The full upstream README, mirrored here for reference. Install config, tool schemas, adoption signals, and an original overview live on the Ui Debugger MCP listing page.
An MCP server that debugs UIs autonomously — so the AI that wrote your app can also test it, without a human clicking through every flow.
AI coding agents (Claude, etc.) are great at writing code. They're bad at knowing if the UI actually works. For backend code there are unit and integration tests. For UI, a human still has to open the app, log in, click around, and report what's broken. That human-in-the-loop is slow, boring, and the main bottleneck when an entire product is built by AI.
Eliminate the human from the UI-debug loop with an MCP server.
Unlike playwright-mcp — where the smart model issues every single click itself — here the smart model stays high-level and delegates the whole clicking loop to the small agent.
| playwright-mcp | UI Debugger MCP | |
|---|---|---|
| Who clicks | smart model, one action per call | small agent, on its own |
| Tools exposed | many (click, type, snapshot…) | few (give a story, get findings) |
| Smart model cost | high (chatty) | low (high-level) |
| Output | raw page state | structured findings + evidence |
Picture a boss, a fast blind driver, and a describer with eyes:
look to ask
"does this look right? is the button centred?" and gets a description back.
Default: glm. Spent only when visual judgment is needed.One goal: the UI works and looks nice. Full design in docs/idea/.
Every run keeps its screenshots and stitches them into a short captioned
replay video — Claude attaches it to the PR so a reviewer sees the flow working
in ~10 seconds (docs/idea/workspace.md).
One project can expose several debug targets. A large app can have all three:
| Target | Protocol / how it's driven | Reads |
|---|---|---|
| web | CDP (Chrome DevTools Protocol), headless by default | DOM |
| desktop | X11 / Wayland input + AT-SPI | a11y tree / vision |
| mobile | ADB (uiautomator + screencap), Android, headless by default | view hierarchy / vision |
Three adapters, one shared contract. Each runs managed (server launches the
target) or attach (connect to a running one via cdpUrl / adbSerial).
Linux first. iOS is out of scope on Linux (macOS-only tooling).
A managed android run boots the emulator windowless — -no-window with
-gpu swiftshader_indirect -no-audio, because -no-window alone still opens a
host GL context and dies on a box with no display. Set "headless": false on
the target to watch it in a window; append anything else with emulatorArgs.
Install like any local MCP server — one entry in your .mcp.json:
Wiring more than one repo? .mcp.json is usually committed, so don't paste the key
into each one — export it once in your shell and reference it, which keeps every
repo's entry identical and secret-free:
It's also published in the official MCP Registry as
io.github.developerz-ai/ui-debugger-mcp — any client that browses the registry (instead of a
hand-written .mcp.json entry) can find and install it by that name.
Then add a per-project config describing the app to debug (models, targets,
urls) — .dz/ui-debugger/ui-debugger-mcp.json if your repo keeps agent config
under .dz/ (checked first), else the root .ui-debugger-mcp.json (legacy).
The fastest way is the init command:
ui-debugger-mcp init scaffolds a project for debugging (described in
docs/idea/config.md):
./tmp/ui-debugger-mcp/web target stub) if
one doesn't already exist — at .dz/ui-debugger/ui-debugger-mcp.json when the
repo already has a .dz/ dir, else at the root .ui-debugger-mcp.jsontmp/ to .gitignore.mcp.json snippet to paste (it never writes your API key)Config files:
.mcp.json → how to launch the server (command + secret key). Gitignored..dz/ui-debugger/ui-debugger-mcp.json (or root .ui-debugger-mcp.json) →
how to debug this app (models, targets). Committed. When both exist the
.dz/ copy wins and the tool prints a notice naming the ignored root file.Every key of both files is documented in docs/idea/config.md;
every tool's exact input/output shape is in docs/idea/mcp-tools.md.
The server reads the current directory to pick the project session — open it in your repo and it debugs that repo.
This creates ./tmp/ui-debugger-mcp/, writes a starter config (.dz/… or root),
and prints the .mcp.json snippet to paste.
It's a conversation, not a remote control — five fat tools, not one-per-click:
| Tool | What it does |
|---|---|
start_debug | Open a run: { target, goal, url?, as?, replace?, criteria?, timeout? }. url is required when the target has no configured url; as names a login persona; replace: true takes over a project that already has a run. The small agent drives autonomously. Returns { session_id }. |
get_findings | Poll status + structured findings (functional bugs + visual issues) + evidence. Long-poll with wait. A whole-object read caps each list at 20 and says so in truncated ({returned, total} per field) — pass fields for the full lists. |
send_message | Talk to the running agent mid-flight — add work, redirect, or answer a question. |
describe | List the configured targets + models for this project — including each target's auth personas[] and notes — plus session: the run this project currently holds. |
end_session | Close the run, free the browser/profile. |
A run is always time-capped: start_debug's timeout (seconds) overrides the
default 300s, so a session can never hang forever — it auto-ends and frees the
profile lock when the cap fires.
Almost every interesting screen is behind a login, and re-explaining the login
flow in every goal string costs driver steps before the real goal starts. Say it
once, in the target:
The run signs in out-of-band, before the driver's first step: it costs zero
driver steps, and the credentials never enter the model's context — the prompt
only learns "you are already signed in as admin". Values are redacted out of
every log the run writes; describe reports persona names only. A typo'd as
fails loud listing the valid names, and a login that does not take fails the run
rather than handing back a session that will report every page behind it as empty.
Use dev/seed credentials — this file is committed. Details in
docs/idea/config.md.
notesA freshly-migrated app with no seed data renders empty states everywhere, and the driver reports "the table is empty" as the run's headline bug. It is right, and it is useless. Say what's expected once, in the target:
One fact per line, on any target (web, desktop, android). Unlike the per-run
goal, notes is config: it is composed into the driver's prompt once, as its
own "Known about this app" section — treat these as expected, never report one
as a defect, and a screen that contradicts one is still worth reporting.
Capped at 1000 characters, enforced when the config loads (the prompt carrying it
is resent to the model on every step, so an essay here is paid for per step).
One run per project (cwd), so a forgotten end_session used to wedge it. It
doesn't any more: describe reports the run this project holds
(session: { id, status, goal }), so a caller that lost its session_id — a
compacted context, a restarted client — can read it, close it, or take it over
with start_debug({ …, replace: true }). Being refused is still the default, and
the refusal spells out all three calls; a silent takeover would kill a healthy run
somebody else is watching.
Every tool result carries both a pretty-printed text block and a typed
structuredContent payload validated against a declared outputSchema — parse
the structured half, don't scrape the text. Tools also declare MCP annotations
(readOnlyHint, destructiveHint, idempotentHint, openWorldHint) so clients
can render/gate them correctly, and evidence paths (screenshots, replay.mp4,
logs) ride as resource_link content items, not inline strings. Full shapes in
docs/reference.md.
Typical loop from a smart agent:
You can also drive it headless from a script with claude -p — see
docs/claude/SKILL.md for the CLI recipe (MCP config,
allowed tools, output formats).
You get findings, but the agent's evidence is what makes them actionable:
| HTTP | Whole exchanges — method, url, status, durationMs, and for fetch/xhr the request and response bodies plus headers. A 4xx carries the server's own reason ({"error":"password too short"}), not just a status. Credential header values are redacted to <redacted, N chars>: presence stays diagnostic, the secret never reaches the model, the logs, or your transcript. |
| DOM | Roles, names, bounds, live value/checked on form controls, data-testid, and WCAG contrast per text node — so "is the box ticked?" and "is this text readable?" are answered structurally, without spending vision. |
| iframes | Embedded documents are read too. Nodes carry frame, and their bounds are page coordinates, so clicking works the same as anywhere else. |
| Tabs | A target="_blank" click is noticed and followable, instead of reading as "nothing happened". |
| Console | Errors and uncaught exceptions, with source locations. |
| Frames | An ordered screenshot per action, stitched into replay.mp4. |
Static assets are held back from a default network read — on a Vite dev server they outnumber real API calls ~15:1 — with a count of what was hidden and the filter to see them. Failed requests are never hidden, whatever their type.
The ui-debugger-mcp binary doubles as a control CLI for the active run
(reads state.json, no API key needed):
Chrome not found
The web adapter launches Chrome via the system PATH. Install Chrome/Chromium, or
set executablePath in the project config (.dz/… or root):
Session locked — "another run is active" One Chrome profile = one run. If a previous run crashed without cleaning up:
Or delete ./tmp/ui-debugger-mcp/<project>/state.json and restart the MCP server.
Run times out with no findings Default cap is 300 s. Raise it per-call:
If the agent is stuck at login, add ?debug-ai=true to your app's login route
(gated by ALLOW_AI_DEBUG_LOGIN) to skip captchas — see CLAUDE.md for the
pattern.
get_findings returns empty bugs[] / visual[]
The run may still be in progress — use wait (ms) to long-poll:
Check ./tmp/ui-debugger-mcp/<project>/sessions/<id>/logs/agent.log for the agent's trace.
Session ids are local timestamps (2026-07-27_14-30-05-0001), so the newest run
sorts last. Only the 5 most recent sessions are kept — each new run prunes the
rest, so evidence from a run six ago is gone by design.
Run fails instantly: "… is not a valid model ID"
The model string in the project config is not a catalog id. OpenRouter takes
provider/model with optional :floor / :nitro routing suffixes — a #…
suffix is rejected outright. Use plain ids (deepseek/deepseek-v4-flash).
Still running an old version after upgrading
npx -y <pkg> REUSES a cached install when one exists, so a bare
"@developerz.ai/ui-debugger-mcp" in .mcp.json keeps booting whatever version
first landed in ~/.npm/_npx — with no signal that anything is stale. There is no
auto-update: nothing in the server checks the registry. Pin @latest in your
args (what init now prints) so every cold start re-resolves:
Already stuck? rm -rf ~/.npm/_npx, then reconnect the server (/mcp in Claude
Code). Check what you're on with npx @developerz.ai/ui-debugger-mcp@latest --version.
Config edits seem to have no effect
Config is read ONCE when the MCP server starts, so an edit cannot reach a running
server. Since v1.3 the server refuses to start a run on a changed file and tells
you to reconnect it (in Claude Code: /mcp → reconnect ui-debugger) rather
than silently running the old settings.
"Another server already has a session" and you can't see it
That run belongs to a different MCP server process on the same project. You can
now READ it — get_findings { session_id: "<the id in the error>" } serves its
on-disk findings — so you can tell a live run from one that settled and is just
waiting to be closed. To take the project over: ui-debugger-mcp stop.
Run ends failed with no bugs and no explanation
That means the driver never called report — it used its whole step budget.
The summary says so explicitly; it is NOT a crash and NOT a clean pass. Narrow
the goal, or raise timeout. Read logs/agent.log and the screenshots for what
it actually did.
replay.mp4 not generated
ffmpeg is optional. Install it and retry, or ignore — findings and screenshots
still land without it.
npx/bunx)All three adapters ship in v1:
| Target | State |
|---|---|
| web | ✅ shipped (CDP, headless + attach) |
| desktop | ✅ shipped (X11/Wayland, AT-SPI + xdotool) |
| android | ✅ shipped (ADB, uiautomator) |
Replay video (replay.mp4, captioned stills → mp4 via ffmpeg) ships with the web adapter.
ffmpeg is optional — absent gracefully, findings still land.
See docs/idea/ for design notes.
docs/idea/overview.md — problem + ideadocs/idea/architecture.md — system designdocs/idea/adapters.md — adapter contract + targetsdocs/idea/desktop-control.md — Linux control tooling (X11/Wayland/mobile)docs/idea/agent-loop.md — the story → findings loopdocs/idea/mcp-tools.md — two tool layers, SQL-like params, in-repo promptsdocs/idea/models.md — the three actors (smart agent / fast guy / vision guy)docs/idea/config.md — config filesdocs/idea/workspace.md — per-project space + logsdocs/claude/SKILL.md — driving claude as a headless CLI tool (generic)CLAUDE.md — instructions for AI agents working on this repoai-task-master — build template (orchestrator + subagents), reference repo, not publishedgold-standards-in-ai — MCP & code conventions, reference repo, not publishedclaude-code-bible — agent-first patterns (sebyx07/claude-code-bible)