FastAPI POST endpoint with a Pydantic body parameter where all fields have defaults (e.g. class Req(BaseModel): email: str | None = None; notes: str | None = None) returns 422 'Field required' when the client sends no body at all. The route signature def handler(body: Req, ...) makes the body parameter required at the HTTP level even though every Pydantic field is optional. Tests that call client.post('/endpoint') without json={} hit 422 instead of the expected 200/400.
The Pydantic model is a required parameter — FastAPI distinguishes between 'all fields have defaults' (model-level) and 'the parameter itself is required' (route-level). Two fixes:
In tests: always pass json={} (empty dict) to POST endpoints that expect a body model, even when you want all defaults.
In the route: make the body parameter itself optional with a default: body: Req = Body(default_factory=Req) or body: Req | None = None (then handle None → create empty model). This makes bodyless POSTs work but changes the OpenAPI spec.
Fix 1 is usually correct — the OpenAPI contract says a body is required, so clients should send one. Fix 2 is only appropriate if you genuinely want to accept bodyless requests.