Agent Skills: Create MCP App

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/client@^2.0.0 @modelcontextprotocol/server@^2.0.0 @modelcontextprotocol/node@^2.0.0 @modelcontextprotocol/express@^2.0.0 zod@^4.2.0 express cors
npm install -D typescript vite vite-plugin-singlefile concurrently cross-env @types/node @types/express @types/cors

ext-apps 2.x requires the split base MCP SDK packages (@modelcontextprotocol/client, server, node, express) at ^2.0.0; @modelcontextprotocol/core comes in transitively. Do not add the legacy @modelcontextprotocol/sdk v1 package or substitute unpublished local packages.

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 } });