^{Originally posted by atlasale December 11, 2025}

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Example Code

from typing import Annotated, Union
from pydantic import BaseModel, Tag, Discriminator
from fastapi import FastAPI, Body

class Cat(BaseModel):
    pet_type: str = "cat"
    meows: int

class Dog(BaseModel):
    pet_type: str = "dog"
    barks: float

def get_pet_type(v):
    if isinstance(v, dict):
        return v.get("pet_type", "")
    return getattr(v, "pet_type", "")

# Define discriminated union
Pet = Annotated[
    Union[
        Annotated[Cat, Tag("cat")],
        Annotated[Dog, Tag("dog")]
    ],
    Discriminator(get_pet_type)
]

app = FastAPI()

# ✅ THIS WORKS in 0.124.1+
@app.post("/pet/works")
async def create_pet_works(pet: Pet = Body(...)):
    return pet

# ❌ THIS BREAKS in 0.124.1+ (worked in 0.124.0)
@app.post("/pet/broken")
async def create_pet_broken(pet: Annotated[Pet, Body(...)]):
    return pet

Description

I'm experiencing a breaking change when upgrading from FastAPI 0.124.0 to
0.124.1/0.124.2.

What works:

 - FastAPI 0.124.0 - Both endpoint patterns work correctly
 - FastAPI 0.124.1+ - Only pet: Pet = Body(...) works

What breaks:

 - FastAPI 0.124.1+ - The pattern pet: Annotated[Pet, Body(...)] fails

with:

 pydantic.errors.PydanticUserError: `Tag` not provided for choice {
   'type': 'tagged-union',
   'choices': {
     'cat': {'type': 'definition-ref', 'metadata':

{'pydantic_internal_union_tag_key': 'cat'}},
'dog': {'type': 'definition-ref', 'metadata':
{'pydantic_internal_union_tag_key': 'dog'}}
},
'discriminator': <function get_pet_type at 0x...>
} used with Discriminator

 For further information visit

https://errors.pydantic.dev/2.12/u/callable-discriminator-no-tag

Root Cause: This appears to be caused by commit 42b250d (https://github.com
/fastapi/fastapi/commit/42b250d14dd42d3c0c24dd085fa53878172a985f) which
fixed arbitrary_types_allowed=True but introduced a regression in how
discriminated unions are handled.

The change in fastapi/_compat/v2.py now includes *self.field_info.metadata
when creating TypeAdapters, which causes discriminator metadata to be
duplicated/nested when the union is wrapped in Annotated[..., Body(...)].

Workaround: Use pet: Pet = Body(...) instead of pet: Annotated[Pet,
Body(...)]

Operating System

Linux, macOS

Operating System Details

No response

FastAPI Version

0.124.2

Pydantic Version

2.12.5

Python Version

3.12

Additional Context

• The error shows tags ARE present ('pydantic_internal_union_tag_key':
  'cat'), so Pydantic sees them but validation fails

  • This only affects callable discriminators with Tag(), not field-based
    discriminators with Literal
  • Both patterns (= Body(...) and Annotated[..., Body(...)]) should be
    functionally equivalent
  • This is a breaking change in a patch release

  Is this a known issue? Should I file a bug report?