extensiondev/mcp

[npm-version-url]: https://www.npmjs.com/package/@extension.dev/mcp
[npm-downloads-image]: https://img.shields.io/npm/dm/%40extension.dev%2Fmcp.svg?color=26FFB8
[npm-downloads-url]: https://www.npmjs.com/package/@extension.dev/mcp
[discord-image]: https://img.shields.io/discord/1253608412890271755?label=Discord&logo=discord&style=flat&color=26FFB8
[discord-url]: https://discord.gg/v9h2RgeTSN
# @extension.dev/mcp [![Version][npm-version-image]][npm-version-url] [![Downloads][npm-downloads-image]][npm-downloads-url] [![Discord][discord-image]][discord-url]
> Give your AI agent hands for browser extension development. 37 MCP tools that scaffold, run, inspect, debug, test, and build cross-browser extensions on your machine, plus a platform lane (private alpha) to share and publish them.
bash
claude mcp add extension-dev npx @extension.dev/mcp
Works with Claude Code, Claude Desktop, Cursor, and any MCP client.
extension.dev · Docs · Templates · 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. These tools give the agent eyes on the live browser, so it debugs from evidence instead of guessing.
Scaffold from the 50+ templates behind templates.extension.dev, or add a popup, sidebar, or content script to an existing project
Run the dev server with HMR in Chrome, Firefox, Edge and every browser below, plus Safari on macOS (no HMR yet)
See the live DOM, logs from every extension context, chrome.storage, and the loaded-extension list
Act: evaluate code in any context, trigger the action button and commands, reload the extension
Test: state expectations about the running extension and get one verdict each
Ship: validate the manifest cross-browser and build for production. On the platform lane: share a preview link, promote a build, and submit to the stores
Built on Extension.js, the open source framework extension.dev sponsors. Every engine command runs the project's own nodemodules/.bin/extension when the project has one, else npx extension@ at the version this server pins (EXTENSIONMCPCLIVERSION changes the pin). To run a fix that has not shipped yet, set EXTENSIONMCPENGINEDIR to a compiled Extension.js checkout root (the folder holding programs/extension/bin/extension.cjs) or to a built extension package folder (holding bin/extension.cjs): every spawn then runs node , the project's engine and the pin are bypassed, extensiondoctor's project-engine check names the path with its version, sha and branch, and a path with no bin refuses every spawn instead of falling back. EXTENSIONMCPENGINEBIN names the exact bin file instead.
## Watch it work
## Browsers
extensiondev opens the extension in the browser you name.
| Google Chrome | Chromium | Microsoft Edge | Brave | Opera | Mozilla Firefox | Apple Safari |
The Chromium family and Firefox get HMR, control, eval, inspect and assert (Firefox over its debugger protocol and the agent bridge, no CDP). Safari gets dev and build on macOS, no HMR and no inspect; the full list of what works there is under Safari. Plus Vivaldi, Yandex, Waterfox, Zen and Floorp, and any Chromium or Gecko binary by path.