Response Headers

Use a Response parameter

You can declare a parameter of type Response in your path operation function (as you can do for cookies).

And then you can set headers in that temporal response object.

from fastapi import FastAPI
from starlette.responses import Response

app = FastAPI()

def get_headers(response: Response):
    response.headers["X-Cat-Dog"] = "alone in the world"
    return {"message": "Hello World"}

And then you can return any object you need, as you normally would (a dict, a database model, etc).

And if you declared a response_model, it will still be used to filter and convert the object you returned.

FastAPI will use that temporal response to extract the headers (also cookies and status code), and will put them in the final response that contains the value you returned, filtered by any response_model.

You can also declare the Response parameter in dependencies, and set headers (and cookies) in them.

Return a Response directly

You can also add headers when you return a Response directly.

Create a response as described in Return a Response Directly and pass the headers as an additional parameter:

from fastapi import FastAPI
from starlette.responses import JSONResponse

app = FastAPI()

def get_headers():
    content = {"message": "Hello World"}
    headers = {"X-Cat-Dog": "alone in the world", "Content-Language": "en-US"}
    return JSONResponse(content=content, headers=headers)

Custom Headers

Have in mind that custom proprietary headers can be added using the 'X-' prefix.

But if you have custom headers that you want a client in a browser to be able to see, you need to add them to your CORS configurations (read more in CORS (Cross-Origin Resource Sharing)), using the parameter expose_headers documented in Starlette's CORS docs.