Pydantic 2.12.0: Method decorated with `model

The Fix

pip install pydantic==2.12.1

Based on closed pydantic/pydantic issue #12353 · 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

@@ -2,6 +2,7 @@
 from __future__ import annotations as _annotations
 
+import sys
 import types
 from collections import deque

repro.py

from pydantic import BaseModel, model_validator


class Model(BaseModel):
    @model_validator(mode="before")
    def check(self) -> Model:
        return self

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==2.12.1\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 Model(BaseModel):
Mechanism: Methods decorated with `model_validator` incorrectly handle return type annotations when using the containing class's name
Why the fix works: Fixes an issue where methods decorated with `model_validator` would run into errors when annotated with the containing class's name as the return type. (first fixed release: 2.12.1).

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 3.14 in real deployments (not just unit tests).
Methods decorated with `model_validator` incorrectly handle return type annotations when using the containing class's name
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #12353
Fix PR: https://github.com/pydantic/pydantic/pull/12355
First fixed release: 2.12.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.43

Discussion

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

Jump to Sources Open on GitHub

“This will be fixed in the next patch release. I assumed you meant to use an _after_ validator in your example? If not, note that…”

@Viicos · 2025-10-08 · confirmation · source

“Yup, I ran into the issue with after, but tried it with before to see if there was any difference. Thank you and I agree…”

@bijlpieter · 2025-10-08 · source

“This is not considered as a regression, as Python 3.14 wasn't supported before 2.12 and you are making use of a 3.14 specific feature (deferred…”

@Viicos · 2025-10-08 · source

Failure Signature (Search String)

class Model(BaseModel):

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "/home/pieter/foo.py", line 10, in <module>
    class Model(BaseModel):
    ...<2 lines>...
            return self
  File "/home/pieter/foo.py", line 11, in Model
    @model_validator(mode="before")
     ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/home/pieter/.cache/uv/environments-v2/foo-af0ca88c5d3839c8/lib/python3.14/site-packages/pydantic/functional_validators.py", line 723, in dec
    f = _decorators.ensure_classmethod_based_on_signature(f)
  File "/home/pieter/.cache/uv/environments-v2/foo-af0ca88c5d3839c8/lib/python3.14/site-packages/pydantic/_internal/_decorators.py", line 711, in ensure_classmethod_based_on_signature
    ) and _is_classmethod_from_sig(function):
          ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/home/pieter/.cache/uv/environments-v2/foo-af0ca88c5d3839c8/lib/python3.14/site-packages/pydantic/_internal/_decorators.py", line 717, in _is_classmethod_from_sig
    sig = signature(unwrap_wrapped_function(function))
  File "/home/pieter/.local/share/uv/python/cpython-3.14.0-linux-x86_64-gnu/lib/python3.14/inspect.py", line 3312, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped,
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                   globals=globals, locals=locals, eval_str=eval_str,
                                   ^^^^^^^^^^^^^^^^^^^
... (truncated) ...

Minimal Reproduction

repro.py

from pydantic import BaseModel, model_validator


class Model(BaseModel):
    @model_validator(mode="before")
    def check(self) -> Model:
        return self

Environment

Python: 3.14
Pydantic: 2

What Broke

Users encounter errors when using model validators with specific return type annotations.

Why It Broke

Methods decorated with `model_validator` incorrectly handle return type annotations when using the containing class's name

Fix Options (Details)

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

pip install pydantic==2.12.1

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/12355

First fixed release: 2.12.1

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
2.12.1	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 2.12.0: Method decorated with `model_validator` runs into error when annotated with (Fix)