@extension.dev/mcp

@extension.dev/mcp

Give your AI agent hands for browser extension development. 33 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, Brave, Opera, Vivaldi, Yandex, Waterfox, LibreWolf, or any Chromium- or Gecko-based binary, plus Safari on macOS (no HMR yet), 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.

Clients

Claude Code	Claude Desktop	Cursor

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

Tier	Tool	Description
build	extension_create	Scaffold from a template
build	extension_list_templates	Browse 60+ templates
build	extension_get_template_source	Read template source files
build	extension_add_feature	Add sidebar/popup/content script
build	extension_build	Build for production
run	extension_dev	Dev server with HMR
run	extension_start	Build + preview
run	extension_preview	Preview the production build
run	extension_wait	Poll the dev-server ready contract
run	extension_stop	Stop a dev/start/preview session (server + browser)
see	extension_manifest_validate	Cross-browser manifest validation
see	extension_inspect	Build output analysis
see	extension_source_inspect	Live DOM inspection (CDP)
see	extension_dom_inspect	CDP-free DOM snapshot
see	extension_list_extensions	List loaded extensions (Chromium)
see	extension_logs	Stream logs from every context
see	extension_doctor	Diagnose the dev session leg by leg (ready contract, ports, token, executor, browser)
act	extension_eval	Evaluate in a context (needs allowEval: true on extension_dev)
act	extension_storage	Read/write chrome.storage
act	extension_reload	Reload extension or tab
act	extension_open	Open a surface / trigger action, command
browsers	extension_install_browser	Install a managed browser binary
browsers	extension_uninstall_browser	Remove a managed browser binary
browsers	extension_list_browsers	List managed browsers
browsers	extension_detect_browsers	Detect system browsers
platform	extension_login	GitHub device-code login, stored token
platform	extension_whoami	Show the stored login (never the token)
platform	extension_logout	Remove stored credentials
platform	extension_publish	Publish a shareable preview to extension.dev
platform	extension_release_promote	Promote a build to a release channel, headless
platform	extension_deploy	Submit to the Chrome, Firefox, and Edge stores through extension.dev

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 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. extension_deploy submits a built extension to the Chrome Web Store, Edge Add-ons, and Firefox AMO through extension.dev, which holds your store credentials and dispatches the release from your project's mirror CI, it defaults to a dry run and store credentials are never tool arguments. After a real submission, extension_store_status reads the recorded outcome, per-store credential health, and review state from the project's public registry, so agents and CI can answer "was it approved?" without a console visit. Access tokens live at most 7 days; CI pipelines re-mint them from the console's Access tokens page.

The extension.dev stack

Package	Use it to
@extension.dev/skill	Teach AI agents the judgment half: cross-browser rules, gotchas, playbooks
@extension.dev/artifact-integrity	Verify extension artifacts and gate CI on tampered bytes before they ship

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

Apache-2.0 (c) 2026 Cezar Augusto and the extension.dev collaborators. See LICENSE.