Build an MCP App

// 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");
});

<!-- 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>

// 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]";
});

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