Why is my app running on 3001 instead of 3000?

Most Node-based dev tools (Create React App, Next.js in some setups) automatically fall back to the next available port when 3000 is already in use, and 3001 is the first one they try. It's not a configuration problem; it's the tool being helpful.

Is it safe to kill a process on port 3001?

Yes, in nearly every case. Port 3001 almost always holds a dev server or local API you or a tool started, so killing it just stops that server.

Why do I have both 3000 and 3001 in use?

This is common with full-stack setups: a frontend dev server on 3000 and a companion API or second service on 3001, either started manually or as a paired dev script (concurrently, npm-run-all, etc.).

Port 3001: The Common Node/React Fallback Port

Port 3001 shows up constantly in local development, almost always for one reason: something wanted port 3000 and it was already taken.

What typically listens on port 3001

Node/React dev server fallback: Create React App’s dev server automatically tries the next port when 3000 is busy, landing on 3001.
Companion API servers: Full-stack projects often run a frontend on 3000 and a backend API on 3001 side by side.
Rails API mode: Some Rails setups default an API-only service to 3001 to avoid colliding with a separate frontend on 3000.
Storybook and other dev tools: Various component/dev tooling defaults here as a secondary port.

Is it safe to kill?

Yes. Like its neighbor 3000, port 3001 is essentially always a local development process you or a tool started. Killing it stops that server with no effect on macOS or anything else running.

Is it suspicious?

No. On a development machine this is routine, especially if you’re running a frontend and backend together. It’s only worth a closer look if you see it in use and you’re not running any dev servers at all, in which case something may have been left running from an earlier session.

How to find and free port 3001 on macOS

lsof -i :3001

To stop whatever’s using it:

kill -9 $(lsof -ti :3001)

If you’re juggling a frontend on 3000 and a backend on 3001 and losing track of which is which, Portie lists both with the exact process behind each, so you can kill the right one without guessing from a terminal window full of similar-looking node processes.