Create MCP App

Table of Contents

• Core Concept: Tool + Resource
• Quick Start Decision Tree
• Getting Reference Code
• Critical Implementation Notes
• Common Mistakes to Avoid
• Testing

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

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}/:

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/:

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 */ };
app.onhostcontextchanged = (ctx) => { /* handle context */ };
app.onteardown = async () => { return {}; };
// etc.

// Then connect
await app.connect();

Common Mistakes to Avoid

1. No text fallback - Always provide content array for non-UI hosts
2. Missing CSP configuration - MCP Apps HTML is served as an MCP resource with no same-origin server; ALL network requests—even to localhost—require a CSP configuration
3. CSP or CORS config in wrong _meta object - _meta.ui.csp and _meta.ui.domain go in the contents[] objects returned by registerAppResource()'s read callback, not in registerAppResource()'s config object
4. Handlers after app.connect() - Register ALL handlers BEFORE calling app.connect()
5. No streaming for large inputs - Use ontoolinputpartial to show progress during input generation

Testing

Using basic-host

Test MCP Apps locally with the basic-host example:

# Terminal 1: Build and run your server
npm run build && npm run serve

# Terminal 2: Run basic-host (from cloned repo)
cd /tmp/mcp-ext-apps/examples/basic-host
npm install
SERVERS='["http://localhost:3001/mcp"]' npm run start
# Open http://localhost:8080

Configure SERVERS with a JSON array of your server URLs (default: http://localhost:3001/mcp).

Debug with sendLog

Send debug logs to the host application (rather than just the iframe's dev console):

await app.sendLog({ level: "info", data: "Debug message" });
await app.sendLog({ level: "error", data: { error: err.message } });