# @modelcontextprotocol/ext-apps

This repo contains the SDK and [specification](./main/specification/draft/apps.mdx) for MCP Apps Extension ([SEP-1865](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1865)).

MCP Apps are proposed standard inspired by [MCP-UI](https://mcpui.dev/) and [OpenAI's Apps SDK](https://developers.openai.com/apps-sdk/) to allow MCP Servers to display interactive UI elements in conversational MCP clients / chatbots.

This repo provides:
- [types.ts](./src/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](./examples/simple-example): Example Server + Apps
  - [server.ts](./examples/simple-server/server.ts): MCP server with two tools that declare UI resources of Apps to be show in the chat when called
  - [ui-react.tsx](./examples/simple-server/src/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](./examples/simple-server/src/ui-vanilla.ts): vanilla App returned by the `create-ui-vanilla`
  
- [main/specification/draft/apps.mdx](./main/specification/draft/apps.mdx): The Draft Extension Specification. It's still... in flux! Feedback welcome! (also see discussions in [SEP-1865](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1865)).

- [message-transport](./src/message-transport.ts): `PostMessageTransport` class that uses `postMessage` to exchange JSON-RPC messages between windows / iframes

- [app.ts](./src/app.ts): `App` class used by an App to talk to its host

- [app-bridge.ts](./src/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](https://github.com/MCP-UI-Org/mcp-ui/pull/146) of hosting / iframing / sandboxing logic to the [MCP-UI](https://github.com/idosal/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](https://gist.github.com/ochafik/a9603ba2d6757d6038ce066eded4c354)

## 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:

```bash
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`.