agy-ui-mcp

An MCP (Model Context Protocol) server that delegates frontend / UI work to Google Antigravity's agy CLI (Gemini) - while guaranteeing the agent never touches backend, API, or business logic. It is designed to be shared by Claude Code and Codex as a dedicated "FE/UI worker".

The server exposes two tools:

• ui_implement - an iterative vision loop: screenshot the running app, prompt agy to edit CSS/components toward the target design, diff-gate the result to revert anything out of scope, re-screenshot, and repeat until it converges (or hits max_iters). Edits are applied to your working tree.

• ui_review - serve the app, screenshot it across every target (route × device × theme × state), optionally run accessibility checks, and have agy critique it read-only (any edit agy makes is reverted).

What it can drive

Across these it supports responsive viewports, device emulation, dark mode / prefers-color-scheme, forced-colors (high contrast), print media, RTL, component states (via pre_steps), seeded localStorage, per-target design references, and match-score convergence when design references are provided.

Accessibility: for web targets, ui_review injects the vendored axe-core into the page and returns structured WCAG violations (per target), which also ground agy's critique.

Related MCP server: Gemini Design MCP Server

Use case: realign a drifted frontend

The case this server is built for: you (or Claude Code / Codex) shipped a full-stack project - backend and frontend both done - but the FE drifted from, or doesn't match, the original design (screen mockups, or design tokens with an HTML/CSS demo). You want to redo the FE to match the design without risking the working backend. That is exactly what the diff-gate guarantees: agy realigns the UI, and anything outside your FE allow scope (API, server, business logic) is reverted automatically.

What it's strong at vs. where it needs help - this is an iterative refinement loop, not a from-scratch FE generator:

Fidelity is highest when you provide an HTML/CSS demo or design tokens (exact colors/spacing/fonts) rather than an image alone (values are inferred from pixels). Note the convergence score is agy's own visual self-assessment - always eyeball the returned shots_before/shots_after and diff to sign off.

Workflow

1. Commit your current state (a dirty tree is fine - it's snapshotted and preserved; the only requirement is a git repo with ≥1 commit).

2. Drop a .agy-ui-scope that allows only FE files and denies the backend, declares how to serve the app, and lists one target per screen with that screen's design_ref (see below).

3. Run ui_implement per screen with its design ref; review shots_before/shots_after + diff, then iterate (max_iters).

4. Run ui_review (read-only + a11y) to have agy critique what's left and surface WCAG issues.

5. Human-polish the last ~10-20% and anything structural the loop can't reach within scope.

Sample config - a Vite/React app, realigned screen-by-screen against mockups in ./design/:

model: "gemini-3.5-flash"
platform: web

# FE surface agy may edit/create.
allow:
  - "src/**/*.css"
  - "src/**/*.scss"
  - "src/components/**"
  - "src/**/*.tsx"
  - "index.html"

# Backend / logic - always reverted, even if agy edits them.
deny:
  - "**/api/**"
  - "**/server/**"
  - "**/*.server.*"
  - "**/route.*"

# Sensitive entry points - reverted AND reported for a human to decide.
ambiguous:
  - "src/main.tsx"
  - "src/App.tsx"
  - "vite.config.*"

serve:
  cmd: "npm run dev"
  url: "http://localhost:5173"
  ready_timeout: 30

devices:
  desktop: { width: 1440, height: 900 }
  mobile:  { name: "iPhone 13" }     # full Playwright device emulation

# One capture per screen, each matched against its own design mockup.
# (targets supersedes the simple `viewports` list when present.)
targets:
  - name: "home-desktop"
    route: "/"
    device: "desktop"
    design_ref: "./design/home-desktop.png"     # image OR an HTML/CSS demo render

  - name: "dashboard-desktop"
    route: "/dashboard"
    device: "desktop"
    design_ref: "./design/dashboard-desktop.png"

  - name: "settings-mobile-dark"
    route: "/settings"
    device: "mobile"
    color_scheme: "dark"                          # emulate prefers-color-scheme: dark
    design_ref: "./design/settings-mobile-dark.png"

Then drive each screen, e.g. ui_implement(project_dir=".", task="Match this screen to its design_ref", target_route="/dashboard"). Targets carry the per-screen mockup; target_route picks which one to work on.

Platform support

The server runs on macOS and Linux. It spawns agy (and native flutter run) through a Unix pseudo-terminal (pty) and manages process groups with POSIX-only calls, so native Windows is not supported - run it under WSL2 (Windows Subsystem for Linux) instead.

Notes:

• iOS always requires macOS + Xcode, regardless of host OS.

• WSL2: install the Linux build of agy (and log in) and run playwright install chromium inside WSL. Web and mobile web-targets work out of the box; native Android additionally needs adb/emulator wiring (e.g. connecting to a Windows-side emulator over TCP, or running the emulator inside WSL2).

• A native-Windows port would require replacing the pty layer with ConPTY (e.g. pywinpty) and the POSIX process-group calls; it is not implemented.

How it works

• PTY spawn. agy is run as agy -p "<prompt>" through a Python pseudo-terminal (pty.openpty + subprocess.Popen), because agy drops its stdout when attached to a non-TTY pipe. Output is captured from the PTY master; ANSI escapes and carriage returns are stripped.

• Subscription auth. agy authenticates via your existing Gemini subscription/login - no GEMINI_API_KEY is passed by this server.

• Diff-gate (the real guardrail). Scope is not enforced inside agy. Web runs happen in a throwaway git worktree; after each turn the server classifies every changed path against your scope (deny > ambiguous > allow > default-deny) and reverts anything not allowed (ambiguous paths are reverted and reported as escalations). A staged edit is restored from the baseline, not the index, so it cannot slip through.

• Vision loop. The orchestrator (this server) screenshots to files with Playwright, embeds those paths in the prompt (agy opens them with its own read_file tool - there is no image flag), lets agy edit, applies the diff-gate, re-screenshots, and loops.

• Native runs. Native platforms run in place (no worktree, to reuse the build cache). flutter run is launched under a PTY and hot-reloaded (r) between iterations - with an automatic hot-restart (R) fallback when a reload produces no visual change. A graceful quit (q) lets Flutter release its lockfile cleanly.

• In-place safety (snapshot-restore). Before a native run, the server snapshots your project's current state into a dangling git baseline commit (without touching your index/HEAD/worktree) and records your pre-existing untracked files. The diff-gate and reverts compare against that baseline, so only agy's edits are gated/undone and your uncommitted work is preserved exactly - you do not need to commit or stash first. The only hard requirement is that the project is a git repo with at least one commit; if it isn't, the tool returns a structured {"status": "blocked", ...} result explaining how to fix it (e.g. git init) instead of running unprotected.

Requirements

• Python ≥ 3.10

• The agy CLI, installed and logged in (subscription auth)

• Playwright Chromium for web/a11y captures - auto-installed on first use (or set AGY_UI_CHROME_CHANNEL=chrome to reuse an already-installed Chrome); not needed for native-only use

• .agy-ui-scope is optional - it is auto-detected from your stack when absent. Run ui_init to generate one, and see templates/agy-ui.rule.md for the recommended agent rule block

• Native iOS (ios-sim): macOS + Xcode + a Flutter project, and a booted iOS Simulator

• Native Android (android-emu): the Android SDK platform-tools (adb) and an AVD; the adapter can auto-launch the AVD by name (emulator -avd <name>)

Install

One-liner (easiest)

Runs a self-contained installer straight from the internet (no clone needed). It installs the server, tries to install Chromium, and offers to register with Claude Code:

curl -fsSL https://raw.githubusercontent.com/qdzsh/agy-ui-mcp/main/scripts/bootstrap.sh | bash

From PyPI (recommended)

# 1. Install the server (gives you an `agy-ui-mcp` command on PATH)
pipx install agy-ui-mcp
# or: uv tool install agy-ui-mcp

# 2. Register it with Claude Code
claude mcp add agy-ui --scope user -- agy-ui-mcp

From GitHub (latest main)

To install the latest unreleased changes straight from the repo:

pipx install git+https://github.com/qdzsh/agy-ui-mcp
# or: uv tool install git+https://github.com/qdzsh/agy-ui-mcp

The Chromium browser auto-installs on first use, so there is no manual playwright install chromium step. (To reuse an already-installed Chrome and skip the download, set AGY_UI_CHROME_CHANNEL=chrome.)

From a clone (one command)

git clone https://github.com/qdzsh/agy-ui-mcp.git && cd agy-ui-mcp
./scripts/install.sh          # installs the package + Chromium, and offers to
                              # register with Claude Code

scripts/install.sh is interactive and idempotent; re-run it any time.

Zero-config quick start

You do not need a .agy-ui-scope file to get going - the server auto-detects your stack (Vite/React, Next.js, Expo, Ionic, CRA, Flutter-web, or generic web) and synthesizes a scope on the fly. The minimal flow:

1. Install (any option above).

2. Log into agy once (subscription auth - no API key).

3. Drop a mockup image into your project, e.g. ./design/home.png.

4. Ask the agent to match it by calling:

   ui_implement(project_dir=".", task="Match the running home screen to this mockup",
                design_refs=["./design/home.png"], target_route="/")

That is enough for the loop to run with zero config files. When you want to customize the scope (allow/deny globs, per-screen targets, serve command), call ui_init(project_dir=".") once to detect your stack and write a real, inspectable .agy-ui-scope you can edit (it never overwrites an existing one unless overwrite=True). See templates/agy-ui.rule.md for a copy-paste rule block that teaches your agent how to use this MCP correctly.

The Chromium browser auto-installs on first use (a one-time download; no manual playwright install chromium step). Set AGY_UI_CHROME_CHANNEL=chrome to reuse an already-installed Chrome and skip that download entirely.

Wire into Claude Code

# If installed as a console script (pipx / uv tool / pip):
claude mcp add agy-ui --scope user -- agy-ui-mcp

# Or run the module directly (e.g. from an editable/venv install):
claude mcp add agy-ui --scope user -- python -m agy_ui_mcp

Wire into Codex

Add to ~/.codex/config.toml:

[mcp_servers.agy-ui]
command = "agy-ui-mcp"        # or: command = "python", args = ["-m", "agy_ui_mcp"]

Configure a project

A .agy-ui-scope file is optional - without one the server auto-detects your stack and synthesizes a scope (see "Zero-config quick start" above). Add a real file only when you want to customize the allow/deny globs, serve command, or per-screen targets. The easiest way is ui_init(project_dir="."), which detects your stack and writes a starter .agy-ui-scope you can then edit.

Alternatively, copy the fully annotated template and edit it for your stack:

cp .agy-ui-scope.example /path/to/your/app/.agy-ui-scope

A minimal web scope:

# platform: web            # default; also expo-web / ionic / flutter-web / ios-sim / android-emu
allow:
  - "src/**/*.css"
  - "src/components/**"
deny:
  - "src/api/**"           # backend - agy edits here are always reverted
  - "**/*.server.*"
ambiguous:
  - "src/main.tsx"         # reverted AND reported for a human to decide
serve:
  cmd: "npm run dev"
  url: "http://localhost:5173"
  ready_timeout: 30
viewports: [1440, 768, 390]
model: "gemini-3.5-flash"

A native (iOS) scope uses targets + a device registry instead of viewports:

platform: ios-sim
serve:
  cmd: "flutter run -d <simulator-udid>"   # argv-split (no shell) for native
  url: ""
  ready_timeout: 600                        # first Xcode/gradle build is slow
allow: ["lib/main.dart"]
deny:  ["lib/data.dart"]
devices:
  sim: { name: "iPhone 17" }                # or udid: "..."
targets:
  - { name: order-mobile, device: sim }
model: "gemini-3.5-flash"

See .agy-ui-scope.example for the full set of options (per-target design_ref, theme, rtl, color_scheme, forced_colors, media, full_page, local_storage, pre_steps, serve.reload_cmd, etc.).

Tool reference

Zero-config: every tool works with no .agy-ui-scope present - the server auto-detects your stack (Vite/React, Next.js, Expo, Ionic, CRA, Flutter-web, or generic web) and synthesizes a scope for the run.

ui_init(project_dir=".", overwrite=False) -> detects your stack and writes a starter .agy-ui-scope (never clobbers an existing one unless overwrite=True). Returns status (ok/exists/error), scope_path, written, detected ({framework, platform, serve_cmd, serve_url, package_manager}), allow, deny, design_dir_found, next_steps, and warnings.

ui_implement(project_dir, task, design_refs=None, target_route=None, max_iters=4, apply=True, match_threshold=90) → returns files_changed, diff, escalations, iterations, shots_before/shots_after, targets, applied/applied_files, match_score, match_gaps, warnings. When apply is true the surviving in-scope edits are written to your working tree.

ui_review(project_dir, target_route=None, against_design=None, a11y=True) → returns critique, shots, targets, a11y ({target: [violations]}), warnings. Read-only.

Both may instead return {"status": "blocked", "blocked_reason": "...", ...} when a native/in-place run can't be made safe (non-git or no commit yet) - the blocked_reason tells you exactly what to do.

Notes & limitations

• Native needs a git repo + ≥1 commit. This is by design (the in-place safety snapshot). A dirty working tree is fine and is preserved; only a non-git or commit-less project is refused (with a clear message). Web runs use an isolated worktree and also require git.

• Playwright Chromium auto-installs. Web and a11y features need a Chromium browser; the server installs it automatically on first use (a one-time download, with a short stderr notice). Set AGY_UI_CHROME_CHANNEL=chrome to reuse an installed Chrome/Edge and skip the download, or AGY_UI_NO_BROWSER_AUTOINSTALL=1 to opt out. Native-only use needs no browser.

• Native serve command is argv-split (shlex.split, no shell) so the PTY can deliver r/R/q keystrokes to flutter directly - shell features (&&, env-var expansion, cd) in serve.cmd won't work for native.

• First native build can take minutes (Xcode / Gradle). Set serve.ready_timeout generously (e.g. 300-600s).

• Flutter scaffolding. If flutter run reports a missing ios/ or android/ project, regenerate it with flutter create --platforms=ios . (or android).

Environment variables

Offline / dry runs

Set AGY_UI_DRY_RUN=1 to make the tools skip every external side effect (spawning agy, launching Playwright, running git, starting a dev server) and return a stub payload. The package imports and the scope/diff-gate logic unit-test without agy, Playwright browsers, or a running dev server.

pip install -e ".[dev]" && python -m pytest -q

License

MIT.