Pydantic default_factory passed in annotation Field

The Fix

pip install pydantic==1.10.18

Based on closed pydantic/pydantic issue #9947 · 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

@@ -488,10 +488,10 @@ def _from_dataclass_field(dc_field: DataclassField[Any]) -> FieldInfo:
         default = dc_field.default
         if default is dataclasses.MISSING:
-            default = PydanticUndefined
+            default = _Unset

repro.py

from typing import Annotated
from pydantic import Field
import pydantic.dataclasses

@pydantic.dataclasses.dataclass()
class ExampleReproducer:
    val2: Annotated[int, Field(default_factory=lambda: 3)]

value = ExampleReproducer()

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==1.10.18\nWhen NOT to use: This fix is not applicable if using non-Annotated fields.\n\n

Why This Fix Works in Production

Trigger: value = ExampleReproducer()
Mechanism: Fixes the issue where the default_factory passed in Annotated was silently ignored in pydantic.dataclass.
Why the fix works: Fixes the issue where the default_factory passed in Annotated was silently ignored in pydantic.dataclass. (first fixed release: 1.10.18).

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).
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #9947
Fix PR: https://github.com/pydantic/pydantic/pull/9971
First fixed release: 1.10.18
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.47

Discussion

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

Jump to Sources Open on GitHub

“Interesting, looks like this works for BaseModels but not dataclasses.”

@sydney-runkle · 2024-07-24 · source

“PRs welcome. I can try to address in v2.9 if this hasn't been picked up by then. I'm guessing a fix would be in make_pydantic_fields_compatible…”

@sydney-runkle · 2024-07-24 · source

“Hi @sydney-runkle, I'd be interested in investigating it further”

@AlessandroMiola · 2024-07-24 · source

Failure Signature (Search String)

value = ExampleReproducer()

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "test_pydantic.py", line 11, in <module>
    value = ExampleReproducer()
            ^^^^^^^^^^^^^^^^^^^
  File "/nix/store/fcjbczh74z9hx7ingvb5ydsa9fv5y81q-python3-3.12.4-env/lib/python3.12/site-packages/pydantic/_internal/_dataclasses.py", line 140, in __init__
    s.__pydantic_validator__.validate_python(ArgsKwargs(args, kwargs), self_instance=s)
pydantic_core._pydantic_core.ValidationError: 1 validation error for ExampleReproducer
val2
  Field required [type=missing, input_value=ArgsKwargs(()), input_type=ArgsKwargs]
    For further information visit https://errors.pydantic.dev/2.7/v/missing

Minimal Reproduction

repro.py

from typing import Annotated
from pydantic import Field
import pydantic.dataclasses

@pydantic.dataclasses.dataclass()
class ExampleReproducer:
    val2: Annotated[int, Field(default_factory=lambda: 3)]

value = ExampleReproducer()

Environment

Python: 3.12

What Broke

Instances of ExampleReproducer fail to initialize, raising a validation error for missing fields.

Fix Options (Details)

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

pip install pydantic==1.10.18

When NOT to use: This fix is not applicable if using non-Annotated fields.

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/9971

First fixed release: 1.10.18

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

When NOT to Use This Fix

This fix is not applicable if using non-Annotated fields.

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
1.10.18	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 default_factory passed in annotation Field silently ignored (Fix)