Header-Parameters

Header in FastAPI

In FastAPI, a Header parameter is used to extract and validate values from the HTTP request headers. These are often used for things like:

• Authentication tokens (e.g., Authorization)
• Custom headers (e.g., X-Request-ID)
• Content negotiation (e.g., Accept, User-Agent)

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Basic Example

from fastapi import FastAPI, Header

app = FastAPI()

@app.get("/items/")
def read_items(user_agent: str = Header(None)):
    return {"User-Agent": user_agent}

• This reads the User-Agent header from the request.
• Header(None) makes it optional.
• If not provided, user_agent will be None.

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Required Header Example

@app.get("/secure-data/")
def get_secure_data(api_key: str = Header(...)):
    return {"API-Key": api_key}

• ... makes the api_key header required
• If missing, FastAPI returns a 422 error

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Header Name Conventions

• HTTP headers are case-insensitive and use hyphens (e.g., X-Token)
• FastAPI automatically converts underscore _ to hyphen -

@app.get("/check/")
def check_header(x_token: str = Header(...)):
    return {"X-Token": x_token}

• This will correctly read the X-Token header from the request.

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🧪 Example with Multiple Headers

@app.get("/info/")
def get_info(
    x_token: str = Header(...),
    x_request_id: str = Header(None)
):
    return {"token": x_token, "request_id": x_request_id}

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✅ Summary

Parameter Type	Source	Use Case
Path	URL path	Resource identification
Query	URL query string	Filtering, pagination
Body	Request body (JSON)	Structured data
Header	HTTP headers	Metadata, auth, tracing

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Would you like an example of using headers for JWT authentication or API key validation in FastAPI?