Troubleshooting¶

@validate does nothing¶

• The class must be a @component (or otherwise container-managed): validation runs through pico-ioc method interception, so direct instantiation (MyService()) bypasses it.
• The parameter needs a type annotation containing a BaseModel (directly or inside a generic like list[Item]); unannotated parameters are skipped by design.

ValidationFailedError instead of pydantic.ValidationError¶

That is the contract: the interceptor wraps pydantic's error so callers depend on pico-pydantic's exception, not on pydantic internals. The original error is chained (__cause__).

My dict argument was replaced by a model instance¶

Also the contract: valid input is transformed via TypeAdapter(annotation).validate_python(value) — a dict annotated as Item arrives as Item. Annotate as dict if you want the raw dict.

Validation runs twice in FastAPI controllers¶

FastAPI already validates request bodies against the endpoint signature. Use @validate for service-layer methods called from multiple entry points; controllers usually do not need it.

Async method returns a coroutine that explodes later¶

The interceptor is sync and validates arguments before dispatch; the coroutine result passes through untouched. If you see late failures, the error is in the method body, not in validation.