UI Annotator MCP

English | 廣東話

See what every UI element is called — in any browser, zero extensions.

An MCP server that adds interactive hover annotations to any web page. Open a proxied URL, hover any element, see its name. Tell your AI assistant "make the sidebar wider" — it knows exactly what you mean.

The Problem

When reviewing a web UI with an AI coding assistant, the hardest part isn't the code change — it's describing which element you want changed.

"That thing on the left... the second row... no, the one with the icon..."

You don't know what it's called. The AI doesn't know what you're pointing at. You waste time on miscommunication instead of shipping.

The Solution

Open your page through the annotator proxy. Hover any element — instantly see its name, CSS selector, and dimensions. Now you both speak the same language.

# Start the MCP server
npx @mcpware/ui-annotator

# Open in ANY browser
http://localhost:7077/localhost:3847

That's it. No browser extensions. No code changes. No setup. Works in Chrome, Firefox, Safari, Edge — any browser.

How It Works

Your app (localhost:3847)
        │
        ▼
┌─────────────────────┐
│  UI Annotator Proxy  │ ← Reverse proxy on port 7077
│  (MCP Server)        │
└─────────────────────┘
        │
        ▼
Proxied page with hover annotations injected
        │
        ├──► User sees: hover overlay + tooltip with element names
        └──► AI sees: structured element data via MCP tools

The proxy fetches your page, injects a lightweight annotation script, and serves it back. The script scans the DOM, identifies named elements, and reports them to the MCP server. Your AI assistant queries the server to understand what's on the page.

Features

Hover Annotations

Hover any element to see:

• Element name (pink) — the human-readable identifier
• CSS selector (monospace) — the technical reference
• Content preview — what text the element contains
• Dimensions — width × height in pixels

Inspect Mode

Click the Inspect button in the toolbar (or let your AI toggle it). In inspect mode:

• Click any element → copies its name to clipboard
• All page interactions are paused (clicks don't trigger buttons/links)
• Click Inspect again to return to normal mode

Collapsible Toolbar

The toolbar sits at the top center of the page showing:

• Inspect toggle button
• Element count
• Helpful subtitle explaining what to do
• Collapse button (▲) to minimize when not needed

MCP Tools for AI

Tool	What it does
annotate(url)	Returns proxy URL for user to open in any browser
get_elements()	Returns all detected UI elements with names, selectors, positions
highlight_element(name)	Flash-highlights a specific element so user can confirm
rescan_elements()	Force DOM rescan after page changes
inspect_mode(enabled)	Toggle inspect mode remotely

Why Not Just Use DevTools?

	Browser DevTools	UI Annotator
Target user	Frontend developers who know the DOM	Anyone — QA, PM, designer, junior dev
Learning curve	Need to understand DOM tree, CSS selectors, box model	Hover and read — zero learning
Communication	"The div.flex.gap-4 inside the second child of..."	"The sidebar"
Language	CSS/HTML technical terms	Human-readable names
Setup	Teach people to open DevTools + navigate the DOM	Open a URL
AI integration	None — AI can't see what you're inspecting	MCP — AI sees the same element names you do

DevTools is for debugging. UI Annotator is for communication — giving humans and AI a shared vocabulary for UI elements.

Why Not Use Existing Tools?

Tool	Approach	Why we're different
MCP Pointer	Chrome extension + MCP server	Requires Chrome extension. Click-to-inspect, no hover overlay.
Agentation	npm package embedded in your app	Requires code changes. React 18+ dependency. Not zero-config.
Cursor Visual Editor	Built-in IDE browser	Only works inside Cursor IDE.
Windsurf Previews	Built-in IDE browser	Only works inside Windsurf IDE.
Chrome DevTools MCP	Programmatic DOM access for AI	AI can inspect, but humans don't see visual annotations.
VisBug	Chrome extension for visual editing	No MCP integration. No AI connection. Chrome only.
Marker.io / BugHerd	SaaS visual feedback widgets	Not MCP. Paid. For bug reporting, not AI-assisted development.

Feature Comparison

Feature	UI Annotator	MCP Pointer	Agentation	Cursor	Chrome DevTools MCP
Visual hover annotation	Yes	No	Partial	Yes (IDE only)	No
Shows element names	Yes	Yes	Yes	No (high-level)	Programmatic
Shows dimensions	Yes	Yes	Yes (Detailed)	Yes	Programmatic
MCP server	Yes	Yes	Yes	No	Yes
Zero browser extensions	Yes	No	Yes	N/A	No
Zero code changes	Yes	Yes	No	N/A	Yes
Any browser	Yes	Chrome only	Desktop only	Cursor only	Chrome only
Zero dependencies	Yes	Chrome ext	React 18+	Cursor	Chrome
Click to copy element name	Yes	No	No	No	No

Architecture

Zero external dependencies

• Reverse proxy: Node.js built-in http module
• MCP server: @modelcontextprotocol/sdk (stdio transport)
• Communication: HTTP POST (browser → server) + GET polling (server → browser)
• No WebSocket, no Express, no browser extension

How the proxy works

1. User requests localhost:7077/localhost:3847
2. Proxy fetches http://localhost:3847
3. For HTML responses:
   • Injects fetch() / XMLHttpRequest interceptor (rewrites API paths through proxy)
   • Rewrites href="/..." and src="/..." attributes to route through proxy
   • Injects annotation script before </body>
4. For non-HTML (CSS, JS, images): passes through directly
5. Strips Content-Security-Policy headers to allow injected script

How annotation works

1. Script scans DOM for elements with id, class, semantic roles, or interactive roles
2. On hover: positions overlay border (follows border-radius) + positions tooltip (always within viewport)
3. Reports all detected elements to server via POST /__annotator/elements
4. Polls GET /__annotator/commands every second for server instructions (highlight, rescan, inspect toggle)
5. MutationObserver auto-rescans when DOM changes

Quick Start

With Claude Code

# Add as MCP server
claude mcp add ui-annotator -- npx @mcpware/ui-annotator

# Then in conversation:
# "Annotate my app at localhost:3847"
# → AI returns proxy URL, you open it, hover elements, discuss changes by name

Manual

npx @mcpware/ui-annotator
# Proxy starts on http://localhost:7077
# Open http://localhost:7077/localhost:YOUR_PORT

Environment Variables

Variable	Default	Description
UI_ANNOTATOR_PORT	7077	Port for the proxy server

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License

MIT