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This skill helps you develop and troubleshoot OpenAI MCP apps on Cloudflare Workers, including CORS, widgets, MIME types, and asset bindings.
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---
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, widget 404s, MIME types, ASSETS binding errors, Next.js integration issues, or edge platform limitations.
user-invocable: true
allowed-tools: [Read, Write, Edit, Bash, Glob, Grep]
---
# Building OpenAI Apps with Stateless MCP Servers
**Status**: Production Ready
**Last Updated**: 2026-01-21
**Dependencies**: `cloudflare-worker-base`, `hono-routing` (optional)
**Latest Versions**: @modelcontextprotocol/[email protected], [email protected], [email protected], [email protected]
---
## 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/[email protected] [email protected] [email protected]
npm install -D @cloudflare/[email protected] [email protected]
```
### 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
<!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
```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 **14** 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
<!-- ✅ Works --> <script>console.log('ok');</script>
<!-- ❌ Blocked --> <script src="https://cdn.example.com/lib.js"></script>
```
### Issue #9: Hono Global Response Override Breaks Next.js (v1.25.0-1.25.2)
**Error**: `No response is returned from route handler` (Next.js App Router)
**Source**: [GitHub Issue #1369](https://github.com/modelcontextprotocol/typescript-sdk/issues/1369)
**Affected Versions**: v1.25.0 to v1.25.2
**Fixed In**: v1.25.3
**Why It Happens**: Hono (MCP SDK dependency) overwrites `global.Response`, breaking frameworks that extend it (Next.js, Remix, SvelteKit). NextResponse instanceof check fails.
**Prevention**:
- **Upgrade to v1.25.3+** (recommended)
- **Before fix**: Use `webStandardStreamableHTTPServerTransport` instead
- **Or**: Run MCP server on separate port from Next.js/Remix/SvelteKit app
```typescript
// ✅ v1.25.3+ - Fixed
const transport = new StreamableHTTPServerTransport({
sessionIdGenerator: undefined,
});
// ✅ v1.25.0-1.25.2 - Workaround
import { webStandardStreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/index.js';
const transport = webStandardStreamableHTTPServerTransport({
sessionIdGenerator: undefined,
});
```
### Issue #10: Elicitation (User Input) Fails on Cloudflare Workers
**Error**: `EvalError: Code generation from strings disallowed`
**Source**: [GitHub Issue #689](https://github.com/modelcontextprotocol/typescript-sdk/issues/689)
**Why It Happens**: Internal AJV v6 validator uses prohibited APIs on edge platforms
**Prevention**: Avoid `elicitInput()` on edge platforms (Cloudflare Workers, Vercel Edge, Deno Deploy)
**Workaround**:
```typescript
// ❌ Don't use on Cloudflare Workers
const userInput = await server.elicitInput({
prompt: "What is your name?",
schema: { type: "string" }
});
// ✅ Use tool parameters instead
server.setRequestHandler(CallToolRequestSchema, async (request) => {
const { name } = request.params.arguments as { name: string };
// User provides via tool call, not elicitation
});
```
**Status**: Requires MCP SDK v2 to fix properly. Track [PR #844](https://github.com/modelcontextprotocol/typescript-sdk/pull/844).
### Issue #11: SSE Transport Statefulness Breaks Serverless Deployments
**Error**: `400: No transport found for sessionId`
**Source**: [GitHub Issue #273](https://github.com/modelcontextprotocol/typescript-sdk/issues/273)
**Why It Happens**: `SSEServerTransport` relies on in-memory session storage. In serverless environments (AWS Lambda, Cloudflare Workers), the initial `GET /sse` request may be handled by Instance A, but subsequent `POST /messages` requests land on Instance B, which lacks the in-memory state.
**Prevention**: Use **Streamable HTTP transport** (added in v1.24.0) instead of SSE for serverless deployments
**Solution**: For stateful SSE, deploy to non-serverless environments (VPS, long-running containers)
**Official Status**: Fixed by introducing Streamable HTTP (v1.24+) - now the **recommended standard** for serverless.
### Issue #12: OAuth Configuration Requires TWO Separate Apps
**Source**: [Cloudflare Remote MCP Server Docs](https://developers.cloudflare.com/agents/guides/remote-mcp-server/)
**Why It Happens**: OAuth providers validate redirect URLs strictly. Localhost and production have different URLs, so they need separate OAuth client registrations.
**Prevention**:
```bash
# Development OAuth App
Callback URL: http://localhost:8788/callback
# Production OAuth App
Callback URL: https://my-mcp-server.workers.dev/callback
```
**Additional Requirements**:
- KV namespace for auth state storage (create manually)
- `COOKIE_ENCRYPTION_KEY` env var: `openssl rand -hex 32`
- Client restart required after config changes
### Issue #13: Widget State Over 4k Tokens Causes Performance Issues (Community-sourced)
**Source**: [OpenAI Apps SDK - ChatGPT UI](https://developers.openai.com/apps-sdk/build/chatgpt-ui/)
**Why It Happens**: Widget state persists only to a single widget instance tied to one conversation message. State is reset when users submit via the main chat composer instead of widget controls.
**Prevention**: Keep state payloads under **4k tokens** for optimal performance
```typescript
// ✅ Good - Lightweight state
window.openai.setWidgetState({ selectedId: "item-123", view: "grid" });
// ❌ Bad - Will cause performance issues
window.openai.setWidgetState({
items: largeArray, // Don't store full datasets
history: conversationLog, // Don't store conversation history
cache: expensiveComputation // Don't cache large results
});
```
**Best Practice**:
- Store only UI state (selected items, view mode, filters)
- Fetch data from MCP server on widget mount
- Use tool calls to persist important data
### Issue #14: Widget-Initiated Tool Calls Fail Without Permission Flag (Community-sourced)
**Source**: [OpenAI Apps SDK - ChatGPT UI](https://developers.openai.com/apps-sdk/build/chatgpt-ui/)
**Why It Happens**: Components initiating tool calls via `window.openai.callTool()` require the tool marked as "able to be initiated by the component" on the MCP server. Without this flag, calls fail silently.
**Prevention**: Mark tools as `widgetCallable: true` in annotations
```typescript
// MCP Server - Mark tool as widget-callable
server.setRequestHandler(ListToolsRequestSchema, async () => ({
tools: [{
name: 'update_item',
description: 'Update an item',
inputSchema: { /* ... */ },
annotations: {
openai: {
outputTemplate: 'ui://widget/item.html',
// ✅ Required for widget-initiated calls
widgetCallable: true
}
}
}]
}));
// Widget - Now allowed to call tool
window.openai.callTool({
name: 'update_item',
arguments: { id: itemId, status: 'completed' }
});
```
---
## Widget Development Best Practices
### File Upload Limitations (Community-sourced)
**Source**: [OpenAI Apps SDK - ChatGPT UI](https://developers.openai.com/apps-sdk/build/chatgpt-ui/)
`window.openai.uploadFile()` only supports 3 image formats: `image/png`, `image/jpeg`, and `image/webp`. Other formats fail silently.
```typescript
// ✅ Supported
window.openai.uploadFile({ accept: 'image/png,image/jpeg,image/webp' });
// ❌ Not supported (fails silently)
window.openai.uploadFile({ accept: 'application/pdf' });
window.openai.uploadFile({ accept: 'text/csv' });
```
**Alternative for Other File Types**:
1. Use base64 encoding in tool arguments
2. Request user paste text content
3. Use external upload service (S3, R2) and pass URL
### Tool Performance Targets (Community-sourced)
**Source**: [OpenAI Apps SDK - Troubleshooting](https://developers.openai.com/apps-sdk/deploy/troubleshooting)
Tool calls exceeding "a few hundred milliseconds" cause UI sluggishness in ChatGPT. Official docs recommend profiling backends and implementing caching for slow operations.
**Performance Targets**:
- **< 200ms**: Ideal response time
- **200-500ms**: Acceptable but noticeable
- **> 500ms**: Sluggish, needs optimization
**Optimization Strategies**:
```typescript
// 1. Cache expensive computations
const cache = new Map();
if (cache.has(key)) return cache.get(key);
const result = await expensiveOperation();
cache.set(key, result);
// 2. Use KV/D1 for pre-computed data
const cached = await env.KV.get(`result:${id}`);
if (cached) return JSON.parse(cached);
// 3. Paginate large datasets
return {
content: [{ type: 'text', text: 'First 20 results...' }],
_meta: { hasMore: true, nextPage: 2 }
};
// 4. Move slow work to async tasks
// Return immediately, update via follow-up
```
---
## MCP SDK 1.25.x Updates (December 2025)
**Breaking Changes** from @modelcontextprotocol/[email protected] → 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 [email protected] → 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": "^1.25.3",
"hono": "^4.11.3",
"zod": "^4.3.5"
},
"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 14 known issues prevented, zero errors in production
---
## Community Resources
### Deployment Tools
**Cloudflare One-Click Deploy**: Deploy MCP servers to Cloudflare Workers with pre-built templates and auto-configured CI/CD. Includes OAuth wrapper and Python support.
- Docs: https://developers.cloudflare.com/agents/guides/remote-mcp-server/
- Blog: https://blog.cloudflare.com/model-context-protocol/
### Frameworks
**Skybridge** (Community): React-focused framework with HMR support for widgets and enhanced MCP server helpers. Unofficial but actively maintained.
- GitHub: https://github.com/alpic-ai/skybridge
- Docs: https://www.skybridge.tech/
> **Note**: Community frameworks are not officially supported. Use at your own discretion
This skill shows how to build ChatGPT/OpenAI Apps using stateless MCP (Model Context Protocol) servers on Cloudflare Workers. It focuses on registering tools, serving interactive HTML/JS widgets from the ASSETS binding, and avoiding common edge-platform pitfalls. The guidance is production-ready and emphasizes compatibility, MIME, CORS, and transport choices for serverless deployments.
You register tools and resources with an MCP Server that responds to JSON-RPC 2.0 requests from ChatGPT. Widgets are served from the Workers ASSETS binding and referenced using ui://widget/ URIs; initial widget data is passed via _meta.initialData and accessed with window.openai.getInitialData(). The skill covers transport choices (streamable HTTP vs SSE), CORS requirements, MIME types, and workarounds for edge limitations like code-eval restrictions and global Response overrides.
Why does my widget return 404?
Ensure the tool's annotation uses a ui://widget/ URI, the widget file exists in ASSETS, and the ASSETS binding name matches wrangler.jsonc and your TypeScript Bindings type.
Why does HTML show as text instead of rendered widget?
Set the resource mimeType to text/html+skybridge when listing resources so ChatGPT renders the content as a widget.
How do I avoid session/state errors in serverless?
Use the Streamable HTTP transport (v1.24+) instead of SSE to avoid in-memory session reliance across stateless instances.