CORS error FastAPI

Why This Happens

In FastAPI, cross-origin requests are blocked by the browser. This is one of the most common FastAPI errors developers encounter. Understanding the root cause helps you fix it quickly and prevent it from recurring.

The Problem

# Frontend on localhost:3000 can't reach API on :8000

The Fix

from fastapi.middleware.cors import CORSMiddleware
app.add_middleware(CORSMiddleware, allow_origins=['*'])

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: CORS blocked. This tells you exactly what went wrong.

2. 2

   Find the cause

   Check the stack trace to find which line of your FastAPI code triggered this error.

3. 3

   Apply the fix

   Use the corrected code pattern shown above. Test to confirm the error is resolved.