baasith6/cursor-skills/detecting-port-conflicts

Detecting Port Conflicts

When a dev server fails to start because a port is already in use, diagnose and resolve it.

Detection

Scan terminal output for these patterns:

• EADDRINUSE
• address already in use
• Port XXXX is already in use
• bind: address already in use
• OSError: [Errno 98] Address already in use

Extract the port number from the error message.

Diagnosis

Find what's using the port:

lsof -i :<PORT> -P -n

This shows the PID, process name, and user. Common culprits:

• A previous dev server that didn't shut down cleanly
• Another project's dev server
• A Docker container
• A system service

Resolution Options

Option 1: Kill the blocking process

kill <PID>
# If it doesn't stop:
kill -9 <PID>

Then restart the original server.

Option 2: Use a different port

Suggest the next available port:

# Check if port+1 is free
lsof -i :<PORT+1> -P -n

Update the dev server config or start command:

• Next.js: next dev -p <PORT>
• Vite: vite --port <PORT>
• Express: set PORT env var
• Django: python manage.py runserver <PORT>

Option 3: Kill all node processes (nuclear option)

killall node

Only suggest this if the user confirms — it kills everything.

Tips

• On macOS, ports below 1024 require root
• Docker containers bind ports that persist even if the container is stopped — check docker ps
• If lsof shows nothing, the port may be in TIME_WAIT state — just wait 30 seconds or use a different port