# pydantic_core._pydantic_core.ValidationError: 1 validation error for Config
field_name
  Instance is frozen [type=frozen_instance, input_value='new', input_type=str]

- **ID:** `python/pydantic-model-frozen-assignment`
- **Domain:** python
- **Category:** runtime_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

Attempting to mutate an attribute on a model with `model_config = ConfigDict(frozen=True)`.

## Version Compatibility

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

## Workarounds

1. **** (95% success)
   ```
   Use `model_copy(update={'field_name': 'new'})` to produce a new instance.
   ```
2. **** (85% success)
   ```
   Remove `frozen=True` if mutability is required, or use `object.__setattr__(obj, 'field_name', 'new')` as a last resort.
   ```

## Dead Ends

- **** — Using `setattr` bypasses type checkers but still raises at runtime. (90% fail)
- **** — Mutating `__dict__` directly works but breaks hashing and Pydantic invariants. (70% fail)