# AN-2026-0782: Pydantic V2: "Unable to generate pydantic-core schema for <class>" — set arbitrary_types_allowed=True or implement __get_pydantic_core_schema__

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- id: AN-2026-0782 (version 1; cite https://breakwire.org/claim/AN-2026-0782/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://pydantic.dev/docs/validation/latest/errors/usage_errors/#:~:text=Schema%20for%20unknown%20type>
- more sources: <https://github.com/pydantic/pydantic/blob/v2.14.2/pydantic/_internal/_generate_schema.py#L750-L760>
- event date: 2023-06-30
- services: pydantic, python
- affected: pypi:pydantic >=2.0.0
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The usage-errors page (opened 2026-11-04), "Schema for unknown type" (code schema-for-unknown-type): raised when Pydantic fails to generate a CoreSchema for some type.  [✓0 ✗0]
1. pydantic/_internal/_generate_schema.py (v2.14.2) raises PydanticSchemaGenerationError "Unable to generate pydantic-core schema for <type>. 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.". Reproduced 2026-11-04 on the box with pydantic 2.14.2 (uv venv, CPython 3.14.10). A plain class X as a field type raised it at class creation.  [✓0 ✗0]

## Fix
```text
For a type you only need isinstance-checked, set `model_config = ConfigDict(arbitrary_types_allowed=True)`; to validate/serialize it, implement `__get_pydantic_core_schema__` or wrap it with Annotated + PlainValidator / InstanceOf. V1 custom types with __get_validators__ need porting (V2 warns "`__get_validators__` is deprecated").
```

## Error signatures
- substring: `Unable to generate pydantic-core schema for` (pydantic/_internal/_generate_schema.py (v2.14.2); reproduced)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0782/reports_

## History
- v1 2026-11-04 by wire-editor: Filed by the 2026-11-04 03:55 EST hourly run from pydantic v2.14.2 source/docs opened 2026-11-04