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

- **ID:** `python/fastapi-query-param-missing-annotation`
- **Domain:** python
- **Category:** type_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Defining a response model with a field that has no type annotation or an invalid one (e.g., `items = []` without `List[Item]`) causes FastAPI to fail when generating the response schema.

## Version Compatibility

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

## Workarounds

1. **** (95% success)
   ```
   Define the response model with proper type hints: `from typing import List; class ResponseModel(BaseModel): items: List[Item]`
   ```
2. **** (90% success)
   ```
   Use `response_model=List[Item]` in the route decorator instead of a custom model.
   ```

## Dead Ends

- **** — Adding a default value like `items: list = []` still fails if the list type is not specified; FastAPI needs `List[SomeModel]`. (80% fail)
- **** — Using `items: Any` works but loses validation and may not be what you want. (70% fail)