# PydanticUserError: `@validator` cannot be applied to instance methods

- **ID:** `python/fastapi-pydantic-v1-v2-migration-validator`
- **Domain:** python
- **Category:** type_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Code migrated to Pydantic v2 but still uses `@validator` without `@classmethod`, or uses removed v1 APIs.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 2.x | active | — | — |

## Workarounds

1. **** (95% success)
   ```
   from pydantic import field_validator
class M(BaseModel):
    x: int
    @field_validator('x')
    @classmethod
    def check(cls, v):
        return v
   ```
2. **** (90% success)
   ```
   @validator('x')
@classmethod
def check(cls, v): ...
   ```

## Dead Ends

- **** — Blocks FastAPI updates and other deps; temporary only. (60% fail)
- **** — Unrelated to validator decorator usage. (90% fail)