WhatThePort

Guides

Practical guides to finding what’s running on localhost, fixing “port already in use” and EADDRINUSE, cleaning up dev servers left by Claude Code and Codex, and keeping memory in check on a Mac.

  1. Port 3000 already in use on Mac: find what’s using it and stop itHow to find the process holding port 3000 (or any port) on macOS with lsof, stop it safely, and keep forgotten dev servers from taking your ports again.
  2. EADDRINUSE: address already in use, explainedWhat EADDRINUSE means in Node.js, Next.js, nodemon, Python and Rails, why it happens on macOS, and how to fix it for good instead of killing ports by hand.
  3. How to see every dev server running on your MacList every localhost server and listening port on macOS from the terminal, work out which project each one belongs to, and keep them in view from the menu bar.
  4. Claude Code left a dev server running? Find and clean up agent-started serversCoding agents like Claude Code, Codex and Cursor start dev servers in the background and leave them running. How to find which agent session started each server, stop the leftovers, and stop agents starting duplicates.
  5. Running parallel coding agents in git worktrees without port chaosWhen several Claude Code, Codex or Conductor agents each run a dev server from their own git worktree, ports collide and old servers pile up. How to give every worktree its own port and clean up after deleted ones.
  6. Why Vite starts on 5174 and Next.js on 3001, and how to find the server on the old port“Port 5173 is in use, trying another one” and “Port 3000 is in use, trying 3001 instead” mean an older dev server is still running. How to find it, stop it, and make your dev server fail instead of drifting.
  7. Dev servers eating your Mac’s RAM: finding leaks in Next.js, Vite and NodeWhy local dev servers grow to gigabytes of memory, how to measure a whole server’s process tree on macOS, how to tell a leak from normal growth, and what to do about it.
  8. Port 5000 already in use on Mac? It’s AirPlay ReceiverOn macOS 12 Monterey and later, ControlCenter listens on ports 5000 and 7000 for AirPlay Receiver. How to confirm it, turn it off, or move Flask and other apps to a different port.
  9. Tools for managing localhost ports and dev servers on a MacA practical overview of the ways to see and stop what’s running on localhost on macOS, from lsof and kill-port to Raycast extensions and menu bar apps, and which fits which workflow.