Initial Checks

• I confirm that I'm using Pydantic V2

Description

Field validators receive an incorrect value for field_name for fields defined with a type alias made with a type statement.

The following code uses a type alias to reuse a field validator in multiple fields.

from typing import Annotated

from pydantic import AfterValidator, BaseModel, ValidationInfo


def validate_my_field(value: str, info: ValidationInfo):
    print(f"{info.field_name} = {value}")
    return value


type MyField = Annotated[str, AfterValidator(validate_my_field)]


class MyModel(BaseModel):
    field1: MyField
    field2: MyField


MyModel.model_validate(
    {
        "field1": "value1",
        "field2": "value2",
    }
)

However, the input is incorrect:

field1 = value1
field1 = value2

As you can see, the validator function is called twice with field_name set to field1.

Notice that the bug only happens with Python 3.12's type statement but doesn't occur if we use TypeAlias:

MyField: TypeAlias = Annotated[str, AfterValidator(validate_my_field)]

After changing this line, the output is correct:

field1 = value1
field2 = value2

Python, Pydantic & OS Version

             pydantic version: 2.11.3
        pydantic-core version: 2.33.1
          pydantic-core build: profile=release pgo=false
                 install path: E:\Git\throwaways\pydantic-field-name-bug\.venv\Lib\site-packages\pydantic
               python version: 3.12.10 (tags/v3.12.10:0cc8128, Apr  8 2025, 12:21:36) [MSC v.1943 64 bit (AMD64)]
                     platform: Windows-11-10.0.26100-SP0
             related packages: typing_extensions-4.13.2
                       commit: unknown