AFAIK it's not documented, but schema_extra parameter of SQLModel's Field is for passing parameters that are not present in the Field, but supported by Pydantic's Field.

For example, there is no validation_alias parameter, but you can pass it the following way:

id_: in = Field(schema_extra={"validation_alias": "item_id"})

The class Config must be nested in your class model, in your example the class is sitting outside:

class MlFlow(BaseModel):
    appId: str
    sessionId: str
    timestamp: str
    type: str
    payload: MlFlowData

    class Config:
       arbitrary_types_allowed = True
       schema_extra = {}

Tip: Setting your properties to Optional AND = None is redundant. User either one, you don't need both.

After digging a bit deeper into the pydantic code I found a nice little way to prevent this. There is a method called field_title_should_be_set(...) in GenerateJsonSchema which can be subclassed and provided to model_json_schema(...).

I'm not sure if the way I've overwritten the method is sufficient for each edge case but at least for this little test class it works as intended.

from pydantic import BaseModel
from pydantic._internal._core_utils import is_core_schema, CoreSchemaOrField
from pydantic.json_schema import GenerateJsonSchema


class Test(BaseModel):
    a: int

class GenerateJsonSchemaWithoutDefaultTitles(GenerateJsonSchema):
    def field_title_should_be_set(self, schema: CoreSchemaOrField) -> bool:
        return_value = super().field_title_should_be_set(schema)
        if return_value and is_core_schema(schema):
            return False
        return return_value

json_schema = Test.model_json_schema(schema_generator=GenerateJsonSchemaWithoutDefaultTitles)
assert "title" not in json_schema["properties"]["a"]