FastAPI in Layers: A Production Structure That Makes Testing Trivial

Approx. 20 min read What started all of this was wanting to build an LLM-powered application. Not a toy. Something with real moving parts — a backend that could handle API calls, talk to a database, manage configuration, and eventually wire into an AI layer on top. I had a rough idea of what I wanted to build, but every time I sat down to start, I felt like I was building on sand. For the longest time before that, my API projects started clean and then quietly fell apart. Not broken — they worked fine. But after a few weeks of adding features, I'd open the codebase and spend more time figuring out where something lived than actually writing code. Business logic bleeding into route handlers. Database calls scattered everywhere. A utils.py that became a graveyard for things I didn't know where else to put. I kept hearing "structure your project properly" but nobody explained why a structure works — just what folders to create. So I paused on the LLM part and went back to basics. I want t