Canonical URL: https://medium.com/workspai/your-code-was-fine-your-debugging-order-was-wrong-d02420c38870 TL;DR If your FastAPI endpoint returns 500 and the code looks fine: Don’t touch the code first Check env, dependencies, wiring, and contracts Most failures are state issues, not logic This checklist saves hours. The Night It Happened Everything looked correct. The endpoint still returned 500. I spent 5 hours changing code… The bug had nothing to do with the code. What Actually Broke The issue wasn’t logic. It was setup drift . Something in the system state had changed: environment config dependency availability runtime wiring Code edits only delayed the real fix. That’s when I changed how I debug.…