Pydantic PEP 695 types with reference to `Enum` inside

The Fix

pip install pydantic==1.10.20

Based on closed pydantic/pydantic issue #10877 · 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

@@ -1967,12 +1967,25 @@ def _computed_field_schema(
         """Generate schema for an Annotated type, e.g. `Annotated[int, Field(...)]` or `Annotated[int, Gt(0)]`."""
         FieldInfo = import_cached_field_info()
-
-        source_type, *annotations = self._get_args_resolving_forward_refs(
-            annotated_type,

repro.py

from enum import Enum
from typing import Annotated

from pydantic import BaseModel
from typing_extensions import Doc


class MyEnum(Enum):
    A = "a"
    B = "b"


type MyEnumType = Annotated[MyEnum, Doc("My enum type.")]


class MyClass(BaseModel):
    my_attr: MyEnumType

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: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\n

Why This Fix Works in Production

Trigger: class MyClass(BaseModel):
Mechanism: PEP 695 type aliases were not unpacked correctly when using the Annotated form in Pydantic
Why the fix works: Unpacks PEP 695 type aliases when using the Annotated form, resolving issues with type annotations in Pydantic. (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).
PEP 695 type aliases were not unpacked correctly when using the Annotated form in Pydantic
Surfaces as: /home/can/.cache/pypoetry/virtualenvs/bikipy-mu4xEx7I-py3.12/bin/python /data/can-data/Projects/bikipy/t.py

Proof / Evidence

GitHub issue: #10877
Fix PR: https://github.com/pydantic/pydantic/pull/11109
First fixed release: 1.10.20
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.41

Discussion

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

Jump to Sources Open on GitHub

“This is fixed on main (might be because of https://github.com/pydantic/pydantic/pull/10809), and https://github.com/pydantic/pydantic/pull/11109 fixes it "officially".”

@Viicos · 2024-12-22 · confirmation · source

“Also under the scope of https://github.com/pydantic/pydantic/issues/9782”

@sydney-runkle · 2024-11-18 · source

Failure Signature (Search String)

class MyClass(BaseModel):

Error Message

Stack trace

error.txt

Error Message
-------------
/home/can/.cache/pypoetry/virtualenvs/bikipy-mu4xEx7I-py3.12/bin/python /data/can-data/Projects/bikipy/t.py 
Traceback (most recent call last):
  File "/data/can-data/Projects/bikipy/t.py", line 16, in <module>
    class MyClass(BaseModel):
  File "/home/can/.cache/pypoetry/virtualenvs/bikipy-mu4xEx7I-py3.12/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 224, in __new__
    complete_model_class(
  File "/home/can/.cache/pypoetry/virtualenvs/bikipy-mu4xEx7I-py3.12/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 595, in complete_model_class
    cls.__pydantic_validator__ = create_schema_validator(
                                 ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/can/.cache/pypoetry/virtualenvs/bikipy-mu4xEx7I-py3.12/lib/python3.12/site-packages/pydantic/plugin/_schema_validator.py", line 50, in create_schema_validator
    return SchemaValidator(schema, config)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.SchemaError: Definitions error: definition `__main__.MyEnum:888676848` was never filled

Minimal Reproduction

repro.py

from enum import Enum
from typing import Annotated

from pydantic import BaseModel
from typing_extensions import Doc


class MyEnum(Enum):
    A = "a"
    B = "b"


type MyEnumType = Annotated[MyEnum, Doc("My enum type.")]


class MyClass(BaseModel):
    my_attr: MyEnumType

Environment

Python: 3.12
Pydantic: 2

What Broke

Users experienced schema validation errors when using PEP 695 types with Enum in annotations.

Why It Broke

PEP 695 type aliases were not unpacked correctly when using the Annotated form in Pydantic

Fix Options (Details)

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

pip install pydantic==1.10.20

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

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/11109

First fixed release: 1.10.20

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

When NOT to Use This Fix

Do not use if it changes public behavior or if the failure cannot be reproduced.

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.

Cluster: pydantic:configuration-error Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Data consistency OpenAPI schema Serialization

Sources

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

Pydantic PEP 695 types with reference to `Enum` inside annotation breaks `pydantic_core` (Fix)