Pydantic v2.12 no longer supports ABC Union (Fix)

The Fix

pip install pydantic==2.12.0

Based on closed pydantic/pydantic issue #12230 · 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

@@ -19,7 +19,7 @@
 
 # Keep this in sync with the version constraint in the `pyproject.toml` dependencies:
-_COMPATIBLE_PYDANTIC_CORE_VERSION = '2.38.0'
+_COMPATIBLE_PYDANTIC_CORE_VERSION = '2.40.1'

repro.py

from pydantic import BaseModel
from abc import ABC, abstractmethod

class ABC_1(BaseModel, ABC):
    @abstractmethod
    def do_something(self):
        pass

class ABC_2(BaseModel, ABC):
    @abstractmethod
    def do_something_else(self):
        pass

class A1(ABC_1):
    def do_something(self):
        print("Doing something in A1")

class A2(ABC_2):
    def do_something_else(self):
        print("Doing something else in A2")

class X(BaseModel):
    x: ABC_1 | ABC_2

a1 = A1()
a2 = A2()

x1 = X(x=a1)
x2 = X(x=a2)

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==2.12.0\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: Yes this seems like a regression, feel free to take a look although it might not be trivial. In any case, I'll look into it before the final release.
Mechanism: Bumped `pydantic-core` to v2.40.1 to resolve issues with ABC Union support in Pydantic v2.12.
Why the fix works: Bumped `pydantic-core` to v2.40.1 to resolve issues with ABC Union support in Pydantic v2.12. (first fixed release: 2.12.0).

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

Production symptom (often without a traceback): Yes this seems like a regression, feel free to take a look although it might not be trivial. In any case, I'll look into it before the final release.

Proof / Evidence

GitHub issue: #12230
Fix PR: https://github.com/pydantic/pydantic/pull/12314
First fixed release: 2.12.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.53

Discussion

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

Jump to Sources Open on GitHub

“Bisected to https://github.com/pydantic/pydantic-core/commit/4760f383a037e5fe91b854f9d883c7475b0f53f5 (https://github.com/pydantic/pydantic-core/pull/1692). Fixed in core, regression test in https://github.com/pydantic/pydantic/pull/12314.”

@Viicos · 2025-10-02 · confirmation · source

“I want to work on this task and I think it makes sense. Do you agree that we should have this feature? @Viicos”

@pipinstalled · 2025-09-09 · source

“Yes this seems like a regression, feel free to take a look although it might not be trivial. In any case, I'll look into it…”

@Viicos · 2025-09-09 · source

Failure Signature (Search String)

Yes this seems like a regression, feel free to take a look although it might not be trivial. In any case, I'll look into it before the final release.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Yes this seems like a regression, feel free to take a look although it might not be trivial. In any case, I'll look into it before the final release.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Yes this seems like a regression, feel free to take a look although it might not be trivial. In any case, I'll look into it before the final release.

Minimal Reproduction

repro.py

from pydantic import BaseModel
from abc import ABC, abstractmethod

class ABC_1(BaseModel, ABC):
    @abstractmethod
    def do_something(self):
        pass

class ABC_2(BaseModel, ABC):
    @abstractmethod
    def do_something_else(self):
        pass

class A1(ABC_1):
    def do_something(self):
        print("Doing something in A1")

class A2(ABC_2):
    def do_something_else(self):
        print("Doing something else in A2")

class X(BaseModel):
    x: ABC_1 | ABC_2

a1 = A1()
a2 = A2()

x1 = X(x=a1)
x2 = X(x=a2)

Environment

Pydantic: 2

What Broke

Users experienced failures when using ABC Union in Pydantic models, leading to application errors.

Fix Options (Details)

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

pip install pydantic==2.12.0

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/12314

First fixed release: 2.12.0

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
2.12.0	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.