**Description** Pydantic broke JSON serialization of custom types (https://github.com/pydantic/pydantic/issues/12348#issuecomment-3380596458) in release 2.12 (https://github.com/pydantic/pydantic/issues/12348). Don't know if there plans to be a backwards-compatible fix. Still investigating. **Reproduction** See logs such as [here](https://github.com/openforcefield/openff-interchange/actions/runs/18346143812/job/52253240618?pr=1340) The error commonly looks like ``` pydantic_core._pydantic_core.PydanticSerializationError: Unable to serialize unknown type: <class 'pint.Quantity'>
Description
Pydantic broke JSON serialization of custom types (pydantic/pydantic#12348 (comment)) in release 2.12 (pydantic/pydantic#12348). Don't know if there plans to be a backwards-compatible fix.
Still investigating.
Reproduction
See logs such as here
The error commonly looks like