@modelcontextprotocol/ext-apps

This repo contains the SDK and specification for MCP Apps Extension (SEP-1865).

MCP Apps are proposed standard inspired by MCP-UI and OpenAI's Apps SDK to allow MCP Servers to display interactive UI elements in conversational MCP clients / chatbots.

This repo provides:

• types.ts: Types of JSON-RPC messages used to communicate between Apps & their host

  • Note that MCP Apps also use some standard MCP messages (e.g. tools/call for the App to trigger actions on its originating Server - these calls are proxied through the Host), but these types are the additional messages defined by the extension

• examples/simple-example: Example Server + Apps

  • server.ts: MCP server with two tools that declare UI resources of Apps to be show in the chat when called
  • ui-react.tsx: React App returned by the create-ui-react tool shows how to use the useApp hook to register MCP callbacks
  • ui-vanilla.tsx: vanilla App returned by the create-ui-vanilla

• specification/draft/apps.mdx: The Draft Extension Specification. It's still... in flux! Feedback welcome! (also see discussions in SEP-1865).

• message-transport: PostMessageTransport class that uses postMessage to exchange JSON-RPC messages between windows / iframes

• app.ts: App class used by an App to talk to its host

• app-bridge.ts: AppBridge class used by the host to talk to a single App

• Soon: more examples!

What this repo does NOT provide:

• There's no host implementation here (beyond the AppBridge just used for communications).
  • We have contributed a tentative implementation of hosting / iframing / sandboxing logic to the MCP-UI repository, and expect OSS clients may use it, while other clients might roll their own hosting logic.
  • A prior iteration of an e2e prototype w/ client, server and hosting parts is available in this gist

Installation

This repo is in flux and isn't published to npm (when it is, it will use the @modelcontextprotocol/ext-apps package). Please install it from git for now:

npm install git+https://github.com/modelcontextprotocol/ext-apps.git

Development Notes

Build tools in dependencies

The build tools (esbuild, tsx, typescript) are in dependencies rather than devDependencies. This is intentional: it allows the prepare script to run when the package is installed from git, since npm doesn't install devDependencies for git dependencies.

Once the package is published to npm with pre-built dist/, these can be moved back to devDependencies.