DevTool MCP

What DevTool MCP does

DevTool MCP MCP server gives an MCP client a control and diagnostics layer for local web development. It manages development processes and a background daemon, starts reverse proxies with frontend instrumentation, launches Chrome or Chromium, and exposes browser state to an AI coding agent.

The proxy records HTTP traffic, browser errors, performance measurements, interactions, DOM mutations, diagnostic events, screenshots, and JavaScript execution results. Page sessions group the document request, resources, errors, metrics, interactions, and mutations for later inspection. The server also supports persistent scoped key-value storage, public tunnels, visual snapshot comparisons, walkthrough overlays, and session messaging.

How it works

A typical workflow starts a process or proxy for a local application, then opens that application through the proxy. The proxy injects the development instrumentation and keeps separate logs and WebSocket connections for each proxy. Browser automation can operate directly on a URL or use a proxy identifier; when a requested proxy is absent, the browser tool can start it from the supplied URL.

Inspection tools return compact summaries by default to limit context size. Detailed sections can be requested for errors, HTTP requests, performance data, interactions, mutations, resources, or other recorded events. Several audits depend on data collected during browsing: API and loading audits require a fresh page load, and responsive audits load hidden frames sequentially at selected viewport sizes.

The daemon preserves process and proxy state across MCP client connections. It starts automatically when needed, and its health-check action can report service problems. Processes can run in the background, be restarted cleanly, emit readiness signals, and be grouped with dependency ordering.

Setup and configuration

Install the command-line package with npm, pip, or uv, or build it from source:

• npm install -g @standardbeagle/agnt
• pip install agnt
• uv pip install agnt

Register the MCP server with an MCP client using the agnt mcp command:

{
  "mcpServers": {
    "agnt": {
      "command": "agnt",
      "args": ["mcp"]
    }
  }
}

The agnt skills command can install agent skills and register the MCP server. It requires Node.js and npx. Project setup can generate .agnt.kdl automatically for recognized application shapes, including Node, Go/Wails, .NET, Docker Compose, Django, Rails, Laravel, Phoenix, Hugo, Jekyll, MkDocs, and static sites. agnt init runs project setup without launching a coding session.

Chrome or Chromium is required for browser automation and browser management. Headless mode requires Chrome 109 or newer. Tunnel providers additionally require their corresponding cloudflared, ngrok, or tailscale binary and configuration.

Tools and capabilities

• proc, run, detect, and daemon manage scripts, processes, dependencies, output, restart behavior, project detection, and service health.
• proxy and proxylog provide reverse-proxy lifecycle control, JavaScript execution, navigation, resizing, traffic logs, and aggregated diagnostics.
• browser and automation launch Chrome sessions, navigate pages, evaluate scripts, inspect session state, and capture viewport, full-page, element, or clipped screenshots.
• currentpage, get_incidents, diagnose, and verify_change support framework triage, layout diagnosis, incident retrieval, dead-click investigation, and targeted rechecks.
• responsive_audit, api_audit, loading_audit, release_qa, and snapshot cover responsive behavior, request patterns, loading sequences, release gates, and visual regressions.
• tunnel, store, session, walkthrough, publish, and watch handle public access, persistent values, agent sessions, browser demonstrations, share management, and event-monitoring commands.

Limitations and notes

The API and loading audits analyze buffers populated by a page load; an empty buffer produces a clean score with a reload instruction rather than findings. evaluate targets the application frame by default and can target the outer proxy shell explicitly. Demo recording relies on the repository’s demo engine and therefore needs the relevant checkout tree, not only the installed binary. Tunnel behavior depends on the selected provider and its local binary. Channel mode is experimental and limited to Claude Code with additional development configuration.