What is middleware in FastAPI and how do you add custom middleware?

Middleware is a function that runs on every request before it reaches a route handler and on every response before it's sent to the client. FastAPI uses Starlette's middleware system — you can add it with @app.middleware('http') or app.add_middleware().

from fastapi import FastAPI, Request
from fastapi.middleware.cors import CORSMiddleware
from fastapi.middleware.gzip import GZipMiddleware
import time

app = FastAPI()

# Custom middleware using decorator
@app.middleware("http")
async def add_process_time_header(request: Request, call_next):
    start = time.time()
    response = await call_next(request)  # passes request to next handler
    duration = time.time() - start
    response.headers["X-Process-Time"] = str(duration)
    return response

# CORS — allow cross-origin requests (e.g. from a React frontend)
app.add_middleware(
    CORSMiddleware,
    allow_origins=["https://myfrontend.com", "http://localhost:3000"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

# GZip compression for large responses
app.add_middleware(GZipMiddleware, minimum_size=1000)

Common built-in middleware:

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