Pydantic TypeError when using BeforeValidator (Fix)

The Fix

pip install pydantic==1.10.19

Based on closed pydantic/pydantic issue #10459 · 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

@@ -279,7 +279,7 @@ def _apply_constraint_with_incompatibility_info(
 
             schema = cs.no_info_after_validator_function(
-                partial(get_constraint_validator(constraint), **{'constraint_value': value}), schema
+                partial(get_constraint_validator(constraint), **{constraint: value}), schema
             )

repro.py

from datetime import timedelta
from typing import Annotated

from annotated_types import Le
from pydantic import BaseModel, BeforeValidator


def validator(v):
    if isinstance(v, (int, float)):
        return timedelta(days=v)
    return v


class BeforeValidatorAfterLe(BaseModel):
    v: Annotated[timedelta, Le(timedelta(days=365)), BeforeValidator(validator)]


class BeforeValidatorBeforeLe(BaseModel):
    v: Annotated[timedelta, BeforeValidator(validator), Le(timedelta(days=365))]


try:
    BeforeValidatorAfterLe(v=366)  # raises ValueError, as expected
except ValueError as ex:
    print(1, ex)

try:
    BeforeValidatorBeforeLe(v=366)  # raises TypeError in pydantic>=2.9.0
except ValueError as ex:  # ValueError (as expected) in 2.8.2
    print(2, "ValueError:", ex)
except TypeError as ex:  # UNEXPECTED TypeError in pydantic>=2.9.0
    print(2, "TypeError:", ex)

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==1.10.19\nWhen NOT to use: Do not use this fix if the application relies on the previous behavior of BeforeValidator.\n\n

Why This Fix Works in Production

Trigger: TypeError when using BeforeValidator
Mechanism: TypeError occurs when using BeforeValidator with constraints from annotated_types in Pydantic V2
Why the fix works: Fixes a TypeError when using BeforeValidator with constraints from annotated_types in Pydantic V2. (first fixed release: 1.10.19).

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).
TypeError occurs when using BeforeValidator with constraints from annotated_types in Pydantic V2
Surfaces as: TypeError when using BeforeValidator

Proof / Evidence

GitHub issue: #10459
Fix PR: https://github.com/pydantic/pydantic/pull/10490
First fixed release: 1.10.19
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.47

Discussion

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

Jump to Sources Open on GitHub

“Looks like an issue with annotation application, will look into this today!”

@sydney-runkle · 2024-09-25 · source

Failure Signature (Search String)

TypeError when using BeforeValidator

Error Message

Stack trace

error.txt

Error Message
-------------
TypeError when using BeforeValidator

Minimal Reproduction

repro.py

from datetime import timedelta
from typing import Annotated

from annotated_types import Le
from pydantic import BaseModel, BeforeValidator


def validator(v):
    if isinstance(v, (int, float)):
        return timedelta(days=v)
    return v


class BeforeValidatorAfterLe(BaseModel):
    v: Annotated[timedelta, Le(timedelta(days=365)), BeforeValidator(validator)]


class BeforeValidatorBeforeLe(BaseModel):
    v: Annotated[timedelta, BeforeValidator(validator), Le(timedelta(days=365))]


try:
    BeforeValidatorAfterLe(v=366)  # raises ValueError, as expected
except ValueError as ex:
    print(1, ex)

try:
    BeforeValidatorBeforeLe(v=366)  # raises TypeError in pydantic>=2.9.0
except ValueError as ex:  # ValueError (as expected) in 2.8.2
    print(2, "ValueError:", ex)
except TypeError as ex:  # UNEXPECTED TypeError in pydantic>=2.9.0
    print(2, "TypeError:", ex)

Environment

Python: 3.12
Pydantic: 2

What Broke

Unexpected TypeError raised during model validation in production, causing application errors.

Why It Broke

TypeError occurs when using BeforeValidator with constraints from annotated_types in Pydantic V2

Fix Options (Details)

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

pip install pydantic==1.10.19

When NOT to use: Do not use this fix if the application relies on the previous behavior of BeforeValidator.

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/10490

First fixed release: 1.10.19

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

When NOT to Use This Fix

Do not use this fix if the application relies on the previous behavior of BeforeValidator.

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
1.10.19	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.