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Extensiondev MCP

by extensiondev

Develop, debug, test, build, and publish cross-browser extensions through MCP tools.

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Install Extensiondev MCP

Pick your client, then add it in one click or paste the config.

STDIONode.js
~/Library/Application Support/Claude/claude_desktop_config.json
claude_desktop_config.json
{
  "mcpServers": {
    "mcp-291": {
      "command": "npx",
      "args": [
        "@extension.dev/mcp"
      ]
    }
  }
}

Settings → Developer → Edit Config, merge this into mcpServers, then fully quit and reopen Claude Desktop.

Install not yet sandbox-tested— we haven't run this command through our automated sandbox yet. Not a red flag; we're working through the catalog.

Overview

extensiondev/mcp connects AI clients to local browser-extension development workflows built on Extension.js. Its tools scaffold projects, start development sessions, inspect browser state, evaluate code, test behavior, validate manifests, and create production builds. Local tools cover Chromium-family browsers and Firefox, while Safari supports development and builds on macOS; optional platform tools handle sharing and publishing workflows. Reach for it when an agent needs observable, actionable access to an extension running in a real browser.

Use cases

  • Scaffold a cross-browser extension from a template
  • Inspect a running extension’s DOM, logs, and storage
  • Debug content scripts and extension contexts with live browser evidence
  • Validate manifests and build an extension for production
  • Share previews or submit approved builds to browser stores

Key features

  • Template-based extension scaffolding
  • Live browser inspection and code evaluation
  • Cross-browser development sessions with HMR
  • Runtime assertions and extension controls
  • Manifest validation and production builds
  • Optional preview and publishing workflows

Security & health

What it can reach and how reliably it runs — from our own checks, not the maintainer’s claims.

ActiveLast checked 1d ago

Recent health check succeeded.

8/8 checks healthy over the last 43d

Supply-chain signal

No high-severity advisories surfaced by our automated scan.

Critical 0High 0Medium 0Low 0

Scanned 2d ago via OSV.dev · @extension.dev/mcp (npm)

Adoption & maintenance

Factual signals from GitHub, npm, and our automated checks — not a rating.

npm downloads2.7kPackage downloads in the last 30 days.
Last commit2d agoMost recent push to the default branch.

How Extensiondev MCP works

What extensiondev/mcp does

extensiondev/mcp exposes browser-extension development operations through the Model Context Protocol. It is built on Extension.js and is intended for agents working with extensions on a developer’s machine. The server can create a project from a template, add common extension features, run the project, inspect runtime behavior, exercise the extension, and prepare builds for distribution.

Its debugging focus is live browser evidence. Agents can work with the DOM, extension-context logs, chrome.storage, and the list of loaded extensions rather than relying only on source inspection. The toolset also supports actions such as evaluating code in extension contexts, invoking an action button or command, and reloading an extension.

How it works

The package runs as a local stdio MCP server, typically through npx @extension.dev/mcp. MCP clients send tool calls to the process, which manages local development sessions and communicates with supported browsers. By default, the server enables the local tools available on the machine; the optional platform group can be enabled with --features=local,platform.

Development sessions can target Chrome, Chromium, Edge, Brave, Opera, Firefox, Safari, Vivaldi, Yandex, Waterfox, Zen, Floorp, or a Chromium or Gecko binary supplied by path. Chromium-based browsers and Firefox support hot module replacement along with control, evaluation, inspection, and assertions. Safari supports development and building on macOS, but not hot module replacement or inspection.

Setup and configuration

Add the server to a client with a command such as:

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

The same npx command can be configured in Claude Desktop, Cursor, VS Code, Codex, or another MCP client that accepts stdio servers. Claude Code also provides a plugin installation path with extension-related slash commands.

Useful options include:

  • --features=local,platform enables the platform tools in addition to local tools.
  • --no-ship blocks publishing, promotion, shared previews, revocation, and real store submissions.
  • --project <workspace>/<project> restricts the server to one project.
  • EXTENSION_DEV_NO_AUTO_OPEN=1 prevents approval pages from opening automatically.
  • EXTENSION_DEV_NO_TELEMETRY=1 or DO_NOT_TRACK=1 disables the documented telemetry.

The corresponding environment settings include EXTENSION_DEV_FEATURES, EXTENSION_DEV_NO_SHIP, and EXTENSION_DEV_PROJECT.

Tools and capabilities

The local tool group includes 24 tools by default, with 11 additional platform tools available when enabled. Core capabilities include:

  • Scaffolding from more than 50 templates and reading template source.
  • Searching Extension.js and extension.dev documentation.
  • Adding an options page, popup, sidebar, or content script to an existing project.
  • Starting browser development sessions with HMR where supported.
  • Inspecting pages, logs, storage, loaded extensions, and runtime state.
  • Evaluating code, triggering extension commands or action buttons, and reloading extensions.
  • Expressing runtime expectations and receiving assertion verdicts.
  • Checking manifests across browsers and producing production builds.
  • Sharing previews, promoting builds, and submitting to stores through the platform lane.

The platform workflow is marked private alpha, and store submissions or stable promotions require human approval.

Read the full README View source on GitHub

Reviews

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Frequently Asked Questions about Extensiondev MCP

extensiondev/mcp is an MCP server that connects AI clients to Extension.js-based browser-extension workflows on a local machine. Its main tools scaffold projects, run extensions, inspect browser state, evaluate code, test behavior, validate manifests, build releases, and optionally share or publish extensions through platform tools.

Install it by configuring your MCP client to run `npx @extension.dev/mcp`. For example, Claude Code uses `claude mcp add extension-dev -- npx @extension.dev/mcp --features=local,platform`; Cursor, VS Code, Codex, Claude Desktop, and other stdio-capable clients can use the same command and arguments.

Yes, extensiondev/mcp works with Cursor. Add it through Cursor’s MCP installation link or place an MCP server entry in `.cursor/mcp.json` using `npx` with `@extension.dev/mcp` and the desired feature flags.

It supports Chrome, Chromium, Edge, Brave, Opera, Firefox, Safari, Vivaldi, Yandex, Waterfox, Zen, and Floorp, plus Chromium or Gecko binaries specified by path. Chromium browsers and Firefox provide the broadest development and inspection support; Safari supports development and builds on macOS.

Start it with `--no-ship` or set `EXTENSION_DEV_NO_SHIP=1`. That setting rejects calls that publish, promote, share a preview, revoke access, or perform a real store submission.

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