Pydantic Dataclass field inconsistencies (Fix)

The Fix

pip install pydantic==2.11.8

Based on closed pydantic/pydantic issue #12045 · 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

@@ -2,10 +2,15 @@
 from __future__ import annotations as _annotations
 
+import copy
+import dataclasses
+import sys

repro.py

# Stdlib dataclasses
from dataclasses import dataclass, field

class BareClass:
    a: int
    b: int = field(kw_only=True)


@dataclass
class DC(BareClass):
    pass

# As documented in https://docs.python.org/3/library/dataclasses.html#inheritance:
list(DC.__dataclass_fields__.keys())
# []

# Pydantic dataclasses
from pydantic.dataclasses import dataclass
from pydantic import Field

class BareClass:
    a: int
    # This time using Pydantic's Field():
    b: int = Field(kw_only=True)

@dataclass
class DC(BareClass):
    pass

list(DC.__dataclass_fields__.keys())
#> ['b']

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==2.11.8\nWhen NOT to use: Do not apply this fix if using non-Pydantic dataclasses without Field().\n\n

Why This Fix Works in Production

Trigger: Users experienced incorrect field types in Pydantic dataclasses, leading to runtime errors.
Mechanism: Inconsistencies in handling Pydantic's Field() function in dataclasses caused unexpected behavior
Why the fix works: Refactor logic to support Pydantic's `Field()` function in dataclasses, addressing inconsistencies in dataclass field handling. (first fixed release: 2.11.8).

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

Inconsistencies in handling Pydantic's Field() function in dataclasses caused unexpected behavior
Production symptom (often without a traceback): Users experienced incorrect field types in Pydantic dataclasses, leading to runtime errors.

Proof / Evidence

GitHub issue: #12045
Fix PR: https://github.com/pydantic/pydantic/pull/12051
First fixed release: 2.11.8
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.60

Verified Execution

We executed the runnable minimal repro in a temporary environment and captured exit codes + logs.

Status: PASS
Ran: 2026-02-11T16:52:29Z
Package: pydantic
Fixed: 2.11.8
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

Discussion

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

Jump to Sources Open on GitHub

“First issue: Second issue:”

Issue thread · issue description · source

Failure Signature (Search String)

Users experienced incorrect field types in Pydantic dataclasses, leading to runtime errors.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Users experienced incorrect field types in Pydantic dataclasses, leading to runtime errors.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Users experienced incorrect field types in Pydantic dataclasses, leading to runtime errors.

Minimal Reproduction

repro.py

# Stdlib dataclasses
from dataclasses import dataclass, field

class BareClass:
    a: int
    b: int = field(kw_only=True)


@dataclass
class DC(BareClass):
    pass

# As documented in https://docs.python.org/3/library/dataclasses.html#inheritance:
list(DC.__dataclass_fields__.keys())
# []

# Pydantic dataclasses
from pydantic.dataclasses import dataclass
from pydantic import Field

class BareClass:
    a: int
    # This time using Pydantic's Field():
    b: int = Field(kw_only=True)

@dataclass
class DC(BareClass):
    pass

list(DC.__dataclass_fields__.keys())
#> ['b']

What Broke

Users experienced incorrect field types in Pydantic dataclasses, leading to runtime errors.

Why It Broke

Inconsistencies in handling Pydantic's Field() function in dataclasses caused unexpected behavior

Fix Options (Details)

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

pip install pydantic==2.11.8

When NOT to use: Do not apply this fix if using non-Pydantic dataclasses without Field().

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/12051

First fixed release: 2.11.8

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

When NOT to Use This Fix

Do not apply this fix if using non-Pydantic dataclasses without Field().

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
2.11.8	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.