflicts, a verified stop that frees `DEV3_PORT0`, the Dev Server button in the task UI that
shows the user what is running, and the `show-image` / `attention` routing rules below. If you
need the app to behave differently (a throwaway board, an env flag), change what the dev-server
runs — see "Scoped QA" — do not route around it.

## The whole flow

This task's dev-server **is** the web UI: `bun run dev` serves the full app in local remote
mode at a stable per-machine token and a CLI-derivable port — no separate `dev3 remote`. The
loop is always the same four beats: **values → server → browser → clean up.**

```bash
# 1. Values. AGENT_BROWSER_SESSION isolates THIS task's browser from every other agent's —
#    without it all agents share one global "default" session and stomp each other (see
#    Gotchas). Derived from the always-present $DEV3_TASK_ID, so this exact line is
#    copy-paste-safe at the top of ANY block that calls agent-browser.
export AGENT_BROWSER_SESSION="dev3-${DEV3_TASK_ID%%-*}"
CODE=$(cat "$HOME/.dev3.0/dev-web-access-code" 2>/dev/null || bun scripts/dev-web-code.ts)
PORT=${DEV3_PORT0:-$(dev3 dev-server status | grep -oE 'DEV3_PORT0=[0-9]+' | cut -d= -f2)}

# 2. Start a FRESH dev-server and wait for it to come up. (Skip the start only if one is
#    already running for THIS task — but see the build-snapshot gotcha: stale code needs a
#    restart, so when in doubt restart.)
dev3 dev-server start
until curl -sf "http://localhost:$PORT/?token=$CODE" >/dev/null; do sleep 2; done

# 3. Drive it. Every agent-browser call inherits AGENT_BROWSER_SESSION, so it all runs in
#    this task's own session. (Load /agent-browser for the full command set.) The screenshot
#    path is task-scoped too, so parallel agents never overwrite each other's PNG.
#    `&streamer=on` is MANDATORY: it enables streamer mode (privacy masking), so screenshots
#    can't leak the developer's real emails/accounts/paths/tunnel URLs (see Gotchas).
#    Keep `set viewport` BEFORE `open`, and keep the width ≥ 1024 for desktop QA — the app's
#    mobile gate reads `screen.width` (see Gotchas).
agent-browser set viewport 1440 900
agent-browser open "http://localhost:$PORT/?token=$CODE&streamer=on"
agent-browser wait --load networkidle
sleep 2                                       # networkidle can still land mid-render — let it settle
agent-browser snapshot -i -d 6                # prove it's DRIVABLE, not just screenshot-able
agent-browser screenshot "/tmp/dev3-ui-${DEV3_TASK_ID%%-*}.png"   # then Read it back to look
agent-browser errors                          # confirm no console errors

# 4. Always clean up what you started. `close` closes only THIS session's browser.
agent-browser close
dev3 dev-server stop          # the port frees a second or two later (graceful shutdown)
```

That's it. `DEV3_REMOTE_PORT=${DEV3_PORT0:-0}` is wired into the repo's `dev` script and
`portCount: 1` is committed in `.dev3/config.json`, so the dev app binds the exact port shown
above (see [decision 093](../../../decisions/2026/06/30/dev-remote-port-from-pool.md)).

## Linux without a display: the dev-server goes headless

On Linux with no `DISPLAY`/`WAYLAND_DISPLAY`, or without `libwebkit2gtk-4.1` (WSL without WSLg,
SSH, containers), the desktop window cannot open. `bun run dev` detects that and serves the same
UI through `dev3 remote`, run from source, on the same port and access code. The flow above does
not change: `agent-browser` only ever talked to the web server. The pane prints
`[dev] headless (<reason>)` when this happens.

- **Headless defaults to the scoped `seeded` board**, because the real `~/.dev3.0` usually
  belongs to an installed `dev3 remote` on the same machine. `--env DEV3_QA_SCOPE=0` opts back
  into the real board.
- **Headless on request** where the window could open:
  `dev3 dev-server start --wait --env DEV3_DEV_HEADLESS=1`.
- **What it cannot show:** the native window, the application menu and OS notifications.
  Everything rendered in the page is the same code.

## Scoped QA: a throwaway board instead of the real one

`dev3 dev-server start` boots a full dev3 instance on **your real board** — another task's
"Branch Merged — mark completed?" dialog is live and clickable in your browser, and another task's
terminal is reachable by navigation. It stays the default anyway, because it is the build the user
runs; reach for the scoped board when the QA would touch real tasks, real accounts, or a live dialog
— creating or launching a task from the New Task dialog counts.

The switch is an env var the `dev` script reads (`DEV3_QA_SCOPE`, see `scripts/dev.ts` and
`scripts/qa-scope.ts`), and the dev-server passes caller-supplied variables straight through. So
the scoped board is **the same dev-server**, one flag longer:

```bash
dev3 dev-server start --wait --env DEV3_QA_SCOPE=seeded    # or virgin
```

`seeded` = one fixture project, zero tasks; `virgin` = completely empty home, the first-run state.
Steps 1, 3 and 4 of the flow above are unchanged — same port, same access code, same `stop`. The
dev-server pane prints the scoped `DEV3_HOME` and an `rm -rf` reset line; the scoped root is stable
per worktree, so the board survives a restart.

Three things worth knowing about the flag:

- **A plain `dev3 dev-server restart` stays on the scoped board** — a restart with no `--env`
  reuses the last start's env, which is also what the Dev Server button in the task UI does.
- **A plain `dev3 dev-server start` goes back to the real board.** A start defines its
  configuration whole, and a `stop` clears the remembered env — so there is nothing to delete
  and nothing left to forget about.
- **`dev3 dev-server status` names the extra keys** (names only, never values), so you can see at
  a glance which board the running server is on.

Do **not** put `DEV3_QA_SCOPE` in `.dev3/config.local.json`. That was the old recipe and its trap
is exactly what the flag removes: the file also reaches the agent sessions of this worktree, and a
forgotten one silently boots the next QA run on the throwaway board.

**What it does NOT isolate** — name these rather than assuming a clean room: the tmux socket
directory (only `TMUX_TMPDIR` moves it, and dev3 sets it nowhere), the PowerShell history file on
Windows, the user's own `~/.codex` / `~/.claude` agent configs, and `dev3` CLI commands run from
inside a REAL worktree (cwd-based task detection outranks `$DEV3_HOME` by design).

## Gotchas

- **The browser is a machine-global singleton — isolate per task or agents stomp each other.**
  Every `agent-browser` call with no session lands in one shared `"default"` session: one
  browser process, one global viewport. When two task agents QA at the same time they collide
  — agent B's `open` silently replaces agent A's page, so A's next `screenshot` captures B's
  UI. The fix is step 1: `export AGENT_BROWSER_SESSION="dev3-${DEV3_TASK_ID%%-*}"` gives each
  task its own isolated session/profile (verify with `agent-browser session` / `session
  list`), and `agent-browser close` then closes only your session. **The Bash tool
  reinitializes the shell per call, so an `export` does not carry across separate
  invocations** — the line derives from the always-present `$DEV3_TASK_ID`, so just repeat it
  at the top of each block, or pass `--session "dev3-${DEV3_TASK_ID%%-*}"` on every command.
  (If you ever must share one browser machine-wide instead, serialize QA across agents so only
  one drives at a time.)
- **The dev-server is a build snapshot — no watch/HMR.** Your code only appears after a
  (re)start. After changing code, `dev3 dev-server restart`, re-wait for the port, then
  `agent-browser reload` — a bare reload re-serves the *old* bundle. Don't keep a stale server
  around; never hand-run `vite build`.
- **Is the running build actually yours?** An already-running app/remote is often production or
  *another* worktree — it won't have your changes. (Re)start THIS task's dev-server and confirm
  with `dev3 --version` (commit hash should match `git log -1`) + that your change actually
  renders. Don't assume.
- **Tell the user before you start the app any way at all** (visible side effect), and stop it
  after (step 4) unless they want it kept.
- **`dev3` says `app not running`? Stop and tell the user.** Every route in this skill needs the
  CLI, so there is nothing to fall back to. Never launch the app yourself — a bare `bun run dev`
  (with or without `--qa`, in your shell or in a `dev3 pane run`) opens a native window on the
  user's screen that no dev-server owns, so nothing can `status` or `stop` it for you.
- **No `DEV3_PORT0`?** (portCount 0, or an older worktree where it was never allocated) — run
  your own fixed-port server instead:
  `dev3 remote --no-detach --no-tunnel --static-code $CODE --port 47823` → `:47823/?token=$CODE`.
- **Every screenshot in streamer mode — no exceptions by default.** The app shows the
  developer's REAL identity (account emails, orgs, home-dir paths, tunnel URLs, QR codes), and
  a QA screenshot easily ends up in a PR/issue/recording. `&streamer=on` in the URL forces the
  privacy masking on for the whole session (it persists in this session's localStorage;
  `&streamer=off` forces it back off). The masking is CSS blur on `.streamer-private` elements
  — see `src/mainview/streamer-mode.tsx` and decision 161. Only capture unmasked when the task
  is explicitly about those values (e.g. testing the accounts UI itself), and say so when
  presenting the image.
- **App renders but nothing clicks? That's the mobile gate, not the tooling.** The app decides
  mobile from the *physical* `screen.width` (`< 1024` → mobile; `src/mainview/hooks/useMobile.tsx`,
  deliberately not `innerWidth`), and a mobile device held in landscape gets
  `MobilePortraitGate`: a "rotate your device" overlay plus `inert` on the whole app — screenshots
  still work, clicks do nothing. `agent-browser` 0.6.0 emulated `screen.*` together with the
  viewport, so `set viewport 1440 900` before `open` was enough. **0.34.0 does not** — `screen`
  stays at the headless display's 800×600 no matter the viewport (`--window-size` in `--args`
  does not move it either), so every desktop QA run lands in the gate. Override it with an init
  script instead, registered before the first navigation:

  ```bash
  printf 'for (const k of ["width","availWidth"]) Object.defineProperty(screen, k, { get: () => 1600, configurable: true });\nfor (const k of ["height","availHeight"]) Object.defineProperty(screen, k, { get: () => 1000, configurable: true });\n' > /tmp/screen-desktop.js
  agent-browser close                      # a running session keeps its old init scripts
  agent-browser set viewport 1600 1000
  agent-browser open "http://localhost:$PORT/?token=$CODE&streamer=on" --init-script /tmp/screen-desktop.js
  ```

  With that, `screen.width` reports 1600 and the gate stays off. You otherwise only hit the gate
  by asking for a phone-sized landscape viewport — that is the app working as designed.
  Measured across the common sizes: `2560×1440`, `1920×1080`, `1600×900`, `1440×900`, `1366×768`,
  `1280×720`, `1024×768`, `768×1024` and phone portrait `390×844` all render and click fine
  (`screen.width` always equals the requested width); only landscape `844×390` shows the gate, goes
  `inert`, and reports **zero** interactive elements in `snapshot -i` — that count is the fastest
  tell. Diagnose in one call:
  `agent-browser eval "JSON.stringify({sw:screen.width,gate:!!document.querySelector('[data-testid=\"mobile-portrait-gate\"]'),inert:!!document.querySelector('[inert]')})"`.
  If a future browser tool pins `screen.*` to the real headless display and desktop QA goes inert,
  that's the moment to add an escape hatch to the app — not before (see
  [the headless-QA decision](../../../decisions/2026/08/14/headless-qa-mobile-gate-stays-as-is.md)).
- **No native dialogs** in browser mode. If a confirm/file-picker flow silently no-ops, that's
  an app bug, not a tooling problem — report it.
- **Show images to the user AFTER you stop this dev-server, not while it runs.**
  `dev3 show-image` (and `dev3 attention` / `dev3 notify`) route to the app instance that owns
  the task — and this QA dev-server is itself a full dev3 instance. While it's up, those
  UI-attention calls can land in *it* (the browser only you see) instead of the user's main
  app, so the user never sees the image. Screenshots persist on disk, so the correct order is
  **capture → `dev3 dev-server stop` (confirm `State: stopped`) → `dev3 show-image`**.