Extensiondev MCP

@extension.dev/mcp [][npm-version-url] [][npm-downloads-url] [][discord-url]

Give your AI agent hands for browser extension development. 38 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.

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 node_modules/.bin/extension when the project has one, else npx extension@ at the version this server pins (EXTENSION_MCP_CLI_VERSION changes the pin). To run a fix that has not shipped yet, set EXTENSION_MCP_ENGINE_DIR 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, extension_doctor'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. EXTENSION_MCP_ENGINE_BIN names the exact bin file instead.

Watch it work

Browsers

extension_dev 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.

Clients

| | | | | | | :-: | :-: | :-: | :-: | :-: | | Claude Code | Claude Desktop | Cursor | VS Code | Codex |

Setup

Claude Code

claude mcp add extension-dev -- npx @extension.dev/mcp --features=local,platform

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

Or paste the .mcp.json block below into .cursor/mcp.json.

VS Code

code --add-mcp '{"name":"extension-dev","command":"npx","args":["@extension.dev/mcp","--features=local,platform"]}'

Or paste it into .vscode/mcp.json under servers, with "type": "stdio" on the entry.

Codex

codex mcp add extension-dev -- npx @extension.dev/mcp --features=local,platform

Or write it into ~/.codex/config.toml as a [mcp_servers.extension-dev] table with the same command and args.

Other clients (Claude Desktop and .mcp.json)

.mcp.json:

{

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

Flags

• By default the server exposes the 25 of its 38 tools that work on this machine (the 13 platform tools are off). --features=local,platform adds the platform group. Env: EXTENSION_DEV_FEATURES.
• --no-ship refuses every call that puts something in front of other people (publish, promote, a real submit, a shared preview, a revoke). Env: EXTENSION_DEV_NO_SHIP=1.
• --project / pins the server to one project. Env: EXTENSION_DEV_PROJECT.
• When a platform act answers approval-required on a desktop (not headless, not CI, and on Linux with a display), the server opens the approval page once in the default browser. --no-ship or EXTENSION_DEV_NO_AUTO_OPEN=1 turns that off.

A refused call answers E_TOOL_DISABLED with the flag to change. Human approval for store submissions and stable promotions, the MCP annotations every tool carries, and the fence around page-written text are described under Server flags.

Telemetry

extension_create sends one draft_seeded event to PostHog at us.i.posthog.com after a successful scaffold (template slug, source and commit, a random per-process session id, never a path or a name), and the platform tools send a per-install id header with their calls. EXTENSION_DEV_NO_TELEMETRY=1 or DO_NOT_TRACK=1 stops both. The full payload is listed under Telemetry.

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, in the open Agent Skills format. The package also ships Claude Code rules and the four slash commands under claude/.

npm i -D @extension.dev/skill

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

Tools

On by default, the 25 that work on this machine:

On with --features=local,platform, the 12 that reach the extension.dev platform (private alpha):

| Tier | Tool | Description | |