Initial Checks
Description
New type aliases prefixed with the type keyword are not supported as the type annotation for a callable discriminator.
Related:
Example Code
import random
from typing import Annotated
from pydantic import BaseModel, Discriminator, Field, Tag, TypeAdapter
discriminator = Discriminator(lambda _: random.choice(["a", "b", None]))
OldUnion = Annotated[int, Tag("a")] | Annotated[str, Tag("b")]
type NewUnion = Annotated[int, Tag("a")] | Annotated[str, Tag("b")]
TypeAdapter[OldUnion](Annotated[OldUnion, discriminator]) # OK
TypeAdapter[NewUnion](Annotated[NewUnion, discriminator]) # TypeError: Discriminator must be used with a Union type, not NewUnion
class M(BaseModel):
u1: OldUnion = Field(discriminator=discriminator) # OK
u2: NewUnion = Field(discriminator=discriminator) # pydantic.errors.PydanticUserError: `Tag` not provided for choice {'type': 'definition-ref', 'schema_ref': '__main__.NewUnion:140737344689696'} used with `Discriminator`Python, Pydantic & OS Version
pydantic version: 2.12.5
pydantic-core version: 2.41.5
pydantic-core build: profile=release pgo=false
python version: 3.14.0 (main, Oct 14 2025, 21:27:55) [Clang 20.1.4 ]
platform: Linux-5.15.0-157-generic-x86_64-with-glibc2.35
related packages: fastapi-0.129.2 email-validator-2.3.0 typing_extensions-4.15.0 mypy-1.19.1
commit: unknown
Initial Checks
Description
New type aliases prefixed with the
typekeyword are not supported as the type annotation for a callable discriminator.Related:
Example Code
Python, Pydantic & OS Version