Header Parameters

You can define Header parameters the same way you define Query, Path and Cookie parameters.

First import Header:

from typing import Annotated

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(user_agent: Annotated[str | None, Header()] = None):
    return {"User-Agent": user_agent}

Then declare the header parameters using the same structure as with Path, Query and Cookie.

You can define the default value as well as all the extra validation or annotation parameters:

from typing import Annotated

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(user_agent: Annotated[str | None, Header()] = None):
    return {"User-Agent": user_agent}

Header has a little extra functionality on top of what Path, Query and Cookie provide.

Most of the standard headers are separated by a "hyphen" character, also known as the "minus symbol" (-).

But a variable like user-agent is invalid in Python.

So, by default, Header will convert the parameter names characters from underscore (_) to hyphen (-) to extract and document the headers.

Also, HTTP headers are case-insensitive, so, you can declare them with standard Python style (also known as "snake_case").

So, you can use user_agent as you normally would in Python code, instead of needing to capitalize the first letters as User_Agent or something similar.

If for some reason you need to disable automatic conversion of underscores to hyphens, set the parameter convert_underscores of Header to False:

from typing import Annotated

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(
    strange_header: Annotated[str | None, Header(convert_underscores=False)] = None,
):
    return {"strange_header": strange_header}

It is possible to receive duplicate headers. That means, the same header with multiple values.

You can define those cases using a list in the type declaration.

You will receive all the values from the duplicate header as a Python list.

For example, to declare a header of X-Token that can appear more than once, you can write:

from typing import Annotated

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(x_token: Annotated[list[str] | None, Header()] = None):
    return {"X-Token values": x_token}

If you communicate with that path operation sending two HTTP headers like:

X-Token: foo
X-Token: bar

The response would be like:

{
    "X-Token values": [
        "bar",
        "foo"
    ]
}

Declare headers with Header, using the same common pattern as Query, Path and Cookie.

And don't worry about underscores in your variables, FastAPI will take care of converting them.