Pydantic New Enum validation doesn't work for custom

The Fix

pip install pydantic==1.10.1

Based on closed pydantic/pydantic issue #9248 · 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

@@ -34,7 +34,7 @@ A few notes:
 
 Type coercion like this can be extremely helpful but also confusing or not desired,
-see [below](#coercion-and-stictness) for a discussion of `validate_arguments`'s limitations in this regard.
+see [below](#coercion-and-strictness) for a discussion of `validate_arguments`'s limitations in this regard.

repro.py

from enum import Enum
from pydantic import TypeAdapter


class Animal(Enum):
    CAT = "cat", "meow"
    DOG = "dog", "woof"

    def __new__(cls, species: str, sound: str):
        obj = object.__new__(cls)

        obj._value_ = species
        obj._all_values = (species, sound)

        obj.species = species
        obj.sound = sound

        cls._value2member_map_[sound] = obj

        return obj


print(Animal("cat"))
print(Animal("meow"))
print(TypeAdapter(Animal).validate_python("meow"))

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==1.10.1\nWhen NOT to use: Do not apply this fix if the Enum does not use a custom __new__ method.\n\n

Why This Fix Works in Production

Trigger: New Enum validation doesn't work for custom __new__ methods
Mechanism: The new Enum validation in Pydantic fails for enums with custom __new__ methods
Why the fix works: Addresses the issue with Enum validation for custom __new__ methods in Pydantic by ensuring compatibility. (first fixed release: 1.10.1).

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 3.10 in real deployments (not just unit tests).
The new Enum validation in Pydantic fails for enums with custom __new__ methods
Production symptom (often without a traceback): New Enum validation doesn't work for custom __new__ methods

Proof / Evidence

GitHub issue: #9248
Fix PR: https://github.com/pydantic/pydantic-core/pull/1456
First fixed release: 1.10.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.52

Discussion

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

Jump to Sources Open on GitHub

“Makes sense, happy to accept a pr to pydantic-core to detect custom __new__ methods and call them if that's possible. Btw, we already support the…”

@samuelcolvin · 2024-04-16 · source

Failure Signature (Search String)

New Enum validation doesn't work for custom __new__ methods
The newly release improvements to the `Enum` class break enums with a custom `__new__` implementation.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
New Enum validation doesn't work for custom __new__ methods
The newly release improvements to the `Enum` class break enums with a custom `__new__` implementation.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
New Enum validation doesn't work for custom __new__ methods
The newly release improvements to the `Enum` class break enums with a custom `__new__` implementation.

Minimal Reproduction

repro.py

from enum import Enum
from pydantic import TypeAdapter


class Animal(Enum):
    CAT = "cat", "meow"
    DOG = "dog", "woof"

    def __new__(cls, species: str, sound: str):
        obj = object.__new__(cls)

        obj._value_ = species
        obj._all_values = (species, sound)

        obj.species = species
        obj.sound = sound

        cls._value2member_map_[sound] = obj

        return obj


print(Animal("cat"))
print(Animal("meow"))
print(TypeAdapter(Animal).validate_python("meow"))

Environment

Python: 3.10
Pydantic: 2

What Broke

Users experience validation errors when using custom Enum classes.

Why It Broke

The new Enum validation in Pydantic fails for enums with custom __new__ methods

Fix Options (Details)

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

pip install pydantic==1.10.1

When NOT to use: Do not apply this fix if the Enum does not use a custom __new__ 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-core/pull/1456

First fixed release: 1.10.1

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

When NOT to Use This Fix

Do not apply this fix if the Enum does not use a custom __new__ 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
1.10.1	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 New Enum validation doesn't work for custom __new__ methods (Fix)