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Agents

Playwright MCP

Setup

The Playwright MCP connects to a headless Chrome running in a separate Docker container (chrome) via CDP. Configuration is in .mcp.json. Example:

{
    "mcpServers": {
      "playwright": {
        "command": "npx",
        "args": ["@playwright/mcp@latest", "--cdp-endpoint", "http://<CHROME_DOCKER_IP>:9222"]
      }
    }
}

Networking

  • The app runs inside the app container, Chrome runs in the chrome container.
  • Do not use localhost or the app hostname to navigate — Chrome cannot resolve them properly.
  • Chrome CDP rejects non-IP Host headers — Chromium hardcodes a check that the HTTP Host header is an IP or localhost. There is no flag to disable this. Always use resolved IPs (not hostnames) in CDP endpoint URLs.
  • Resolve container IPs first with getent hosts <container>, then use the IP:
getent hosts app    # for navigation URLs
getent hosts chrome # for CDP endpoint in .mcp.json

Checklist

  1. Start the dev server: pnpm run dev (run in background)
  2. Wait for the server to be ready: curl -s -o /dev/null -w "%{http_code}" http://localhost:4200
  3. Resolve the app IP: getent hosts app
  4. Navigate with Playwright: browser_navigate to http://<resolved-ip>:4200
  5. Use browser_snapshot (preferred over screenshots) to inspect the page