Show HN: Step-by-step instructions to build your first ChatGPT Apps SDK app (github.com)

This post is a hands‑on walkthrough showing how to build a minimal ChatGPT Apps SDK (MCP) backend in Go with the mcp-server-go SDK. The author steps through Auth0 configuration (creating an API whose Identifier matches the server URL, enabling RFC 9068 JWT profile, making a connection a “domain connection,” setting Default Audience, and enabling Dynamic Client Registration per the 2025-06-18 MCP spec), then scaffolds a Go module that defines a tool, a UI resource (text/html+skybridge widget), and an MCP Service. The example tool (test123) accepts structured input, returns structured output plus text (to satisfy ChatGPT MCP client expectations), and the server is wired with an auth provider that discovers JWKS to validate access tokens and extract the sub claim. Why it matters: it’s a compact, end‑to‑end template for building ChatGPT-integrated apps that need secure token validation, dynamic client onboarding, interactive UI widgets, and scalable session hosting. Key technical takeaways include using Redis as a session host for multi‑instance scaling (vs. in‑memory for single instance), the importance of matching API identifiers to MCP endpoints, the RFC 9068 JWT approach for token validation via JWKS, how to expose UI resources to ChatGPT using annotations, and running the server via a streaming HTTP handler for long‑lived connections. The guide also highlights practical local development (Cloudflare Tunnel) and provides concrete code snippets that accelerate building secure, production‑ready ChatGPT Apps.