Pydantic Dataclass with `__pydantic_config_

The Fix

pip install pydantic==1.10.20

Based on closed pydantic/pydantic issue #11201 · 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

@@ -45,7 +45,7 @@ except ValidationError as e:
 ```
 
-1. See the [Extra Attributes](models.md#extra-fields) section for more details.
+1. See the [Extra data](models.md#extra-data) section for more details.

repro.py

from typing import Any
from pydantic import ConfigDict, Field
from pydantic.dataclasses import dataclass


@dataclass
class Dataclass:
    x: int

    __pydantic_config__ = ConfigDict(extra="allow")
    __pydantic_extra__: dict[str, Any] = Field(init=False)


d = Dataclass(x=1, y=2)
print(d.__pydantic_extra__)

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 is not applicable if you require `__pydantic_extra__` for dataclasses.\n\n

Why This Fix Works in Production

Trigger: pydantic.errors.PydanticUserError: Field __pydantic_extra__ has `init=False` and dataclass has config setting `extra="allow"`. This combination is not allowed.
Mechanism: The combination of `__pydantic_config__` with `extra='allow'` and `__pydantic_extra__` with `init=False` is not supported in Pydantic dataclasses
Why the fix works: Updated documentation to clarify that `__pydantic_extra__` is not supported for dataclasses and to provide insights on handling extra data. (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 3.11 in real deployments (not just unit tests).
The combination of `__pydantic_config__` with `extra='allow'` and `__pydantic_extra__` with `init=False` is not supported in Pydantic dataclasses
Surfaces as: pydantic.errors.PydanticUserError: Field __pydantic_extra__ has `init=False` and dataclass has config setting `extra="allow"`. This combination is not allowed.

Proof / Evidence

GitHub issue: #11201
Fix PR: https://github.com/pydantic/pydantic/pull/11213
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: 0.64

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: 1.10.20
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

default=PydanticUndefined extra={'init': False}
\n\n[stderr]\n/tmp/ss-exec-tjk6_vht/repro.py:7: RuntimeWarning: fields may not start with an underscore, ignoring "__pydantic_extra__"
  class Dataclass:

Discussion

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

Jump to Sources Open on GitHub

“In the meantime I implement __pydantic_extra__ with a somewhat naive property:”

@pawamoy · 2024-12-31 · source

“I see, thank you :slightly_smiling_face:”

@pawamoy · 2025-01-03 · source

“__pydantic_extra__ is actually not supported for dataclasses”

@Viicos · 2025-01-03 · source

Failure Signature (Search String)

pydantic.errors.PydanticUserError: Field __pydantic_extra__ has `init=False` and dataclass has config setting `extra="allow"`. This combination is not allowed.

Error Message

Stack trace

error.txt

Error Message
-------------
pydantic.errors.PydanticUserError: Field __pydantic_extra__ has `init=False` and dataclass has config setting `extra="allow"`. This combination is not allowed.

Stack trace

error.txt

Error Message
-------------
AttributeError: 'Dataclass' object has no attribute '__pydantic_extra__'. Did you mean: '__pydantic_config__'?

Minimal Reproduction

repro.py

from typing import Any
from pydantic import ConfigDict, Field
from pydantic.dataclasses import dataclass


@dataclass
class Dataclass:
    x: int

    __pydantic_config__ = ConfigDict(extra="allow")
    __pydantic_extra__: dict[str, Any] = Field(init=False)


d = Dataclass(x=1, y=2)
print(d.__pydantic_extra__)

Environment

Python: 3.11
Pydantic: 2

What Broke

Users encounter errors when trying to use extra fields in Pydantic dataclasses.

Why It Broke

The combination of `__pydantic_config__` with `extra='allow'` and `__pydantic_extra__` with `init=False` is not supported in Pydantic 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 is not applicable if you require `__pydantic_extra__` for dataclasses.

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/11213

First fixed release: 1.10.20

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

When NOT to Use This Fix

This fix is not applicable if you require `__pydantic_extra__` for dataclasses.

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 Dataclass with `__pydantic_config__` extra=allow and `__pydantic_extra__` field (Fix)