Header

API reference for the FastAPI Header parameter function

Header

Declare a header parameter for a path operation. Header parameters are extracted from HTTP request headers.

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}

Signature

def Header(
    default: Any = Undefined,
    *,
    convert_underscores: bool = True,
    alias: str | None = None,
    title: str | None = None,
    description: str | None = None,
    gt: float | None = None,
    ge: float | None = None,
    lt: float | None = None,
    le: float | None = None,
    min_length: int | None = None,
    max_length: int | None = None,
    pattern: str | None = None,
    examples: list[Any] | None = None,
    deprecated: bool | str | None = None,
    include_in_schema: bool = True,
    json_schema_extra: dict[str, Any] | None = None,
) -> Any

Parameters

default · Any · default: "Undefined"

Default value if the parameter field is not set.

convert_underscores · bool · default: "True"

Automatically convert underscores to hyphens in the parameter field name. For example, user_agent becomes user-agent.

alias · str | None · default: "None"

An alternative name for the parameter field. This will be used to extract the data and for the generated OpenAPI.

title · str | None · default: "None"

Human-readable title for the parameter.

description · str | None · default: "None"

Human-readable description for the parameter.

gt · float | None · default: "None"

Greater than validation. If set, the value must be greater than this. Only applicable to numbers.

ge · float | None · default: "None"

Greater than or equal validation. If set, the value must be greater than or equal to this. Only applicable to numbers.

lt · float | None · default: "None"

Less than validation. If set, the value must be less than this. Only applicable to numbers.

le · float | None · default: "None"

Less than or equal validation. If set, the value must be less than or equal to this. Only applicable to numbers.

min_length · int | None · default: "None"

Minimum length for strings.

max_length · int | None · default: "None"

Maximum length for strings.

pattern · str | None · default: "None"

RegEx pattern for strings.

discriminator · str | None · default: "None"

Parameter field name for discriminating the type in a tagged union.

strict · bool | None · default: "None"

If True, strict validation is applied to the field.

multiple_of · float | None · default: "None"

Value must be a multiple of this. Only applicable to numbers.

allow_inf_nan · bool | None · default: "None"

Allow inf, -inf, nan. Only applicable to numbers.

max_digits · int | None · default: "None"

Maximum number of allowed digits for numbers.

decimal_places · int | None · default: "None"

Maximum number of decimal places allowed for numbers.

examples · list[Any] | None · default: "None"

Example values for this field.

deprecated · bool | str | None · default: "None"

Mark this parameter field as deprecated. It will affect the generated OpenAPI (visible at /docs).

include_in_schema · bool · default: "True"

Whether to include this parameter field in the generated OpenAPI.

json_schema_extra · dict[str, Any] | None · default: "None"

Any additional JSON schema data.

Examples

Basic Header Parameter

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}

By default, user_agent will automatically be converted to User-Agent when reading from headers due to convert_underscores=True.

Disable Automatic Conversion

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

Required Header

@app.get("/items/")
async def read_items(
    x_token: Annotated[str, Header()]
):
    return {"X-Token": x_token}

A header parameter is required when there is no default value.

Multiple Header Values

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

This allows receiving multiple headers with the same name. For example, multiple X-Token headers.

Header with Validation

@app.get("/items/")
async def read_items(
    x_token: Annotated[str, Header(min_length=10, max_length=100)]
):
    return {"X-Token": x_token}

Custom Header Name with Alias

@app.get("/items/")
async def read_items(
    token: Annotated[str | None, Header(alias="X-API-Key")] = None
):
    return {"token": token}

Use alias when you want to use a different variable name in Python than the actual header name.

Common Use Cases

Authentication Token

@app.get("/users/me")
async def read_user(
    authorization: Annotated[str, Header()]
):
    return {"token": authorization}

Content Type

@app.post("/items/")
async def create_item(
    content_type: Annotated[str | None, Header()] = None
):
    return {"Content-Type": content_type}

Custom Headers

@app.get("/items/")
async def read_items(
    x_request_id: Annotated[str | None, Header()] = None,
    x_correlation_id: Annotated[str | None, Header()] = None
):
    return {
        "X-Request-ID": x_request_id,
        "X-Correlation-ID": x_correlation_id
    }

Header names are case-insensitive according to the HTTP specification, but FastAPI will convert them for consistency.