When building an API with FastAPI, it’s deceptively easy to dump every endpoint into main.py. A few @app.get() decorators here, a couple of @app.post() decorators there, and everything works.
Then your project grows. Suddenly, main.py is an 800-line monolith, team members are constantly hitting Git merge conflicts, and nobody knows where the user authentication logic ends and the billing logic begins.
FastAPI solves this with APIRouter. Here is how to structure your routing cleanly from day one.
1. The Core Tool: APIRouter
Think of APIRouter as a mini FastAPI app that doesn’t run on its own. It groups related path operations together, which you then mount onto your main application.
