Mcp MCP Server

@extension.dev/mcp

Give your AI agent hands for browser extension development. 30 MCP tools that scaffold, run, inspect, debug, and publish cross-browser extensions.

claude mcp add extension-dev npx @extension.dev/mcp

Works with Claude Code, Claude Desktop, Cursor, and any MCP client.

extension.dev · Documentation · Templates · Examples · Discord

Why an MCP server for extensions

Extensions fail silently: content scripts that never inject, panels that never open, permissions that return undefined with no error. An agent editing files blind will happily "fix" all of them without noticing none of them work.

These tools give agents eyes on the live browser, so they debug from evidence instead of guessing:

• Scaffold from the 60+ template catalog behind templates.extension.dev, or add a popup, sidebar, or content script to an existing project
• Run the dev server with HMR in Chrome, Edge, Firefox, Safari, Brave, Opera, Vivaldi, Yandex, Waterfox, LibreWolf, or any Chromium- or Gecko-based binary, no build config
• See the live DOM, unified logs from every extension context, chrome.storage contents, and the loaded-extension list
• Act: evaluate code in any context, trigger the action button and commands, reload the extension, replay events
• Ship: validate the manifest cross-browser, build for production, publish a shareable preview, and promote builds to release channels headlessly

Built on Extension.js, the open-source cross-browser extension framework.

Setup

Claude Code

claude mcp add extension-dev npx @extension.dev/mcp

Or install it as a plugin — the MCP server plus the /extension, /extension-add, /extension-debug, and /extension-publish commands in one step:

/plugin marketplace add extensiondev/mcp
/plugin install extension-mcp@extensiondev-mcp

Cursor

Claude Desktop / .mcp.json

{
  "mcpServers": {
    "extension-dev": {
      "command": "npx",
      "args": ["@extension.dev/mcp"]
    }
  }
}

Pair with the skill

This server gives agents hands; @extension.dev/skill gives them judgment: the cross-browser rules, silent-failure gotchas, debugging playbooks, and store checklist, packaged in the open Agent Skills format. With both installed, agents know to verify against the live browser instead of guessing, and these tools make that a one-call operation.

npm i -D @extension.dev/skill
mkdir -p .claude/skills && cp -R node_modules/@extension.dev/skill/skills/extension-dev .claude/skills/

Claude Code project integration

The package ships drop-in instructions, slash commands, and rules for extension projects:

# Rules (how Claude understands your project)
cp node_modules/@extension.dev/mcp/claude/CLAUDE.md ~/my-extension/.claude/CLAUDE.md

# Slash commands (/extension, /extension-add, /extension-debug, /extension-publish)
mkdir -p ~/my-extension/.claude/commands
cp node_modules/@extension.dev/mcp/claude/commands/*.md ~/my-extension/.claude/commands/

Tools

Browser-launching tools (dev, start, preview) shell out to the extension CLI — the project's own node_modules/.bin/extension when present, otherwise npx extension@<pinned> at the version this package is verified against; everything else runs in-process.

From preview to store

The four platform tools connect agents to extension.dev: extension_login runs a GitHub device-code flow and stores a project-scoped token locally (never returned to the agent), extension_publish turns a build into a shareable preview URL, and extension_release_promote promotes a tested build to a release channel from CI or an agent session, no browser required. To automate store submission itself (Chrome Web Store, Edge Add-ons, Firefox AMO), pair with @extension.dev/deploy.

The extension.dev stack

All of it rides on Extension.js, the open-source cross-browser extension framework.

Community

• Join the Discord for help and feedback
• Browse production-ready examples
• Report Extension.js framework issues on GitHub

License

MIT (c) Cezar Augusto and the extension.dev collaborators