python validation_error ai_generated true

422 Unprocessable Entity: {"detail":[{"type":"missing","loc":["body"],"msg":"Field required"}]}

ID: python/fastapi-422-validation-error

Also available as: JSON · Markdown
91%Fix Rate
90%Confidence
80Evidence
2020-01-01First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
311 active — — —

Root Cause

Almost always a content-type mismatch between client and server, or a Pydantic v1-to-v2 migration issue where validators silently stopped working. The 422 response includes detailed field-level error information.

generic

Workarounds

  1. 93% success Verify Content-Type header matches the endpoint parameter type 
    Use application/json for Body/BaseModel parameters, application/x-www-form-urlencoded for Form() fields. Check the client is sending the right Content-Type. In curl: -H 'Content-Type: application/json' -d '{"field": "value"}'. With Form() fields, use -d 'field=value' (no JSON).

    Sources: https://fastapi.tiangolo.com/tutorial/body/

  2. 87% success Update Pydantic v1 validators to v2 syntax 
    For FastAPI >=0.100 with Pydantic v2: change @validator to @field_validator, change class Config to model_config = ConfigDict(...), change orm_mode=True to from_attributes=True. Pydantic v2 silently ignores v1-style validators.

    Sources: https://docs.pydantic.dev/latest/migration/

  3. 85% success Use Body(embed=True) for single-field JSON bodies 
    FastAPI expects {"field": value} for models, but single-parameter endpoints may receive just the value. Add Body(embed=True) to force wrapping: async def endpoint(item: str = Body(embed=True)).

    Sources: https://fastapi.tiangolo.com/tutorial/body-multiple-params/

Dead Ends

Common approaches that don't work:

  1. Adding Optional to the field type to make it not required 68% fail

    If the client is sending data in the wrong Content-Type (e.g., form data instead of JSON), the field is 'missing' from the JSON body even though it was sent. Making the field Optional masks the root cause and allows None values through validation.

  2. Switching from Pydantic BaseModel to plain dict parameter 80% fail

    Loses all validation and type safety. The error shifts downstream to application logic where it is harder to debug. The 422 is actually helpful — it tells you exactly what is wrong with the request.

  3. Adding response_model_exclude_unset=True to the endpoint 95% fail

    response_model_exclude_unset controls response serialization, not request validation. It has zero effect on incoming 422 errors.

Error Chain

Frequently confused with: