c5b2543115
Install instructions now list the split base MCP SDK packages at ^2.0.0 by role (core comes in transitively, server only for server authors), require Node 20+, and state that the wire protocol is unchanged so 2.x Views and 1.x hosts interoperate. The agent skills lose the exact prerelease wording and add-app-to-server gains a v1 to v2 import mapping. docs/migrate-to-2.md lists the breaking changes, the peer matrix and the host-side error deltas.
103 lines
4.8 KiB
Markdown
103 lines
4.8 KiB
Markdown
# MCP Apps SDK
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## Project Overview
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MCP Apps SDK (`@modelcontextprotocol/ext-apps`) enables MCP servers to display interactive UIs in conversational clients.
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Key abstractions:
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- **View** - UI running in an iframe, uses `App` class with `PostMessageTransport` to communicate with host
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- **Host** - Chat client embedding the iframe, uses `AppBridge` class to proxy MCP requests
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- **Server** - MCP server that registers tools/resources with UI metadata
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Specification (stable): `specification/2026-01-26/apps.mdx`
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## Commands
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```bash
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# Install dependencies
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npm install
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# Build the SDK only (generates schemas + bundles, does not build examples)
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npm run build
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# Build everything (SDK + all examples)
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npm run build:all
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# Type check + build a single example
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npm run --workspace examples/<example-name> build
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# Run all examples (starts server at http://localhost:8080)
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npm start
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# Run E2E tests (primary testing mechanism - starts examples server automatically)
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npm run test:e2e
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# Run unit tests (E2E tests have broader coverage; unit tests cover specific modules)
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npm test
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# Check JSDoc comment syntax and `{@link}` references
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npm exec typedoc -- --treatValidationWarningsAsErrors --emit none
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# Regenerate package-lock.json
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# Note: repo .npmrc pins registry to npmjs.org, so a plain `npm i` is safe even
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# if your global npm config points elsewhere. The Docker step below is optional
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# — it locks linux-amd64 optionalDependencies (sharp, rollup, bun) for CI.
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rm -fR package-lock.json node_modules && \
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docker run --rm -it --platform linux/amd64 -v $PWD:/src:rw -w /src node:latest npm i && \
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rm -fR node_modules && \
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npm i --cache=~/.npm-mcp-apps --registry=https://registry.npmjs.org/
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```
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## Architecture
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### SDK Entry Points
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- `@modelcontextprotocol/ext-apps` - Main SDK for Apps (`App` class, `PostMessageTransport`)
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- `@modelcontextprotocol/ext-apps/react` - React hooks (`useApp`, `useHostStyleVariables`, etc.)
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- `@modelcontextprotocol/ext-apps/app-bridge` - SDK for hosts (`AppBridge` class)
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- `@modelcontextprotocol/ext-apps/server` - Server helpers (`registerAppTool`, `registerAppResource`)
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### Key Source Files
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- `src/app.ts` - `App` subclasses `Protocol` from `@modelcontextprotocol/client`, handles View initialization, tool calls, and messaging
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- `src/app-bridge.ts` - `AppBridge` subclasses `Protocol` from `@modelcontextprotocol/client` for the iframe channel and proxies through a separate outer `Client`
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- `src/server/index.ts` - Helpers for MCP servers to register tools/resources with UI metadata
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- `src/types.ts` - Protocol types re-exported from `spec.types.ts` and Zod schemas from `generated/schema.ts` (auto-generated during build)
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- `src/message-transport.ts` - `PostMessageTransport` for iframe communication
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- `src/react/` - React hooks: `useApp`, `useHostStyles`, `useAutoResize`, `useDocumentTheme`
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### Protocol Flow
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```
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View (App/Protocol) <--PostMessageTransport--> Host (AppBridge/Protocol) <--separate MCP Client--> MCP Server
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```
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1. Host creates iframe with view HTML
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2. View creates `App` instance and calls `connect()` with `PostMessageTransport`
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3. View sends `ui/initialize` (the only handshake on the iframe channel) and receives authoritative Apps capabilities and context
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4. Host sends `sendToolInput()` with tool arguments after initialization
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5. View can call server tools via `app.callServerTool()` or send messages via `app.sendMessage()`
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6. Host sends `sendToolResult()` when tool execution completes
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7. Host calls `teardownResource()` before unmounting iframe
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## Documentation
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JSDoc `@example` tags should pull type-checked code from companion `.examples.ts` files (e.g., `app.ts` → `app.examples.ts`). Use ` ```ts source="./file.examples.ts#regionName" ` fences referencing `//#region regionName` blocks; region names follow `exportedName_variant` or `ClassName_methodName_variant` pattern (e.g., `useApp_basicUsage`, `App_hostCapabilities_checkAfterConnection`). For whole-file inclusion (any file type), omit the `#regionName`. Run `npm run sync:snippets` to sync.
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Standalone docs in `docs/` (listed in `typedoc.config.mjs` `projectDocuments`) can also have type-checked companion `.ts`/`.tsx` files using the same pattern.
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## Full Examples
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Uses npm workspaces. Full examples in `examples/` are separate packages:
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- `basic-server-*` - Starter templates (vanillajs, react, vue, svelte, preact, solid). Use these as the basis for new examples.
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- `basic-host` - Reference host implementation
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- Other examples showcase specific features (charts, 3D, video, etc.)
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## Claude Code Plugin
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The `plugins/mcp-apps/` directory contains a Claude Code plugin distributed via the plugin marketplace. It provides the following Claude Code skills files:
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- `plugins/mcp-apps/skills/create-mcp-app/SKILL.md` — for creating an MCP App
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- `plugins/mcp-apps/skills/migrate-oai-app/SKILL.md` — for migrating an app from the OpenAI Apps SDK to the MCP Apps SDK
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