Middleware

You can add middleware to FastAPI applications.

A "middleware" is a function that works with every request before it is processed by any specific path operation. And also with every response before returning it.

• It takes each request that comes to your application.
• It can then do something to that request or run any needed code.
• Then it passes the request to be processed by the rest of the application (by some path operation).
• It then takes the response generated by the application (by some path operation).
• It can do something to that response or run any needed code.
• Then it returns the response.

To create a middleware you use the decorator @app.middleware("http") on top of a function.

The middleware function receives:

• The request.
• A function call_next that will receive the request as a parameter.
  • This function will pass the request to the corresponding path operation.
  • Then it returns the response generated by the corresponding path operation.
• You can then further modify the response before returning it.

import time

from fastapi import FastAPI, Request

app = FastAPI()


@app.middleware("http")
async def add_process_time_header(request: Request, call_next):
    start_time = time.perf_counter()
    response = await call_next(request)
    process_time = time.perf_counter() - start_time
    response.headers["X-Process-Time"] = str(process_time)
    return response

You can add code to be run with the request, before any path operation receives it.

And also after the response is generated, before returning it.

For example, you could add a custom header X-Process-Time containing the time in seconds that it took to process the request and generate a response:

import time

from fastapi import FastAPI, Request

app = FastAPI()


@app.middleware("http")
async def add_process_time_header(request: Request, call_next):
    start_time = time.perf_counter()
    response = await call_next(request)
    process_time = time.perf_counter() - start_time
    response.headers["X-Process-Time"] = str(process_time)
    return response

When you add multiple middlewares using either @app.middleware() decorator or app.add_middleware() method, each new middleware wraps the application, forming a stack. The last middleware added is the outermost, and the first is the innermost.

On the request path, the outermost middleware runs first.

On the response path, it runs last.

For example:

app.add_middleware(MiddlewareA)
app.add_middleware(MiddlewareB)

This results in the following execution order:

• Request: MiddlewareB → MiddlewareA → route

• Response: route → MiddlewareA → MiddlewareB

This stacking behavior ensures that middlewares are executed in a predictable and controllable order.

You can later read more about other middlewares in the Advanced User Guide: Advanced Middleware.

You will read about how to handle CORSCross-Origin Resource Sharing with a middleware in the next section.