# OpenAI Apps MCP
> 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.
- Skill: `majiayu000/openai-apps-mcp-2` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add majiayu000/openai-apps-mcp-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/openai-apps-mcp-2/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/majiayu000/openai-apps-mcp-2
---
# Building OpenAI Apps with Stateless MCP Servers
**Status**: Production Ready
**Last Updated**: 2026-01-03
**Dependencies**: `cloudflare-worker-base`, `hono-routing` (optional)
**Latest Versions**: @modelcontextprotocol/sdk@1.25.1, hono@4.11.3, zod@4.1.13, wrangler@4.54.0
---
## 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
```bash
npm create cloudflare@latest my-openai-app -- --type hello-world --ts --git --deploy false
cd my-openai-app
npm install @modelcontextprotocol/sdk@1.25.1 hono@4.11.3 zod@4.1.13
npm install -D @cloudflare/vite-plugin@1.17.1 vite@7.2.4
```
### 2. Configure wrangler.jsonc
```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`)
```typescript
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`)
```html
Loading...
```
### 5. Deploy
```bash
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/`)
```typescript
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`)
```typescript
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
```jsonc
{ "assets": { "binding": "ASSETS" } } // wrangler.jsonc
```
```typescript
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)
```typescript
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
```typescript
// ✅ 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`
```typescript
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.
```html
```
## MCP SDK 1.25.x Updates (December 2025)
**Breaking Changes** from @modelcontextprotocol/sdk@1.24.x → 1.25.x:
- Removed loose type exports (Prompts, Resources, Roots, Sampling, Tools) - use specific schemas
- ES2020 target required (previous: ES2018)
- `setRequestHandler` is now typesafe - incorrect schemas throw type errors
**New Features**:
- **Tasks** (v1.24.0+): Long-running operations with progress tracking
- **Sampling with Tools** (v1.24.0+): Tools can request model sampling
- **OAuth Client Credentials** (M2M): Machine-to-machine authentication
**Migration**: If using loose type imports, update to specific schema imports:
```typescript
// ❌ Old (removed in 1.25.0)
import { Tools } from '@modelcontextprotocol/sdk/types.js';
// ✅ New (1.25.1+)
import { ListToolsRequestSchema, CallToolRequestSchema } from '@modelcontextprotocol/sdk/types.js';
```
---
## 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:
```typescript
// 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
```json
{
"dependencies": {
"@modelcontextprotocol/sdk": "^1.25.1",
"hono": "^4.11.3",
"zod": "^4.1.13"
},
"devDependencies": {
"@cloudflare/vite-plugin": "^1.17.1",
"@cloudflare/workers-types": "^4.20260103.0",
"vite": "^7.2.4",
"wrangler": "^4.54.0"
}
}
```
## 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