ChatGPT App Creator

Last verified: January 2026. Cross-reference with official docs for latest information.

Quick Context

The ChatGPT Apps SDK enables developers to build interactive applications that run inside ChatGPT. Based on available documentation, apps consist of three components:

1. MCP Server: Exposes tools and resources via the Model Context Protocol
2. Widgets: HTML/JS UI components rendered in iframes within ChatGPT
3. OAuth Provider: Authentication where you are the OAuth provider (ChatGPT is the client)

Critical Mental Model: Unlike typical OAuth integrations where your app consumes someone else’s OAuth (like “Sign in with Google”), ChatGPT Apps flip this around. You run the OAuth server, and ChatGPT is the OAuth client connecting to you. This is the #1 source of confusion based on developer reports.

Architecture Overview

┌─────────────────────────────────────────────────────────┐
│                      ChatGPT                            │
│  ┌─────────────────┐         ┌───────────────────────┐  │
│  │   Chat Message  │         │   Widget (iframe)     │  │
│  │   with Tool     │◄───────►│   window.openai API   │  │
│  │   Results       │         │                       │  │
│  └────────┬────────┘         └───────────┬───────────┘  │
└───────────┼──────────────────────────────┼──────────────┘
            │                              │
            ▼                              ▼
┌───────────────────────┐      ┌───────────────────────┐
│    Your MCP Server    │      │   Your OAuth Server   │
│  - tools              │      │  /.well-known/...     │
│  - resources          │      │  /authorize, /token   │
│  - widget HTML        │      │  /register (DCR)      │
└───────────────────────┘      └───────────────────────┘

Key Technical Details

• Widgets are served as resources with MIME type text/html+skybridge
• PKCE S256 is mandatory for OAuth
• Dynamic Client Registration (DCR) generates fresh client credentials per session
• Tools should have descriptions starting with “Use this when…” for better discovery
• Tool annotations are required: readOnlyHint, openWorldHint, destructiveHint
• Tool responses use three-part structure:
  • structuredContent: Concise data for the model (keep small)
  • content: Text narration shown in chat
  • _meta: Widget-only data including openai/outputTemplate for widget rendering

Common Tasks

Creating an MCP Server

See scripts/mcp-server.ts (Node.js) or scripts/mcp-server.py (Python) for complete examples.

Transport Modes (critical for production):

• stdio: For local testing with MCP Inspector only
• http: For production with ChatGPT—exposes HTTP/SSE endpoints

# Local testing (stdio)
npx tsx mcp-server.ts

# Production (HTTP/SSE) - required for ChatGPT
MCP_TRANSPORT=http npx tsx mcp-server.ts

Key points:

• Use descriptive tool names and descriptions
• Return structured data for widgets to consume
• Handle CORS headers on the MCP endpoint (done automatically in http mode)
• Include resource references in tool responses to trigger widget rendering

Implementing OAuth

See references/oauth-setup.md for comprehensive guide.

Required endpoints:

1. /.well-known/oauth-protected-resource – Declares your authorization server
2. /.well-known/oauth-authorization-server – OAuth server metadata
3. /authorize – User login and consent
4. /token – Token exchange (must handle Basic Auth headers)
5. /register – Dynamic Client Registration

Widget Development

Widgets communicate via the window.openai API. See references/window-openai-api.md for full reference.

Key APIs:

• window.openai.toolOutput – Data from tool response
• window.openai.widgetState – Persistent state within a message
• window.openai.callTool() – Invoke MCP tools from widget
• window.openai.setWidgetState() – Update persistent state

Critical Gotchas

Most issues developers encounter are already documented. Check references/gotchas.md first when debugging—it covers 20 common problems including OAuth role confusion, PKCE errors, widget caching, CSP violations, tool annotations, and more, each with code fixes.

Debugging

Use the MCP Inspector to test your server:

npx @modelcontextprotocol/inspector

See references/troubleshooting.md for error diagnosis guide.

Submission

Before submitting to the ChatGPT App Store:

• Ensure all tools have correct annotations (top rejection reason)
• Server must be on public HTTPS (no localhost/ngrok)
• Provide test credentials for authenticated apps
• Organization must be verified with Owner role

See references/submission.md for complete requirements and common rejection reasons.

Reference Files

Load these for detailed information:

Script Files

Working code examples:

Assets

Widget templates used by the MCP server examples:

Run with:

# TypeScript
cd scripts && npm install && npx tsx mcp-server.ts

# Python
cd scripts && pip install -r requirements.txt && python mcp-server.py

Quick Reference

Widget MIME type: text/html+skybridge

Tool annotations (all three required):

readOnlyHint: true     # Only reads data, no side effects
openWorldHint: false   # Only affects user's data, not external systems
destructiveHint: false # No irreversible changes

Tool response structure:

structuredContent  → Model reads this (keep concise)
content            → Shown in chat as narration
_meta              → Widget-only (model never sees)
  └─ "openai/outputTemplate": "widget://app/name"

OAuth endpoints:

/.well-known/oauth-protected-resource
/.well-known/oauth-authorization-server
/authorize
/token
/register

Widget APIs:

window.openai.toolOutput          // Tool response data
window.openai.widgetState         // Persistent state
window.openai.theme               // 'light' or 'dark'
window.openai.setWidgetState()    // Update state
window.openai.callTool(name, args) // Call MCP tools

Commands:

# Test MCP server locally
npx @modelcontextprotocol/inspector

# Run TypeScript server (production)
MCP_TRANSPORT=http npx tsx mcp-server.ts

# Run Python server (production)
MCP_TRANSPORT=http python mcp-server.py

Notes

• Cross-reference with official docs for latest changes
• Test incrementally: OAuth → MCP tools → Widgets