/create-mcp-app

name: create-mcp-app
description: This skill should be used when the user asks to "create an MCP App", "add a UI to an MCP tool", "build an interactive MCP View", "scaffold an MCP App", or needs guidance on MCP Apps SDK patterns, UI-resource registration, MCP App lifecycle, or host integration. Provides comprehensive guidance for building MCP Apps with interactive UIs.

Create MCP App

Build interactive UIs that run inside MCP-enabled hosts like Claude Desktop. An MCP App combines an MCP tool with an HTML resource to display rich, interactive content.

Core Concept: Tool + Resource

Every MCP App requires two parts linked together:

1. **Tool** - Called by the LLM/host, returns data 2. **Resource** - Serves the bundled HTML UI that displays the data

The tool's `_meta.ui.resourceUri` references the resource's URI.

Host calls tool → Host renders resource UI → Server returns result → UI receives result.

Quick Start Decision Tree

Framework Selection

| Framework | SDK Support | Best For | |-----------|-------------|----------| | React | `useApp` hook provided | Teams familiar with React | | Vanilla JS | Manual lifecycle | Simple apps, no build complexity | | Vue/Svelte/Preact/Solid | Manual lifecycle | Framework preference |

Project Context

**Adding to existing MCP server:**

• Import `registerAppTool`, `registerAppResource` from SDK
• Add tool registration with `_meta.ui.resourceUri`
• Add resource registration serving bundled HTML

**Creating new MCP server:**

• Set up server with transport (stdio or HTTP)
• Register tools and resources
• Configure build system with `vite-plugin-singlefile`

Getting Reference Code

Clone the SDK repository for working examples and API documentation:

git clone --branch "v$(npm view @modelcontextprotocol/ext-apps version)" --depth 1 https://github.com/modelcontextprotocol/ext-apps.git /tmp/mcp-ext-apps

Framework Templates

Learn and adapt from `/tmp/mcp-ext-apps/examples/basic-server-{framework}/`:

| Template | Key Files | |----------|-----------| | `basic-server-vanillajs/` | `server.ts`, `src/mcp-app.ts`, `mcp-app.html` | | `basic-server-react/` | `server.ts`, `src/mcp-app.tsx` (uses `useApp` hook) | | `basic-server-vue/` | `server.ts`, `src/App.vue` | | `basic-server-svelte/` | `server.ts`, `src/App.svelte` | | `basic-server-preact/` | `server.ts`, `src/mcp-app.tsx` | | `basic-server-solid/` | `server.ts`, `src/mcp-app.tsx` |

Each template includes:

• `server.ts` with `registerAppTool` and `registerAppResource`
• `main.ts` entry point with HTTP and stdio transport setup
• Client-side app (e.g., `src/mcp-app.ts`, `src/mcp-app.tsx`) with lifecycle handlers
• `src/global.css` with global styles and host style variable fallbacks
• `vite.config.ts` using `vite-plugin-singlefile`
• `package.json` with `npm run` scripts and required dependencies
• `.gitignore` excluding `node_modules/` and `dist/`

API Reference (Source Files)

Read JSDoc documentation directly from `/tmp/mcp-ext-apps/src/`:

| File | Contents | |------|----------| | `src/app.ts` | `App` class, handlers (`ontoolinput`, `ontoolresult`, `onhostcontextchanged`, `onteardown`, etc.), lifecycle | | `src/server/index.ts` | `registerAppTool`, `registerAppResource`, helper functions | | `src/spec.types.ts` | All type definitions: `McpUiHostContext`, `McpUiStyleVariableKey` (CSS variable names), `McpUiResourceCsp` (CSP configuration), etc. | | `src/styles.ts` | `applyDocumentTheme`, `applyHostStyleVariables`, `applyHostFonts` | | `src/react/useApp.tsx` | `useApp` hook for React apps |

Advanced Patterns

See `/tmp/mcp-ext-apps/docs/patterns.md` for detailed recipes:

• **App-only tools** — `visibility: ["app"]`, hiding tools from model
• **Polling** — real-time dashboards, interval management
• **Chunked responses** — large files, pagination, base64 encoding
• **Error handling** — `isError`, informing model of failures
• **Binary resources** — audio/video/etc via `resources/read`, blob field
• **Network requests** — assets, fetch, CSP, `_meta.ui.csp`, CORS, `_meta.ui.domain`
• **Host context** — theme, styling, fonts, safe area insets
• **Fullscreen mode** — `requestDisplayMode`, display mode changes
• **Model context** — `updateModelContext`, `sendMessage`, keeping model informed
• **View state** — `viewUUID`, localStorage, state recovery
• **Visibility-based pause** — IntersectionObserver, pausing animations/WebGL
• **Streaming input** — `ontoolinputpartial`, progressive rendering

Reference Host Implementation

`/tmp/mcp-ext-apps/examples/basic-host/` shows one way an MCP Apps-capable host could be implemented. Real-world hosts like Claude Desktop are more sophisticated—use basic-host for local testing and protocol understanding, not as a guarantee of host behavior.

Critical Implementation Notes

Adding Dependencies

**Always** use `npm install` to add dependencies rather than manually writing version numbers:

npm install @modelcontextprotocol/ext-apps @modelcontextprotocol/sdk zod express cors
npm install -D typescript vite vite-plugin-singlefile concurrently cross-env @types/node @types/express @types/cors

This lets npm resolve the latest compatible versions. **Never** specify version numbers from memory.

TypeScript Server Execution

Unless the user has specified otherwise, use `tsx` for running TypeScript server files. For example:

npm install -D tsx

npm pkg set scripts.dev="cross-env NODE_ENV=development concurrently 'cross-env INPUT=mcp-app.html vite build --watch' 'tsx --watch main.ts'"

> [!NOTE] > The SDK examples use `bun` but generated projects should default to `tsx` for broader compatibility.

Handler Registration Order

Register ALL handlers BEFORE calling `app.connect()`:

const app = new App({ name: "My App", version: "1.0.0" });

// Register handlers first
app.ontoolinput = (params) => { /* handle input */ };
app.ontoolresult = (result) => { /* handle result */ };
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