# fastapi.exceptions.FastAPIError: Invalid args for response field

- **ID:** `python/fastapi-multiple-body-params-error`
- **Domain:** python
- **Category:** config_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Defining multiple body parameters without a Pydantic model causes FastAPI to misinterpret the request body, leading to validation issues.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.x | active | — | — |

## Workarounds

1. **** (95% success)
   ```
   Combine parameters into a single Pydantic model: class Item(BaseModel):\n    name: str\n    price: float\nThen use item: Item as the body parameter.
   ```
2. **** (90% success)
   ```
   If multiple bodies are needed, embed them in a parent model: class RequestModel(BaseModel):\n    item: Item\n    user: User
   ```

## Dead Ends

- **** — Using dict as a body parameter type can cause serialization issues and unexpected validation errors. (70% fail)
- **** — Adding Body() to each parameter but not using a single model still confuses FastAPI. (60% fail)