Address already in use Uvicorn FastAPI

Why This Happens

In FastAPI, another process is using the port. 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

uvicorn app:app --port 8000

The Fix

# Kill existing process:
lsof -ti:8000 | xargs kill
uvicorn app:app --port 8000

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: Uvicorn bind error. 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.