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@extension.dev/mcp is the Extension.js MCP server. It exposes the whole Extension.js workflow as Model Context Protocol tools. An AI agent can build your extension, run it, and verify the result in a real browser. The tools are build-aware: they know your project, your generated manifest, and your dev session. Every tool lives under the extension_* namespace, and the server ships 28 of them. The table below covers the ones that you reach for most.

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Install the server

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The core tools

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The session model

extension_dev is the entry point. It is the only tool that unlocks the local control channel that the act verbs ride:
  • extension_storage, extension_reload, and extension_open need a session started with allowControl: true.
  • extension_eval needs a session started with allowEval: true, which writes a session token file with 0600 permissions.
Reading is always free. Anything that changes state is opt-in per session.

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Device login

The server includes a device login for the extension.dev platform:
The flow prints a code and a URL that you authorize at extension.dev/device. Minted tokens live at most 7 days, server-enforced. The stored login is also the third token source for extension publish. The CLI resolves a token in this order: the --token flag, then the EXTENSION_DEV_TOKEN environment variable, then the stored device login.

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Engine pinning for scaffolds

Projects scaffolded through the server’s create tool declare the Extension.js engine version inside the project itself. The project-local pin wins over the server environment, so later sessions build with the version that the scaffold declared.

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Next steps