FastAPI at 1M+ users: the patterns that actually matter

FastAPI is the default Python web framework in 2026 — 38% of Python teams ship on it, up from 29% a year ago. That means a lot of greenfield projects are making the same early mistakes. This post is what I wish I'd known before scaling Savyour (Pakistan's first cashback platform, 1M+ users, 300+ merchant integrations) from 50 RPS to 3,000+ RPS on FastAPI. Everything below is drawn from production. No "hello world" demos. 1. Know your async boundaries FastAPI supports both def and async def endpo