name: OpenAI Apps MCP description: | Build ChatGPT apps with MCP servers on Cloudflare Workers. Extend ChatGPT with custom tools and interactive widgets (HTML/JS UI).

Use when: developing ChatGPT extensions, implementing MCP servers, or troubleshooting CORS blocking (allow chatgpt.com), widget 404s (missing ui://widget/), wrong MIME type (text/html+skybridge), or ASSETS binding undefined. allowed-tools: [Read, Write, Edit, Bash, Glob, Grep]

Building OpenAI Apps with Stateless MCP Servers

Status: Production Ready Last Updated: 2025-11-26 Dependencies: cloudflare-worker-base, hono-routing (optional) Latest Versions: @modelcontextprotocol/sdk@1.23.0, hono@4.10.6, zod@4.1.13, wrangler@4.50.0

Overview

Build ChatGPT Apps using MCP (Model Context Protocol) servers on Cloudflare Workers. Extends ChatGPT with custom tools and interactive widgets (HTML/JS UI rendered in iframe).

Architecture: ChatGPT → MCP endpoint (JSON-RPC 2.0) → Tool handlers → Widget resources (HTML)

Status: Apps available to Business/Enterprise/Edu (GA Nov 13, 2025). MCP Apps Extension (SEP-1865) formalized Nov 21, 2025.

Quick Start

1. Scaffold & Install

npm create cloudflare@latest my-openai-app -- --type hello-world --ts --git --deploy false
cd my-openai-app
npm install @modelcontextprotocol/sdk@1.23.0 hono@4.10.6 zod@4.1.13
npm install -D @cloudflare/vite-plugin@1.15.2 vite@7.2.4

2. Configure wrangler.jsonc

{
  "name": "my-openai-app",
  "main": "dist/index.js",
  "compatibility_flags": ["nodejs_compat"],  // Required for MCP SDK
  "assets": {
    "directory": "dist/client",
    "binding": "ASSETS"  // Must match TypeScript
  }
}

3. Create MCP Server (`src/index.ts`)

import { Hono } from 'hono';
import { cors } from 'hono/cors';
import { Server } from '@modelcontextprotocol/sdk/server/index.js';
import { ListToolsRequestSchema, CallToolRequestSchema } from '@modelcontextprotocol/sdk/types.js';

const app = new Hono<{ Bindings: { ASSETS: Fetcher } }>();

// CRITICAL: Must allow chatgpt.com
app.use('/mcp/*', cors({ origin: 'https://chatgpt.com' }));

const mcpServer = new Server(
  { name: 'my-app', version: '1.0.0' },
  { capabilities: { tools: {}, resources: {} } }
);

// Tool registration
mcpServer.setRequestHandler(ListToolsRequestSchema, async () => ({
  tools: [{
    name: 'hello',
    description: 'Use this when user wants to see a greeting',
    inputSchema: {
      type: 'object',
      properties: { name: { type: 'string' } },
      required: ['name']
    },
    annotations: {
      openai: { outputTemplate: 'ui://widget/hello.html' }  // Widget URI
    }
  }]
}));

// Tool execution
mcpServer.setRequestHandler(CallToolRequestSchema, async (request) => {
  if (request.params.name === 'hello') {
    const { name } = request.params.arguments as { name: string };
    return {
      content: [{ type: 'text', text: `Hello, ${name}!` }],
      _meta: { initialData: { name } }  // Passed to widget
    };
  }
  throw new Error(`Unknown tool: ${request.params.name}`);
});

app.post('/mcp', async (c) => {
  const body = await c.req.json();
  const response = await mcpServer.handleRequest(body);
  return c.json(response);
});

app.get('/widgets/*', async (c) => c.env.ASSETS.fetch(c.req.raw));

export default app;

4. Create Widget (`src/widgets/hello.html`)

<!DOCTYPE html>
<html>
<head>
  <style>
    body { margin: 0; padding: 20px; font-family: system-ui; }
  </style>
</head>
<body>
  <div id="greeting">Loading...</div>
  <script>
    if (window.openai && window.openai.getInitialData) {
      const data = window.openai.getInitialData();
      document.getElementById('greeting').textContent = `Hello, ${data.name}! 👋`;
    }
  </script>
</body>
</html>

5. Deploy

npm run build
npx wrangler deploy
npx @modelcontextprotocol/inspector https://my-app.workers.dev/mcp

Critical Requirements

CORS: Must allow https://chatgpt.com on /mcp/* routes Widget URI: Must use ui://widget/ prefix (e.g., ui://widget/map.html) MIME Type: Must be text/html+skybridge for HTML resources Widget Data: Pass via _meta.initialData (accessed via window.openai.getInitialData()) Tool Descriptions: Action-oriented ("Use this when user wants to...") ASSETS Binding: Serve widgets from ASSETS, not bundled in worker code SSE: Send heartbeat every 30s (100s timeout on Workers)

Known Issues Prevention

This skill prevents 8 documented issues:

Issue #1: CORS Policy Blocks MCP Endpoint

Error: Access to fetch blocked by CORS policy Fix: app.use('/mcp/*', cors({ origin: 'https://chatgpt.com' }))

Issue #2: Widget Returns 404 Not Found

Error: 404 (Not Found) for widget URL Fix: Use ui://widget/ prefix (not resource:// or /widgets/)

annotations: { openai: { outputTemplate: 'ui://widget/map.html' } }

Issue #3: Widget Displays as Plain Text

Error: HTML source code visible instead of rendered widget Fix: MIME type must be text/html+skybridge (not text/html)

server.setRequestHandler(ListResourcesRequestSchema, async () => ({
  resources: [{ uri: 'ui://widget/map.html', mimeType: 'text/html+skybridge' }]
}));

Issue #4: ASSETS Binding Undefined

Error: TypeError: Cannot read property 'fetch' of undefined Fix: Binding name in wrangler.jsonc must match TypeScript

{ "assets": { "binding": "ASSETS" } }  // wrangler.jsonc

type Bindings = { ASSETS: Fetcher };  // index.ts

Issue #5: SSE Connection Drops After 100 Seconds

Error: SSE stream closes unexpectedly Fix: Send heartbeat every 30s (Workers timeout at 100s inactivity)

const heartbeat = setInterval(async () => {
  await stream.writeSSE({ data: JSON.stringify({ type: 'heartbeat' }), event: 'ping' });
}, 30000);

Issue #6: ChatGPT Doesn't Suggest Tool

Error: Tool registered but never appears in suggestions Fix: Use action-oriented descriptions

// ✅ Good: 'Use this when user wants to see a location on a map'
// ❌ Bad: 'Shows a map'

Issue #7: Widget Can't Access Initial Data

Error: window.openai.getInitialData() returns undefined Fix: Pass data via _meta.initialData

return {
  content: [{ type: 'text', text: 'Here is your map' }],
  _meta: { initialData: { location: 'SF', zoom: 12 } }
};

Issue #8: Widget Scripts Blocked by CSP

Error: Refused to load script (CSP directive) Fix: Use inline scripts or same-origin scripts. Third-party CDNs blocked.

<!-- ✅ Works --> <script>console.log('ok');</script>
<!-- ❌ Blocked --> <script src="https://cdn.example.com/lib.js"></script>

Zod 4.0 Migration Notes (MAJOR UPDATE - July 2025)

Breaking Changes from zod@3.x → 4.x:

.default() now expects input type (not output type). Use .prefault() for old behavior.
ZodError: error.issues (not error.errors)
.merge() and .superRefine() deprecated
Optional properties with defaults now always apply

Performance: 14x faster string parsing, 7x faster arrays, 6.5x faster objects

Migration: Update validation code:

// Zod 4.x
try {
  const validated = schema.parse(data);
} catch (error) {
  if (error instanceof z.ZodError) {
    return { content: [{ type: 'text', text: error.issues.map(e => e.message).join(', ') }] };
  }
}

Dependencies

{
  "dependencies": {
    "@modelcontextprotocol/sdk": "^1.23.0",
    "hono": "^4.10.6",
    "zod": "^4.1.13"
  },
  "devDependencies": {
    "@cloudflare/vite-plugin": "^1.15.2",
    "@cloudflare/workers-types": "^4.20250531.0",
    "vite": "^7.2.4",
    "wrangler": "^4.50.0"
  }
}

Official Documentation

MCP Specification: https://modelcontextprotocol.io/ (Latest: 2025-11-25)
MCP SDK: https://github.com/modelcontextprotocol/typescript-sdk
OpenAI Apps SDK: https://developers.openai.com/apps-sdk
MCP Apps Extension (SEP-1865): http://blog.modelcontextprotocol.io/posts/2025-11-21-mcp-apps/
Context7 Library ID: /modelcontextprotocol/typescript-sdk

Production Reference

Open Source Example: https://github.com/jezweb/chatgpt-app-sdk (portfolio carousel widget)

Live in Production: Rendering in ChatGPT Business
MCP Server: Full JSON-RPC 2.0 implementation with tools + resources (~310 lines)
Widget Integration: WordPress API → window.openai.toolOutput → React carousel
Database: D1 (SQLite) for contact form submissions
Stack: Hono 4 + React 19 + Tailwind v4 + Drizzle ORM
Key Files:
- /src/lib/mcp/server.ts - Complete MCP handler
- /src/server/tools/portfolio.ts - Tool with widget annotations
- /src/widgets/PortfolioWidget.tsx - Data access pattern
Verified: All 8 known issues prevented, zero errors in production

OpenAI Apps MCP