designer-mcp

Cursor-style designer pen for Claude Code. Click, marquee, or draw on any webpage in a headed Chromium and Claude gets back the exact source file, line number, CSS selector, and a screenshot — ready to edit and verify.

What it does

Three modes of visual-to-source:

All screenshots are saved as PNG files in /tmp and returned as paths — your MCP client never hits a context limit on base64.

React source resolution works in Next.js dev mode via the _debugSource fiber property (attached by @babel/plugin-transform-react-jsx-source). Production builds strip this; see Production source mapping below.

Related MCP server: Browser Feedback MCP

Demo

You:     "Make this button rounder"
Claude:  [designer_open http://localhost:3000/dashboard]
Claude:  [designer_pick mode=element]
You:     *click the button*
Claude:  → source: Button.tsx:42
Claude:  [Edit Button.tsx add rounded-full]
Claude:  [designer_screenshot selector=#cta-btn]  ← after screenshot for verification

Install

Prereqs: Node 18+, Claude Code, a working macOS/Linux (Playwright Chromium).

git clone https://github.com/YOUR_USER/designer-mcp.git
cd designer-mcp
npm install
npx playwright install chromium      # one-time browser download

Register the MCP with Claude Code (user-scope = available in every session):

claude mcp add --scope user designer-mcp node "$(pwd)/index.js"

Install the Claude skill so future sessions know the workflow:

mkdir -p ~/.claude/skills/designer
cp SKILL.md ~/.claude/skills/designer/SKILL.md

Restart Claude Code. You should see designer_* tools and a designer: skill in your session.

Usage

Start your Next.js dev server (for source mapping):

cd your-nextjs-app && npm run dev

Then, in Claude Code:

"Open http://localhost:3000/settings in the designer and let me pick the header."

Claude will call designer_open(...), then designer_pick({ mode: "element" }). Chromium pops to the front, your cursor becomes crosshairs, you click the header. Claude gets source.fileName + lineNumber and can edit directly.

Modes cheat sheet

• Single element — use element

• Multiple related elements in one region — use area (drag a box; returns every element whose center falls inside)

• Annotate / explain visually — use draw (red pen, Enter to finish, Esc to cancel)

Production source mapping

_debugSource is dev-only. To use the picker on a production build, enable source maps in next.config.js:

module.exports = {
  productionBrowserSourceMaps: true,
  // ...
};

The picker currently returns source: null in prod; a future version will resolve the selector through the deployed sourcemap. PRs welcome.

Tool reference

All tools are exposed over MCP; Claude Code sees them as mcp__designer-mcp__*.

designer_open(url: string)

Launch or reuse the headed Chromium instance and navigate. Foregrounds the window on macOS via bringToFront() + an AppleScript nudge.

designer_pick({ mode?: "element" | "area" | "draw" })

Activate the picker overlay. Returns when the user completes the interaction (or Esc cancels, or 180s timeout).

designer_screenshot({ selector?: string })

PNG of the page or a specific element. Returns { path, bytes }.

designer_close()

Tear down the browser and release Playwright resources.

How it works

1. A Playwright-controlled Chromium is launched headed. Singleton per process.

2. designer_pick injects a small vanilla-JS overlay (picker.js) into the page. The overlay:

   • element mode — tracks mousemove/click, outlines the hover target in blue, resolves a unique-ish CSS selector, walks the React fiber chain for _debugSource, returns to the MCP.

   • area mode — rubber-band marquee; on mouseup, any element whose center falls inside the box is collected (dedup by selector).

   • draw mode — full-viewport canvas overlay; captures strokes as point arrays; Enter finishes.

3. The server polls window.__designerResult every 200ms for up to 180 seconds.

4. On completion, an appropriate screenshot (element / area clip / full viewport) is saved to /tmp and the path returned.

Contributing

• PRs welcome, especially around:

  • Production sourcemap resolution

  • Kestrel/React Native picker (currently web only)

  • Multi-element accumulation in element mode (Cmd-click to add)

  • VS Code "reveal in editor" integration

License

MIT