# pydantic_core._pydantic_core.ValidationError: 1 validation error for Model
  Instance is frozen [type=frozen_instance, input_value=..., input_type=...]

- **ID:** `python/pydantic-immutability-error`
- **Domain:** python
- **Category:** runtime_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Model was defined with model_config = ConfigDict(frozen=True), so any attempt to mutate attributes after creation raises frozen_instance.

## Version Compatibility

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

## Workarounds

1. **** (97% success)
   ```
   new = old.model_copy(update={'name': 'new'})
   ```
2. **** (99% success)
   ```
   class M(BaseModel):
    model_config = ConfigDict(frozen=False)
   ```

## Dead Ends

- **** — Pydantic v2 intercepts __setattr__ at the C level; bypassing breaks hashing invariants. (80% fail)
- **** — Works but loses identity and any registered validators may re-run unexpectedly. (40% fail)