Pydantic Pydantic 2.12.2 issue when using

The Fix

pip install pydantic==2.12.3

Based on closed pydantic/pydantic issue #12413 · 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

@@ -394,6 +394,7 @@ Model()
 * `@root_validator` has been deprecated, and should be replaced with
     [`@model_validator`](api/functional_validators.md#pydantic.functional_validators.model_validator), which also provides new features and improvements.
+    Be aware that the allowed signatures have changed (see the [relevant documentation](./concepts/validators.md#model-validators)).
     * Under some circumstances (such as assignment when `model_config['validate_assignment'] is True`),
         the `@model_validator` decorator will receive an instance of the model, not a dict of values. You may

repro.py

Here is the versioning: /opt/conda/lib/python3.12/site-packages/pydantic/_migration.py:283: UserWarning: `pydantic.utils:version_info` has been moved to `pydantic.version:version_info`.
  warnings.warn(f'`{import_path}` has been moved to `{new_location}`.')
             pydantic version: 2.11.7
        pydantic-core version: 2.33.2
          pydantic-core build: profile=release pgo=false
               python version: 3.12.9 | packaged by conda-forge | (main, Feb 14 2025, 08:00:06) [GCC 13.3.0]
                     platform: Linux-5.10.244-240.970.amzn2.x86_64-x86_64-with-glibc2.35
             related packages: fastapi-0.116.1 pydantic-settings-2.10.1 typing_extensions-4.14.1
                       commit: unknown.

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==2.12.3\nWhen NOT to use: This fix is not applicable if using custom validators that require the previous signature.\n\n

Why This Fix Works in Production

Trigger: AttributeError: 'pydantic_core._pydantic_core.ValidationInfo' object has no attribute 'format'
Mechanism: Raises a warning when an invalid after model validator function signature is used, reverting to the previous behavior to address issues reported in Pydantic 2.12.2.
Why the fix works: Raises a warning when an invalid after model validator function signature is used, reverting to the previous behavior to address issues reported in Pydantic 2.12.2. (first fixed release: 2.12.3).

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: AttributeError: 'pydantic_core._pydantic_core.ValidationInfo' object has no attribute 'format'

Proof / Evidence

GitHub issue: #12413
Fix PR: https://github.com/pydantic/pydantic/pull/12414
First fixed release: 2.12.3
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

“You are using an invalid function signature for _after_ model validators”

@Viicos · 2025-10-17 · source

Failure Signature (Search String)

AttributeError: 'pydantic_core._pydantic_core.ValidationInfo' object has no attribute 'format'

Error Message

Stack trace

error.txt

Error Message
-------------
AttributeError: 'pydantic_core._pydantic_core.ValidationInfo' object has no attribute 'format'

Minimal Reproduction

repro.py

Here is the versioning: /opt/conda/lib/python3.12/site-packages/pydantic/_migration.py:283: UserWarning: `pydantic.utils:version_info` has been moved to `pydantic.version:version_info`.
  warnings.warn(f'`{import_path}` has been moved to `{new_location}`.')
             pydantic version: 2.11.7
        pydantic-core version: 2.33.2
          pydantic-core build: profile=release pgo=false
               python version: 3.12.9 | packaged by conda-forge | (main, Feb 14 2025, 08:00:06) [GCC 13.3.0]
                     platform: Linux-5.10.244-240.970.amzn2.x86_64-x86_64-with-glibc2.35
             related packages: fastapi-0.116.1 pydantic-settings-2.10.1 typing_extensions-4.14.1
                       commit: unknown.

Environment

Python: 3.12
Pydantic: 2.12.2

What Broke

Application fails to load dimensions, causing data retrieval issues.

Fix Options (Details)

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

pip install pydantic==2.12.3

When NOT to use: This fix is not applicable if using custom validators that require the previous signature.

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/12414

First fixed release: 2.12.3

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

When NOT to Use This Fix

This fix is not applicable if using custom validators that require the previous signature.

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
2.12.3	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 Pydantic 2.12.2 issue when using intake.open_esm_datastore (Fix)