Testing FastAPI

A FastAPI app is an ASGI callable, which means a test can hand it a request directly, in the same process, without binding a port. TestClient does that: it wraps the app in an httpx-style client whose get and post methods run the full stack (middleware, routing, validation, dependencies, handlers, serialisation) and return a normal response object. Combined with dependency_overrides from the dependency lesson, a route test needs no database, no network, and no API key table, and finishes in a few milliseconds.

The tests that matter for Relay are the ones that pin the contract: the status codes, the response shapes, the error envelope, and the auth behaviour. Business rules are tested against the service functions directly, which is why the routers were kept thin.