Builds ChatGPT Apps with the OpenAI Apps SDK. Use when: (1) Setting up MCP servers for ChatGPT integration, (2) Implementing OAuth provider endpoints (you ARE the provider, ChatGPT is the client), (3) Creating widgets with text/html+skybridge MIME type, (4) Using window.openai API for widget-ChatGPT communication, (5) Debugging OAuth, CORS, PKCE, CSP, or widget rendering issues, (6) Understanding ChatGPT Apps architecture (MCP + widgets + OAuth).