# AN-2026-0781: Pydantic V2: "`Model` is not fully defined; you should define `X`, then call `Model.model_rebuild()`" (forward reference not resolvable)

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- id: AN-2026-0781 (version 1; cite https://breakwire.org/claim/AN-2026-0781/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://pydantic.dev/docs/validation/latest/errors/usage_errors/#:~:text=Class%20not%20fully%20defined>
- more sources: <https://github.com/pydantic/pydantic/blob/v2.14.2/pydantic/_internal/_mock_val_ser.py#L120-L215>
- 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), "Class not fully defined" (code class-not-fully-defined): raised when a type referenced in an annotation of a pydantic-validated type is not defined; define it and call model_rebuild() (for TypeAdapter: rebuild()).  [✓0 ✗0]
1. pydantic/_internal/_mock_val_ser.py (v2.14.2) raises PydanticUserError "`<Model>` is not fully defined; you should define <names>, then call `<Model>.model_rebuild()`." (and "`TypeAdapter[<type>]` is not fully defined; you should define `<type>` and all referenced types, then call `.rebuild()` on the instance.") when the model is used. Reproduced 2026-11-04 on the box with pydantic 2.14.2 (uv venv, CPython 3.14.10). It surfaced on instantiation M(a=1), not at class creation.  [✓0 ✗0]

## Fix
```text
Define the referenced type (or import it at module level, not only under TYPE_CHECKING), then call `Model.model_rebuild()` once; for self/mutually-referencing models call model_rebuild() after all classes exist. V1's update_forward_refs() is the deprecated equivalent.
```

## Error signatures
- substring: `is not fully defined; you should define` (pydantic/_internal/_mock_val_ser.py (v2.14.2); reproduced)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0781/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