Pydantic "Failed to get discriminator value for tagged

The Fix

pip install pydantic==1.10.1

Based on closed pydantic/pydantic issue #10334 · 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

@@ -1,6 +1,6 @@
 black==19.10b0
 coverage==5.1
-Cython==0.29.16;sys_platform!='win32'
+Cython==0.29.17;sys_platform!='win32'
 flake8==3.7.9

repro.py

from typing import Literal

from pydantic import BaseModel, ConfigDict, Field


def to_pascal_case(s: str) -> str:
    return "".join(x.capitalize() for x in s.lower().split("_"))


class MyBaseModel(BaseModel):
    model_config = ConfigDict(alias_generator=to_pascal_case, populate_by_name=True)


class Cat(MyBaseModel):
    pet_type: Literal["cat"] = "cat"
    meows: int


class Dog(MyBaseModel):
    pet_type: Literal["dog"] = "dog"
    barks: float


class Owner(MyBaseModel):
    name: str
    pet: Cat | Dog = Field(..., discriminator="pet_type")


owner = Owner(name="Alice", pet=Dog(barks=3.14))
owner.model_dump_json(warnings="error")

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: This fix should not be applied if the alias generator is not used in your models.\n\n

Why This Fix Works in Production

Trigger: E pydantic_core._pydantic_core.PydanticSerializationError: Pydantic serializer warnings:
Mechanism: The error occurs due to a bug in tagged union serialization when using an alias generator
Why the fix works: Fixes a bug related to tagged union serialization when using an alias generator in Pydantic models. (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 3.11 in real deployments (not just unit tests).
The error occurs due to a bug in tagged union serialization when using an alias generator
Surfaces as: E pydantic_core._pydantic_core.PydanticSerializationError: Pydantic serializer warnings:

Proof / Evidence

GitHub issue: #10334
Fix PR: https://github.com/pydantic/pydantic-core/pull/1442
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

“@brearleyt, Thanks for reporting - definitely a bug with the new tagged union serialization. I'll take a look at this and roll out a fix…”

@sydney-runkle · 2024-09-06 · source

Failure Signature (Search String)

E pydantic_core._pydantic_core.PydanticSerializationError: Pydantic serializer warnings:

Error Message

Stack trace

error.txt

Error Message
-------------
E   pydantic_core._pydantic_core.PydanticSerializationError: Pydantic serializer warnings:
E     Failed to get discriminator value for tagged union serialization with value `Dog(pet_type='dog', barks=3.14)` - defaulting to left to right union serialization.

Minimal Reproduction

repro.py

from typing import Literal

from pydantic import BaseModel, ConfigDict, Field


def to_pascal_case(s: str) -> str:
    return "".join(x.capitalize() for x in s.lower().split("_"))


class MyBaseModel(BaseModel):
    model_config = ConfigDict(alias_generator=to_pascal_case, populate_by_name=True)


class Cat(MyBaseModel):
    pet_type: Literal["cat"] = "cat"
    meows: int


class Dog(MyBaseModel):
    pet_type: Literal["dog"] = "dog"
    barks: float


class Owner(MyBaseModel):
    name: str
    pet: Cat | Dog = Field(..., discriminator="pet_type")


owner = Owner(name="Alice", pet=Dog(barks=3.14))
owner.model_dump_json(warnings="error")

Environment

Python: 3.11
Pydantic: 2

What Broke

Users experience serialization errors when using tagged unions with alias generators in Pydantic models.

Why It Broke

The error occurs due to a bug in tagged union serialization when using an alias generator

Fix Options (Details)

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

pip install pydantic==1.10.1

When NOT to use: This fix should not be applied if the alias generator is not used in your models.

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/1442

First fixed release: 1.10.1

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

When NOT to Use This Fix

This fix should not be applied if the alias generator is not used in your models.

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.

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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.

Pydantic "Failed to get discriminator value for tagged union serialization" when using (Fix)