@extension.dev/mcp

Give your AI agent hands for browser extension development. One command connects Claude Code, Claude Desktop, Cursor, or any MCP client to 26 tools that scaffold, run, inspect, debug, and publish cross-browser extensions:

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

Extensions fail silently: content scripts that never inject, panels that never open, permissions that return undefined with no error. These tools give agents eyes on the live browser (DOM probes, unified logs from every context, storage access, event replay) so they debug from evidence instead of guessing. Scaffolding draws on the 60+ template catalog of the extension.dev platform, built on Extension.js.

What's in this package

@extension.dev/mcp
  src/              MCP server source (26 tools)
  claude/           Claude Code integration (CLAUDE.md, slash commands, rules)
  bin/              CLI entrypoint

MCP server: programmatic bridge between AI assistants and the extension.dev platform. Imports from published npm packages:

extension-create: project scaffolding
extension-develop: build/dev/preview
extension-install: managed browser binaries

Claude Code integration: drop-in instructions, slash commands, and rules for Claude Code:

claude/CLAUDE.md: project-level instructions for any extension project
claude/commands/: slash commands (/extension, /extension-add, /extension-debug, /extension-publish)
claude/rules/: rules for extension development, cross-browser compat, and MCP tools

Agent Skill: the knowledge companion to this server lives in @extension.dev/skill: a portable Agent Skills-format skill (SKILL.md + references) covering cross-browser rules, silent-failure gotchas, debugging playbooks, and store publishing. This server gives agents hands; the skill gives them judgment. Pair them:

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

Browser-launching tools (dev, start, preview) shell out to the extension CLI since they require the full browser launcher infrastructure.

Setup

Claude Code

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

Claude Desktop / .mcp.json

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

Claude Code integration (manual)

Copy the Claude Code rules and commands into any extension project:

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

# Slash commands
mkdir -p ~/my-extension/.claude/commands
cp node_modules/@extension.dev/mcp/claude/commands/*.md ~/my-extension/.claude/commands/

Tools

Tier	Tool	Integration	Description
1	`extension_create`	`extensionCreate()`	Scaffold from a template
1	`extension_list_templates`	native	Browse 60+ templates
1	`extension_build`	`extensionBuild()`	Build for production
1	`extension_dev`	CLI spawn	Dev server with HMR
1	`extension_start`	CLI spawn	Build + preview
1	`extension_preview`	CLI spawn	Preview production build
2	`extension_get_template_source`	native	Read template source files
2	`extension_manifest_validate`	native	Cross-browser manifest validation
2	`extension_inspect`	native	Build output analysis
2	`extension_source_inspect`	CDP WebSocket	Live DOM inspection
2	`extension_dom_inspect`	agent bridge	CDP-free DOM snapshot
2	`extension_list_extensions`	CDP WebSocket	List loaded extensions (Chromium)
2	`extension_logs`	agent bridge	Stream logs from every context
2	`extension_wait`	native	Poll ready.json contract
2	`extension_add_feature`	native	Add sidebar/popup/content script
act	`extension_eval`	agent bridge	Eval in a context (`--allow-eval`)
act	`extension_storage`	agent bridge	Read/write `chrome.storage`
act	`extension_reload`	agent bridge	Reload extension or tab
act	`extension_open`	agent bridge	Open surface / trigger `action`,`command`
3	`extension_install_browser`	`extensionInstall()`	Install managed browser
3	`extension_list_browsers`	`getManagedBrowsersCacheRoot()`	List managed browsers
3	`extension_detect_browsers`	native	System browser detection
auth	`extension_login`	platform	GitHub device-code → stored token
auth	`extension_whoami`	native	Show stored login (no token)
auth	`extension_logout`	native	Remove stored credentials
auth	`extension_publish`	platform	Publish to extension.dev (token)

Development

pnpm install
pnpm compile    # Build with rslib
pnpm test       # Run tests
pnpm start      # Start MCP server

Publishing

NPM_TOKEN=<token> pnpm publish

Uses prepublishOnly to run tests and compile before publishing.

The extension.dev open source stack

Package	Use it to
`@extension.dev/skill`	Teach agents the cross-browser rules and silent-failure gotchas
`@extension.dev/deploy`	Ship to Chrome, Firefox, and Edge stores from CI
`@extension.dev/artifact-integrity`	Gate releases on artifact verification
`@extension.dev/compiler`	Build extensions in the browser with esbuild-wasm
`@extension.dev/core`	Authenticate and publish to the extension.dev platform

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

License

MIT