@extension.dev/mcp

Give your AI agent hands for browser extension development. 28 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 50+ 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_templates | Browse 50+ templates (list) and read one's source (source) | | 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 + launch the production build (build: false launches the existing dist) | | 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_analyze | Static analysis of the built extension on disk | | see | extension_inspect | Deep live inspection of a running extension (closed shadow roots, probes) | | see | extension_dom_snapshot | Shallow DOM snapshot of a chosen tab or extension surface over the agent bridge | | see | extension_list_extensions | List loaded extensions (Chromium and Firefox) | | see | extension_logs | Stream logs from every context | | see | extension_doctor | Diagnose the dev session leg by leg (ready contract, ports, token, executor, browser) | | see | extension_theme_verify | Verify a Chrome theme manifest against the colors Chrome actually paints | | 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_browsers | Detect, list, install, and uninstall browsers | | platform | extension_auth | Device login at extension.dev, plus login status and logout | | platform | extension_preview_web | Render a build in the web emulator, and share it as a link | | platform | extension_shares | List every link you have shared, and revoke one permanently | | platform | extension_publish | Publish a shareable preview to extension.dev | | platform | extension_release_promote | Promote a build to a release channel, headless | | platform | extension_submit | Submit for store review: Chrome, Firefox, Edge, Safari, through extension.dev | | platform | extension_release_status | Read release channels, recent builds, and store submission and review state |

Browser-launching tools (dev, start) 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.

Sharing a build in progress

An unpacked extension is unusually hard to hand to someone: the only way to look at a colleague's work-in-progress has been to take their zip and run untrusted code with real browser permissions on your own machine. extension_preview_web with share: true uploads the dist/ it just built and returns a link that renders those exact bytes in the emulator. Whoever opens it installs nothing and signs in to nothing, which is what lets a designer, a PM, or a reviewer into the loop at all. Sharing needs auth (extension_auth or EXTENSION_DEV_TOKEN), the link expires, and DELETEing the returned revokeUrl with the same token kills it early. Revocation is permanent and re-sharing mints a new link, so that revokeUrl is the only handle to the link you just made; every share is also appended to .extension.dev/shared-previews.json in the project (gitignored) so it survives losing the tool output. Without share, the tool returns a local-only deep link and uploads nothing.

extension_shares is the other half of that: it lists every link the token has shared, live and dead, with the previewUrl and revokeUrl of each, and revokes one by artifactId or by pasting any of its URLs. Pass projectPath and it reconciles the platform's answer with the project's own record, so a link shared from another machine shows up as remoteOnly and a record with nothing behind it any more shows up under localOnly. It never rewrites the local file.

That is a different job from shipping. Use share for the build you are holding right now; use extension_publish and extension_release_promote below for builds your CI has released.

From preview to store

The platform tools connect agents to extension.dev: extension_auth runs extension.dev's own device flow (you approve the code at extension.dev/device, and GitHub is federated server-side, so no GitHub token ever reaches your machine) and stores a project-scoped token locally (never returned to the agent), extension_publish turns a build your project has already published into a shareable URL, and extension_release_promote promotes a tested build to a release channel from CI or an agent session, no browser required. extension_submit submits a built extension to the Chrome Web Store, Edge Add-ons, Firefox AMO, and the App Store for Safari 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. The two verbs are not interchangeable: extension_publish pushes to the extension.dev platform, extension_submit sends the build into a store's review queue, which is irreversible. After a real submission, extension_release_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.