# pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for <class 'decimal.Decimal'>. Set `arbitrary_types_allowed=True` in the model_config to ignore this error or implement `__get_pydantic_core_schema__` on your type to fully support it.

- **ID:** `python/pydantic-arbitrary-types-not-allowed`
- **Domain:** python
- **Category:** type_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

A field uses a class Pydantic cannot introspect (a custom class or third-party type lacking a schema hook), and `arbitrary_types_allowed` is not enabled.

## Version Compatibility

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

## Workarounds

1. **** (90% success)
   ```
   Enable it: `model_config = ConfigDict(arbitrary_types_allowed=True)` and add a `field_validator` for the type.
   ```
2. **** (85% success)
   ```
   Implement `__get_pydantic_core_schema__` on the custom class to provide a proper schema.
   ```

## Dead Ends

- **** — Adding `arbitrary_types_allowed=True` without `isinstance` checks means the field is never validated, leading to silent bad data. (50% fail)
- **** — Subclassing the third-party type does not help; Pydantic still cannot introspect it. (85% fail)