Fix · Node, npm and build

Port already in use (EADDRINUSE)

What you may see

The dev server says port 3000 is already in use
The server starts on a different port than I expect

Most likely cause

Another program (often an earlier copy of your own dev server) is already listening on that port.

Check first (about two minutes)

Do you have another terminal tab running the same project? Close it with Ctrl+C.

The fix prompt

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The exact error or what you see

What you did before it happened

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Prompt language

You are a patient senior engineer who knows Node.js, npm and build tools and explains them to people who are not developers. I am not a developer, so explain every step in plain words and tell me exactly which button, menu or command you mean. If you are not sure about something, say so; do not guess.

THE PROBLEM
Port already in use (EADDRINUSE)

THE EXACT ERROR OR WHAT I SEE
[paste the exact error here]

WHAT I DID BEFORE IT HAPPENED
[write in a few words what you did before it happened]

MOST LIKELY CAUSE
Another program (often an earlier copy of your own dev server) is already listening on that port.

DO THESE STEPS, IN THIS ORDER
1. Run lsof -i :3000 (use my port number) and show me which program holds the port.
2. If it is MY OWN old dev server, tell me how to stop it by its process number. If it is anything else, do not stop it; start my server on another port instead (for example npm run dev -- -p 3001).
3. Start the server again and open the address it prints.
Stop and tell me if what you see does not match a step. Do not skip ahead.

CHECK THAT IT WORKED
- The server starts and the browser shows my app at the printed address.
Tell me exactly what I should see on the screen when it works.

DO NOT CHANGE
- Do not stop processes you did not start, and do not use kill -9 on everything.
- Anything I did not ask about: the design, the text, other pages, other settings or the data.
- Do not delete files, data or branches, do not reset or force-push, and do not rewrite history, unless you first show me exactly what would be lost and I say yes.
- Do not paste, print or ask me to paste a password or secret key into this chat. If one is needed, tell me which settings page of the right service to store it in.

WHEN YOU FINISH
Explain in simple words what you changed (file by file or setting by setting), why you changed it, and how I can undo it.

Paste it into Claude, ChatGPT, Gemini or Cursor, then follow what it says.

After you tried it, did it work?

Steps

Run lsof -i :3000 (use my port number) and show me which program holds the port.
If it is MY OWN old dev server, tell me how to stop it by its process number. If it is anything else, do not stop it; start my server on another port instead (for example npm run dev -- -p 3001).
Start the server again and open the address it prints.

Do not do this

Do not restart the computer first: it is usually one process.
Do not stop processes you did not start, and do not use kill -9 on everything.

Try another entry