Pydantic "`Foo` is not fully defined; you should define

The Fix

pip install pydantic==1.10.20

Based on closed pydantic/pydantic issue #11031 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -648,62 +648,96 @@ Here are some additional notes on the behavior of [`model_construct()`][pydantic
 ## Generic models
 
-Pydantic supports the creation of generic models to make it easier to reuse a common model structure.
+Pydantic supports the creation of generic models to make it easier to reuse a common model structure. Both the new
+[type parameter syntax][type-params] (introduced by [PEP 695](https://peps.python.org/pep-0695/) in Python 3.12)

repro.py

from __future__ import annotations

from pydantic.dataclasses import dataclass


@dataclass
class Foo[T]:
    value: T


print(Foo(value=1))

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install pydantic==1.10.20\nWhen NOT to use: This fix should not be used if the application does not require PEP 695 support.\n\n

Why This Fix Works in Production

Trigger: pydantic.errors.PydanticUserError: `Foo` is not fully defined; you should define `T`, then call `pydantic.dataclasses.rebuild_dataclass(Foo)`.
Mechanism: The error occurs due to improper handling of PEP 695 generics syntax in dataclasses
Why the fix works: Properly supports PEP 695 generics syntax, fixing the issue where the error occurs when using the new generic syntax with dataclasses. (first fixed release: 1.10.20).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Shows up under Python 3.12 in real deployments (not just unit tests).
The error occurs due to improper handling of PEP 695 generics syntax in dataclasses
Surfaces as: pydantic.errors.PydanticUserError: `Foo` is not fully defined; you should define `T`, then call `pydantic.dataclasses.rebuild_dataclass(Foo)`.

Proof / Evidence

GitHub issue: #11031
Fix PR: https://github.com/pydantic/pydantic/pull/11189
First fixed release: 1.10.20
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.59

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“also happens when a regular dataclass is used as an arbitrary type with a BaseModel:”

@DetachHead · 2024-12-03 · source

Failure Signature (Search String)

pydantic.errors.PydanticUserError: `Foo` is not fully defined; you should define `T`, then call `pydantic.dataclasses.rebuild_dataclass(Foo)`.

Error Message

Stack trace

error.txt

Error Message
-------------
pydantic.errors.PydanticUserError: `Foo` is not fully defined; you should define `T`, then call `pydantic.dataclasses.rebuild_dataclass(Foo)`.

Stack trace

error.txt

Error Message
-------------
pydantic.errors.PydanticUserError: `Bar` is not fully defined; you should define `T`, then call `Bar.model_rebuild()`.

Minimal Reproduction

repro.py

from __future__ import annotations

from pydantic.dataclasses import dataclass


@dataclass
class Foo[T]:
    value: T


print(Foo(value=1))

Environment

Python: 3.12
Pydantic: 2

What Broke

Users encounter a PydanticUserError when using new generic syntax with dataclasses.

Why It Broke

The error occurs due to improper handling of PEP 695 generics syntax in dataclasses

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install pydantic==1.10.20

When NOT to use: This fix should not be used if the application does not require PEP 695 support.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/pydantic/pydantic/pull/11189

First fixed release: 1.10.20

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix should not be used if the application does not require PEP 695 support.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.

Version Compatibility Table

Version	Status
1.10.20	Fixed

Related Issues

No related fixes found.

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Related clusters: Data consistency OpenAPI schema Serialization

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Pydantic "`Foo` is not fully defined; you should define `T`" error when using new generic syntax (Fix)