Build an MCP App

Prerequisites

You'll need Node.js 18 or higher. Familiarity with MCP tools and resources is recommended since MCP Apps combine both primitives. Experience with the MCP TypeScript SDK will help you better understand the server-side patterns.

Getting started

The fastest way to create an MCP App is using an AI coding agent with the MCP Apps skill. If you prefer to set up a project manually, skip to Manual setup.

Using an AI coding agent

AI coding agents with Skills support can scaffold a complete MCP App project for you. Skills are folders of instructions and resources that your agent loads when relevant. They teach the AI how to perform specialized tasks like creating MCP Apps.

The create-mcp-app skill includes architecture guidance, best practices, and working examples that the agent uses to generate your project.

1. Install the skill

   If you are using Claude Code, you can install the skill directly with:

   /plugin marketplace add modelcontextprotocol/ext-apps
   /plugin install mcp-apps@modelcontextprotocol-ext-apps
   

   You can also use the Vercel Skills CLI to install skills across different AI coding agents:

   npx skills add modelcontextprotocol/ext-apps
   

   Alternatively, you can install the skill manually by cloning the ext-apps repository:

   git clone https://github.com/modelcontextprotocol/ext-apps.git
   

   And then copying the skill to the appropriate location for your agent:

   Agent	Skills directory (macOS/Linux)	Skills directory (Windows)
   Claude Code	~/.claude/skills/	%USERPROFILE%\.claude\skills\
   VS Code and GitHub Copilot	~/.copilot/skills/	%USERPROFILE%\.copilot\skills\
   Gemini CLI	~/.gemini/skills/	%USERPROFILE%\.gemini\skills\
   Cline	~/.cline/skills/	%USERPROFILE%\.cline\skills\
   Goose	~/.config/goose/skills/	%USERPROFILE%\.config\goose\skills\
   Codex	~/.codex/skills/	%USERPROFILE%\.codex\skills\
   Cursor	~/.cursor/skills/	%USERPROFILE%\.cursor\skills\

   For example, with Claude Code you can install the skill globally (available in all projects):

   cp -r ext-apps/plugins/mcp-apps/skills/create-mcp-app ~/.claude/skills/create-mcp-app
   

   Copy-Item -Recurse ext-apps\plugins\mcp-apps\skills\create-mcp-app $env:USERPROFILE\.claude\skills\create-mcp-app
   

   Or install it for a single project only by copying to .claude/skills/ in your project directory:

   mkdir -p .claude/skills && cp -r ext-apps/plugins/mcp-apps/skills/create-mcp-app .claude/skills/create-mcp-app
   

   New-Item -ItemType Directory -Force -Path .claude\skills | Out-Null; Copy-Item -Recurse ext-apps\plugins\mcp-apps\skills\create-mcp-app .claude\skills\create-mcp-app
   

   To verify the skill is installed, ask your agent "What skills do you have access to?" — you should see create-mcp-app as one of the available skills.

2. Create your app

   Ask your AI coding agent to build it:

   Create an MCP App that displays a color picker
   

   The agent will recognize the create-mcp-app skill is relevant, load its instructions, then scaffold a complete project with server, UI, and configuration files.

   

3. Run your app

   npm install && npm run build && npm run serve
   

   npm install; npm run build; npm run serve
   

4. Test your app

   Follow the instructions in Testing your app below. For the color picker example, start a new chat and ask Claude to provide you a color picker.

   

Manual setup

If you're not using an AI coding agent, or prefer to understand the setup process, follow these steps.

1. Create the project structure

   A typical MCP App project separates the server code from the UI code:

   my-mcp-app

   package.json

   tsconfig.json

   vite.config.ts

   server.ts

   mcp-app.html

   src

   mcp-app.ts

   The server registers the tool and serves the UI resource. The UI resource will eventually be rendered in a secure iframe with deny-by-default CSP configuration. If your app has CSS and JS assets, you will need to configure CSP, or you can bundle your assets into the HTML with a tool like vite-plugin-singlefile, which is what we will do in this tutorial.

2. Install dependencies

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

   The ext-apps package provides helpers for both the server side (registering tools and resources) and the client side (the App class for UI-to-host communication). Vite with the vite-plugin-singlefile plugin is used here to bundle your UI and assets into a single HTML file for convenience, but this is optional — you can use any bundler or serve unbundled files if you configure CSP.

3. Configure the project

   package.json

   The "type": "module" setting enables ES module syntax. The build script uses the INPUT environment variable to tell Vite which HTML file to bundle. The serve script runs your server using tsx for TypeScript execution.

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

   tsconfig.json

   The TypeScript configuration targets modern JavaScript (ES2022) and uses ESNext modules with bundler resolution, which works well with Vite. The include array covers both the server code in the root and UI code in src/.

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

   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,
       },
     },
   });
   

4. Build the project

   With the project structure and configuration in place, continue to Building an MCP App below to implement the server and UI.

Building an MCP App

Let's build a simple app that displays the current server time. This example demonstrates the full pattern: registering a tool with UI metadata, serving the bundled HTML as a resource, and building a UI that communicates with the server.

Server implementation

The server needs to do two things: register a tool that includes the _meta.ui.resourceUri field, and register a resource handler that serves the bundled HTML. Here's the complete server file:

// server.ts
console.log("Starting MCP App server...");

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import {
  registerAppTool,
  registerAppResource,
  RESOURCE_MIME_TYPE,
} from "@modelcontextprotocol/ext-apps/server";
import cors from "cors";
import express from "express";
import fs from "node:fs/promises";
import path from "node:path";

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

// The ui:// scheme tells hosts this is an MCP App resource.
// The path structure is arbitrary; organize it however makes sense for your app.
const resourceUri = "ui://get-time/mcp-app.html";

// Register the tool that returns the current time
registerAppTool(
  server,
  "get-time",
  {
    title: "Get Time",
    description: "Returns the current server time.",
    inputSchema: {},
    _meta: { ui: { resourceUri } },
  },
  async () => {
    const time = new Date().toISOString();
    return {
      content: [{ type: "text", text: time }],
    };
  },
);

// Register the resource that serves the bundled HTML
registerAppResource(
  server,
  resourceUri,
  resourceUri,
  { mimeType: RESOURCE_MIME_TYPE },
  async () => {
    const html = await fs.readFile(
      path.join(import.meta.dirname, "dist", "mcp-app.html"),
      "utf-8",
    );
    return {
      contents: [
        { uri: resourceUri, mimeType: RESOURCE_MIME_TYPE, text: html },
      ],
    };
  },
);

// Expose the MCP server over HTTP
const expressApp = express();
expressApp.use(cors());
expressApp.use(express.json());

expressApp.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);
});

expressApp.listen(3001, (err) => {
  if (err) {
    console.error("Error starting server:", err);
    process.exit(1);
  }
  console.log("Server listening on http://localhost:3001/mcp");
});

Let's break down the key parts:

• resourceUri: The ui:// scheme tells hosts this is an MCP App resource. The path structure is arbitrary.
• registerAppTool: Registers a tool with the _meta.ui.resourceUri field. When the host calls this tool, the UI is fetched and rendered, and the tool result is passed to it upon arrival.
• registerAppResource: Serves the bundled HTML when the host requests the UI resource.
• Express server: Exposes the MCP server over HTTP on port 3001.

UI implementation

The UI consists of an HTML page and a TypeScript module that uses the App class to communicate with the host. Here's the HTML:

<!-- mcp-app.html -->
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>Get Time App</title>
  </head>
  <body>
    <p>
      <strong>Server Time:</strong>
      <code id="server-time">Loading...</code>
    </p>
    <button id="get-time-btn">Get Server Time</button>
    <script type="module" src="/src/mcp-app.ts"></script>
  </body>
</html>

And the TypeScript module:

// src/mcp-app.ts
import { App } from "@modelcontextprotocol/ext-apps";

const serverTimeEl = document.getElementById("server-time")!;
const getTimeBtn = document.getElementById("get-time-btn")!;

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

// Establish communication with the host
app.connect();

// Handle the initial tool result pushed by the host
app.ontoolresult = (result) => {
  const time = result.content?.find((c) => c.type === "text")?.text;
  serverTimeEl.textContent = time ?? "[ERROR]";
};

// Proactively call tools when users interact with the UI
getTimeBtn.addEventListener("click", async () => {
  const result = await app.callServerTool({
    name: "get-time",
    arguments: {},
  });
  const time = result.content?.find((c) => c.type === "text")?.text;
  serverTimeEl.textContent = time ?? "[ERROR]";
});

The key parts:

• app.connect(): Establishes communication with the host. Call this once when your app initializes.
• app.ontoolresult: A callback that fires when the host pushes a tool result to your app (e.g., when the tool is first called and the UI renders).
• app.callServerTool(): Lets your app proactively call tools on the server. Keep in mind that each call involves a round-trip to the server, so design your UI to handle latency gracefully.

The App class provides additional methods for logging, opening URLs, and updating the model's context with structured data from your app. See the full API documentation.

Testing your app

To test your MCP App, build the UI and start your local server:

npm run build && npm run serve

npm run build; npm run serve

In the default configuration, your server will be available at http://localhost:3001/mcp. However, to see your app render, you need an MCP host that supports MCP Apps. You have several options.

Testing with Claude

Claude (web) and Claude Desktop support MCP Apps. For local development, you'll need to expose your server to the internet. You can run an MCP server locally and use tools like cloudflared to tunnel traffic through.

In a separate terminal, run:

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

Copy the generated URL (e.g., https://random-name.trycloudflare.com) and add it as a custom connector in Claude - click on your profile, go to Settings, Connectors, and finally Add custom connector.

Testing with the basic-host

The ext-apps repository includes a test host for development. Clone the repo and install dependencies:

git clone https://github.com/modelcontextprotocol/ext-apps.git
cd ext-apps/examples/basic-host
npm install

git clone https://github.com/modelcontextprotocol/ext-apps.git
cd ext-apps\examples\basic-host
npm install

Running npm start from ext-apps/examples/basic-host/ will start the basic-host test interface. To connect it to a specific server (e.g., one you're developing), pass the SERVERS environment variable inline:

SERVERS='["http://localhost:3001/mcp"]' npm start

$env:SERVERS='["http://localhost:3001/mcp"]'; npm start

Navigate to http://localhost:8080. You'll see a simple interface where you can select a tool and call it. When you call your tool, the host fetches the UI resource and renders it in a sandboxed iframe. You can then interact with your app and verify that tool calls work correctly.

Learn more

Feedback

MCP Apps is under active development. If you encounter issues or have ideas for improvements, open an issue on the GitHub repository. For broader discussions about the extension's direction, join the conversation in GitHub Discussions.