Pydantic Settings cannot deserialize JSON value of type declared with PEP-695 style type alias

[palle-k]

With the following Pydantic settings model

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

class Bar(BaseModel):
    type: Literal["bar"] = "bar"
    bar: int
    
class Baz(BaseModel):
    type: Literal["baz"] = "baz"
    baz: str
    
type BarOrBaz = Annotated[Bar | Baz, Field(discriminator="type")]

class Foo(BaseSettings):
    foo: BarOrBaz

and the env

export FOO='{"type": "bar", "bar": 42}'

Pydantic Settings fails to deserialize FOO:

Foo()

ValidationError: 1 validation error for Foo
foo
  Input should be a valid dictionary or object to extract fields from [type=model_attributes_type, input_value='{"type": "bar", "bar": 42}', input_type=str]
    For further information visit https://errors.pydantic.dev/2.10/v/model_attributes_type

Declaring BarOrBaz without the PEP-695 style type alias (i.e. leaving out the type keyword) works as expected - validation succeeds.

Version Info

             pydantic version: 2.10.6
        pydantic-core version: 2.27.2
          pydantic-core build: profile=release pgo=false
                 install path: /path/to/my/venv/lib/python3.13/site-packages/pydantic
               python version: 3.13.1 (main, Dec  3 2024, 17:59:52) [Clang 19.1.5 ]
                     platform: macOS-15.3-arm64-arm-64bit-Mach-O
             related packages: mypy-1.15.0 fastapi-0.115.8 pydantic-settings-2.7.1 typing_extensions-4.12.2
                       commit: unknown

[hramezani]

Thanks @palle-k for reporting this bug. pydantic-settings considers foo as a normal field(not complex). So, it doesn't decode it.

Meanwhile, you can use field_serializer to decode the value manually:

import json

from typing import Annotated, Literal
from pydantic import BaseModel, Field, field_validator
from pydantic_settings import BaseSettings

class Bar(BaseModel):
    type: Literal["bar"] = "bar"
    bar: int
    
class Baz(BaseModel):
    type: Literal["baz"] = "baz"
    baz: str
    
type BarOrBaz = Annotated[Bar | Baz, Field(discriminator="type")]

class Foo(BaseSettings):
    foo: BarOrBaz

    @field_validator('foo', mode='before')
    @classmethod
    def decode_foo(cls, v: str) -> dict:
        return json.loads(v)