MCP Apps

MCP Apps Skill

Build interactive UI applications that render directly inside MCP hosts like Claude Desktop, ChatGPT, and VSCode. MCP Apps extend the Model Context Protocol to return rich interfaces instead of plain text.

When to Use This Skill

• Creating MCP tools with interactive UI (forms, dashboards, visualizations)
• Building visual interfaces for AI-assisted workflows
• Adding rich media viewers (PDFs, maps, 3D models) to conversations
• Creating configuration wizards or multi-step workflows
• Building real-time monitoring dashboards

Core Concepts

What Are MCP Apps?

MCP Apps let tools return rich, interactive interfaces instead of plain text. When a tool declares a UI resource, the host renders it in a sandboxed iframe, and users interact with it directly in the conversation.

Key benefits over regular web apps:

• Context preservation - UI lives inside the conversation
• Bidirectional data flow - App can call server tools, host pushes results
• Integration with host capabilities - Delegate actions to the host
• Security guarantees - Sandboxed iframe with controlled permissions

Architecture

┌─────────────────────────────────────────────────────────────┐
│                    MCP Host (Claude, VSCode)                 │
│  ┌─────────────────────────────────────────────────────┐    │
│  │                  Sandboxed Iframe                    │    │
│  │  ┌─────────────────────────────────────────────┐    │    │
│  │  │              Your MCP App UI                 │    │    │
│  │  │     (React, Vue, Svelte, or vanilla)        │    │    │
│  │  └─────────────────────────────────────────────┘    │    │
│  │                         │                            │    │
│  │                   postMessage                        │    │
│  │                         │                            │    │
│  └─────────────────────────┼────────────────────────────┘    │
│                            │                                  │
│                     JSON-RPC                                  │
│                            │                                  │
│  ┌─────────────────────────┼────────────────────────────┐    │
│  │                   MCP Server                          │    │
│  │  • Tools with _meta.ui.resourceUri                   │    │
│  │  • UI Resources (bundled HTML)                       │    │
│  └───────────────────────────────────────────────────────┘    │
└───────────────────────────────────────────────────────────────┘

How It Works

1. Tool declares UI metadata - Tool description includes _meta.ui.resourceUri pointing to a ui:// resource
2. Host preloads UI - Host fetches the bundled HTML before tool execution
3. Sandboxed rendering - UI renders in sandboxed iframe with restricted permissions
4. Bidirectional communication - App and host communicate via JSON-RPC over postMessage

Quick Start

Using the Init Script

# Scaffold a new MCP App project
python scripts/init_mcp_app.py my-mcp-app --path ./projects

This creates a complete project structure with server, UI, and configuration.

Manual Setup

1. Install dependencies:

npm install @modelcontextprotocol/ext-apps @modelcontextprotocol/sdk
npm install -D typescript vite vite-plugin-singlefile express cors @types/express @types/cors tsx

2. Create project structure:

my-mcp-app/
├── package.json
├── tsconfig.json
├── vite.config.ts
├── server.ts           # MCP server
├── mcp-app.html        # UI entry point
└── src/
    └── mcp-app.ts      # UI logic

See references/boilerplate.md for complete file contents.

Server Implementation

Registering a Tool with UI

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { registerAppTool, registerAppResource, RESOURCE_MIME_TYPE } from "@modelcontextprotocol/ext-apps/server";

const server = new McpServer({
  name: "My MCP App Server",
  version: "1.0.0",
});

// The ui:// scheme tells hosts this is an MCP App resource
const resourceUri = "ui://my-tool/app.html";

// Register the tool with UI metadata
registerAppTool(
  server,
  "my-tool",
  {
    title: "My Tool",
    description: "A tool with interactive UI",
    inputSchema: {
      type: "object",
      properties: {
        query: { type: "string" }
      }
    },
    _meta: {
      ui: { resourceUri }
    }
  },
  async (args) => {
    // Tool logic here
    return {
      content: [{ type: "text", text: JSON.stringify(args) }]
    };
  }
);

// Register the UI resource
registerAppResource(
  server,
  resourceUri,
  resourceUri,
  { mimeType: RESOURCE_MIME_TYPE },
  async () => {
    const html = await fs.readFile("dist/mcp-app.html", "utf-8");
    return {
      contents: [{ uri: resourceUri, mimeType: RESOURCE_MIME_TYPE, text: html }]
    };
  }
);

Exposing via HTTP

import express from "express";
import cors from "cors";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";

const app = express();
app.use(cors());
app.use(express.json());

app.post("/mcp", async (req, res) => {
  const transport = new StreamableHTTPServerTransport({
    sessionIdGenerator: undefined,
    enableJsonResponse: true,
  });
  res.on("close", () => transport.close());
  await server.connect(transport);
  await transport.handleRequest(req, res, req.body);
});

app.listen(3001, () => {
  console.log("Server listening on http://localhost:3001/mcp");
});

UI Implementation

Basic App Class Usage

import { App } from "@modelcontextprotocol/ext-apps";

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

// Connect to host
await app.connect();

// Receive initial tool result
app.ontoolresult = (result) => {
  const data = result.content?.find((c) => c.type === "text")?.text;
  console.log("Received:", data);
  renderUI(data);
};

// Call server tools from UI
async function fetchData() {
  const result = await app.callServerTool({
    name: "fetch-data",
    arguments: { query: "example" },
  });
  return result.content?.find((c) => c.type === "text")?.text;
}

// Update model context
async function notifyModel(info: string) {
  await app.updateModelContext({
    content: [{ type: "text", text: info }],
  });
}

// Log for debugging
app.log("info", "App initialized");

// Open links in user's browser
app.openLink("https://example.com");

HTML Entry Point

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <title>My MCP App</title>
  <style>
    body { font-family: system-ui, sans-serif; padding: 1rem; }
    /* Add your styles */
  </style>
</head>
<body>
  <div id="app">Loading...</div>
  <script type="module" src="/src/mcp-app.ts"></script>
</body>
</html>

Configuration Files

vite.config.ts

import { defineConfig } from "vite";
import { viteSingleFile } from "vite-plugin-singlefile";

export default defineConfig({
  plugins: [viteSingleFile()],
  build: {
    outDir: "dist",
    rollupOptions: {
      input: process.env.INPUT,
    },
  },
});

package.json Scripts

{
  "type": "module",
  "scripts": {
    "build": "INPUT=mcp-app.html vite build",
    "serve": "npx tsx server.ts",
    "dev": "npm run build && npm run serve"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "moduleResolution": "bundler",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "outDir": "dist"
  },
  "include": ["*.ts", "src/**/*.ts"]
}

Testing

With Claude Desktop

1. Build and start your server:

   npm run build && npm run serve
   

2. Expose locally with cloudflared:

   npx cloudflared tunnel --url http://localhost:3001
   

3. Add as custom connector in Claude:

   • Profile → Settings → Connectors → Add custom connector
   • Enter the cloudflared URL

4. Chat with Claude and trigger your tool.

With basic-host

The ext-apps repo includes a test host:

git clone https://github.com/modelcontextprotocol/ext-apps
cd ext-apps/examples/basic-host
npm install
SERVERS='["http://localhost:3001/mcp"]' npm start

Navigate to http://localhost:8080 to test.

Security Model

MCP Apps run in sandboxed iframes with:

• No access to parent window DOM
• No cookie or storage access from host
• Cannot navigate parent page
• All communication via auditable JSON-RPC

Requesting Permissions

registerAppTool(server, "camera-tool", {
  // ...
  _meta: {
    ui: {
      resourceUri: "ui://camera/app.html",
      permissions: ["camera", "microphone"], // Request additional permissions
      csp: ["https://cdn.example.com"] // Allow external resources
    }
  }
});

Framework Support

MCP Apps work with any web framework. The ext-apps repo has starters for:

• React
• Vue
• Svelte
• Preact
• Solid
• Vanilla JavaScript

See references/patterns.md for framework-specific patterns.

Common Patterns

Form with Validation

app.ontoolresult = (result) => {
  renderForm(result.content);
};

async function handleSubmit(formData: FormData) {
  const result = await app.callServerTool({
    name: "submit-form",
    arguments: Object.fromEntries(formData),
  });
  
  if (result.isError) {
    showError(result.content);
  } else {
    showSuccess(result.content);
    await app.updateModelContext({
      content: [{ type: "text", text: "User submitted form successfully" }],
    });
  }
}

Real-time Dashboard

let intervalId: number;

app.ontoolresult = async () => {
  // Start polling for updates
  intervalId = setInterval(async () => {
    const result = await app.callServerTool({
      name: "get-metrics",
      arguments: {},
    });
    updateDashboard(result.content);
  }, 5000);
};

// Clean up on disconnect
window.addEventListener("beforeunload", () => {
  clearInterval(intervalId);
});

Multi-step Workflow

let currentStep = 0;
const steps = ["configure", "review", "confirm"];

async function nextStep(data: unknown) {
  const result = await app.callServerTool({
    name: `workflow-${steps[currentStep]}`,
    arguments: data,
  });
  
  currentStep++;
  
  if (currentStep < steps.length) {
    renderStep(currentStep, result.content);
  } else {
    await app.updateModelContext({
      content: [{ type: "text", text: "Workflow completed" }],
    });
    renderComplete(result.content);
  }
}

Troubleshooting

UI Not Rendering

• Ensure tool has _meta.ui.resourceUri in description
• Check that UI resource is registered with correct URI
• Verify bundled HTML is being served correctly
• Check browser console for errors

Communication Errors

• Ensure app.connect() is called before other operations
• Check that server is running and accessible
• Verify CORS is enabled on server

Host Not Recognizing App

• Confirm host supports MCP Apps (Claude, VSCode Insiders, ChatGPT)
• Check that server exposes correct MCP endpoint
• Verify cloudflared tunnel is active for remote testing

References

• references/boilerplate.md - Complete project setup code
• references/patterns.md - Common UI patterns and examples
• ext-apps Repository - Official SDK and examples
• MCP Apps Documentation - Official docs
• API Reference - Full SDK API