While building ChatGPT Apps backed by an MCP server, I kept running into non-obvious failures: 405/406 errors, SSE streams that never flush, invalid session errors, CORS preflights, and Edge vs Serverless quirks.

The documentation explains the protocol, but not how these failures actually surface during deployment.

I put together a minimal, deterministic MCP + SSE starter that deploys cleanly on Vercel and makes the correct behavior explicit. It’s intentionally small and review-friendly, meant as a reference rather than a framework.

Repo: https://github.com/shuddha2021/chatgpt-app-starter-kit