Pydantic AttributeError for model

The Fix

pip install pydantic==2.11.6

Based on closed pydantic/pydantic issue #12110 · 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

@@ -712,8 +712,9 @@ def verify_square(self) -> Self:
 
     def dec(f: Any) -> _decorators.PydanticDescriptorProxy[Any]:
-        # auto apply the @classmethod decorator
-        f = _decorators.ensure_classmethod_based_on_signature(f)
+        # auto apply the @classmethod decorator (except for *after* validators, which should be instance methods):

repro.py

from typing import Optional
from pydantic import model_validator, BaseModel

class A(BaseModel):
     a: Optional[str] = None

     @model_validator(mode="after")
     def check(cls, values):
             assert values.a is not None
 
>> A(a="abc")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.10/site-packages/pydantic/main.py", line 253, in __init__
    validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
  File "<stdin>", line 5, in check
AttributeError: 'pydantic_core._pydantic_core.ValidationInfo' object has no attribute 'a'

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==2.11.6\nWhen NOT to use: This fix should not be applied if the model validator's signature is intended to be a class method.\n\n

Why This Fix Works in Production

Trigger: from typing import Optional
Mechanism: The model_validator was incorrectly treated as a class method instead of an instance method
Why the fix works: Addresses an AttributeError encountered with model validators in Pydantic V2.12.0a1 by ensuring that after model validators are treated as instance methods. (first fixed release: 2.11.6).

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 3.10 in real deployments (not just unit tests).
The model_validator was incorrectly treated as a class method instead of an instance method
Surfaces as: from typing import Optional

Proof / Evidence

GitHub issue: #12110
Fix PR: https://github.com/pydantic/pydantic/pull/11957
First fixed release: 2.11.6
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.42

Discussion

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

Jump to Sources Open on GitHub

“Thanks for testing out the beta release”

@Viicos · 2025-08-01 · source

Failure Signature (Search String)

from typing import Optional

Error Message

Stack trace

error.txt

Error Message
-------------
from typing import Optional
from pydantic import model_validator, BaseModel

class A(BaseModel):
     a: Optional[str] = None

     @model_validator(mode="after")
     def check(cls, values):
             assert values.a is not None
 
>> A(a="abc")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.10/site-packages/pydantic/main.py", line 253, in __init__
    validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
  File "<stdin>", line 5, in check
AttributeError: 'pydantic_core._pydantic_core.ValidationInfo' object has no attribute 'a'

Minimal Reproduction

repro.py

from typing import Optional
from pydantic import model_validator, BaseModel

class A(BaseModel):
     a: Optional[str] = None

     @model_validator(mode="after")
     def check(cls, values):
             assert values.a is not None
 
>> A(a="abc")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.10/site-packages/pydantic/main.py", line 253, in __init__
    validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
  File "<stdin>", line 5, in check
AttributeError: 'pydantic_core._pydantic_core.ValidationInfo' object has no attribute 'a'

Environment

Python: 3.10
Pydantic: 2

Why It Broke

The model_validator was incorrectly treated as a class method instead of an instance method

Fix Options (Details)

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

pip install pydantic==2.11.6

When NOT to use: This fix should not be applied if the model validator's signature is intended to be a class method.

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/11957

First fixed release: 2.11.6

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

When NOT to Use This Fix

This fix should not be applied if the model validator's signature is intended to be a class method.

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
2.11.6	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 AttributeError for model_validator in 2.12.0a1 release (Fix)