Pydantic Model construction fails on non-union types

The Fix

pip install pydantic==2.11.2

Based on closed pydantic/pydantic issue #11661 · 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

@@ -644,6 +644,7 @@ def _apply_discriminator_to_union(
                 schema,
                 discriminator,
+                self.defs._definitions,
             )
         except _discriminated_union.MissingDefinitionForUnionRef:

repro.py

from pydantic import BaseModel, Field
from typing import Annotated, Literal

class Foo(BaseModel):
    type: Literal["foo"] = "foo"

type FooWithDiscriminator = Annotated[Foo, Field(discriminator="type")]

class Container(BaseModel):
    f: FooWithDiscriminator

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==2.11.2\nWhen NOT to use: This fix should not be used if the model is intended to work with union types.\n\nOption B — Safe version pin\npip install pydantic==2.11.0\nWhen NOT to use: Do not use if you need features or security fixes in newer releases.\n\n

Why This Fix Works in Production

Trigger: Model construction fails on non-union types annotated with discriminator.
Mechanism: The discriminator information was lost when using a PEP-695-style type alias
Why the fix works: Provides the available definitions when applying discriminated unions, fixing issues in the schema cleaning process related to discriminator metadata. (first fixed release: 2.11.2).

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 3.13 in real deployments (not just unit tests).
The discriminator information was lost when using a PEP-695-style type alias
Production symptom (often without a traceback): Model construction fails on non-union types annotated with discriminator.

Proof / Evidence

GitHub issue: #11661
Fix PR: https://github.com/pydantic/pydantic/pull/11670
First fixed release: 2.11.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.67

Discussion

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

Jump to Sources Open on GitHub

“Turns out in 2.11.0 the discriminator info was lost, but no error was thrown. This will be fixed in the next patch release.”

@Viicos · 2025-04-01 · confirmation · source

Failure Signature (Search String)

Model construction fails on non-union types annotated with discriminator.
Annotating a type with a `discriminator` that is not a union fails with the following error:

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Model construction fails on non-union types annotated with discriminator.
Annotating a type with a `discriminator` that is not a union fails with the following error:

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Model construction fails on non-union types annotated with discriminator.
Annotating a type with a `discriminator` that is not a union fails with the following error:

Minimal Reproduction

repro.py

from pydantic import BaseModel, Field
from typing import Annotated, Literal

class Foo(BaseModel):
    type: Literal["foo"] = "foo"

type FooWithDiscriminator = Annotated[Foo, Field(discriminator="type")]

class Container(BaseModel):
    f: FooWithDiscriminator

Environment

Python: 3.13
Pydantic: 2

What Broke

Model construction fails with a SchemaError when using a discriminator on non-union types.

Why It Broke

The discriminator information was lost when using a PEP-695-style type alias

Fix Options (Details)

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

pip install pydantic==2.11.2

When NOT to use: This fix should not be used if the model is intended to work with union types.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option B — Safe version pin Backward-compatible pin

pip install pydantic==2.11.0

When NOT to use: Do not use if you need features or security fixes in newer releases.

Use when you can’t upgrade immediately. Plan a follow-up to upgrade (pins can accumulate security/compat debt).

Fix reference: https://github.com/pydantic/pydantic/pull/11670

First fixed release: 2.11.2

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

When NOT to Use This Fix

This fix should not be used if the model is intended to work with union types.
Do not use if you need features or security fixes in newer releases.

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
2.11.0	Working
2.11.2	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 Model construction fails on non-union types annotated with discriminator. (Fix)