Hello!

I've encountered an issue in FastAPI'sopenapi.json generation for parameter inputs (Query,Header,Cookie). When usingalias for field names in Pydantic models, the alias appears correctly in the generated schema. However, when usingvalidation_alias, it is ignored and the original field name is shown instead.

To demonstrate this behavior, I’ve included minimal reproducible examples along with corresponding Swagger UI screenshots.

With alias

frompydanticimportBaseModel,FieldfromtypingimportAnnotatedfromfastapiimportFastAPI,Header,Query,CookieclassDataModel(BaseModel):foo:str=Field(...,alias="bar")app=FastAPI()@app.get("/test")deftest(query:Annotated[DataModel,Query(...)],header:Annotated[DataModel,Header(...)],cookie:Annotated[DataModel,Cookie(...)]):return"Received: "+str(query)+", "+str(header)+", "+str(cookie)

With validation_alias

frompydanticimportBaseModel,FieldfromtypingimportAnnotatedfromfastapiimportFastAPI,Header,Query,CookieclassDataModel(BaseModel):foo:str=Field(...,validation_alias="bar")app=FastAPI()@app.get("/test")deftest(query:Annotated[DataModel,Query(...)],header:Annotated[DataModel,Header(...)],cookie:Annotated[DataModel,Cookie(...)]):return"Received: "+str(query)+", "+str(header)+", "+str(cookie)

As shown above, even though validation_alias="bar" is set, the OpenAPI schema still uses "foo".

Upon inspecting the parameter generation logic, I found that the name used in the schema is determined via the field alias:

This uses Pydantic’s ModelField logic, where alias is returned if present; otherwise, the field name is used:

Proposed Change

This PR modifies the alias resolution logic in ModelField to prioritize validation_alias or serialization_alias, depending on mode, instead of defaulting to alias alone.

Thanks for taking the time to review this!