# fastapi.exceptions.RequestValidationError: 1 validation error for Body
body -> username
  field required (type=value_error.missing)

- **ID:** `python/fastapi-missing-form-data`
- **Domain:** python
- **Category:** data_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Endpoint declares Form(...) but client sends JSON, or declares JSON body but client sends form-urlencoded.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 0.100+ | active | — | — |
| 0.110+ | active | — | — |

## Workarounds

1. **** (95% success)
   ```
   pip install python-multipart  # required for Form/File
   ```
2. **** (90% success)
   ```
   async def login(username: str = Form(...), password: str = Form(...))  # and client must send application/x-www-form-urlencoded
   ```

## Dead Ends

- **** — Server starts but Form parsing still fails at runtime. (80% fail)
- **** — Breaks form clients; still 422 for the other content type. (65% fail)