When FastAPI is the right fit for a product service

FastAPI is useful when a focused service benefits from Python's ecosystem, explicit API contracts, or asynchronous I/O. It is not automatically an argument to split a working application into more pieces.

A service needs a clear boundary

Data processing, a model-serving workflow, or an integration with a distinct scaling profile can make a good boundary. A vague desire for microservices does not.

Keep the contract boring

Typed request models, useful error responses, and measured timeouts make services easier to operate. The goal is a dependable component another engineer can call without needing to learn its internal story.