Port 3000 is already in use

Find the process that holds the port, then stop it. On macOS and Linux:

lsof -nP -iTCP:3000 -sTCP:LISTEN   # the PID is the second column
kill <PID>                         # kill -9 <PID> only if it won't stop

On Windows, in Command Prompt or PowerShell:

netstat -ano | findstr :3000       :: the PID is the last column
taskkill /PID <PID> /T /F          :: /T stops its child processes too

Or, anywhere Node is installed: npx kill-port 3000.

What does “address already in use” mean?

Only one process can listen on a TCP port at a time. When your dev server starts and something already listens on its port, the operating system refuses, and the server stops with an error such as:

• Node.js: Error: listen EADDRINUSE: address already in use :::3000
• Python: OSError: [Errno 48] Address already in use (98 on Linux, WinError 10048 on Windows)
• Rails: Address already in use - bind(2) for "127.0.0.1" port 3000 (Errno::EADDRINUSE)

Next.js and Vite don’t stop: they print Port 3000 is in use, trying 3001 instead. and move on. The app runs, but on a port your browser tab, your proxy or your other Service doesn’t expect.

How do I find what is using a port?

macOS. lsof -nP -iTCP:3000 -sTCP:LISTEN prints the command and PID of whatever listens on port 3000. Without -sTCP:LISTEN it also lists clients connected to it, such as your browser. Ports 5000 and 7000 are often taken by AirPlay Receiver, part of macOS: turn it off in System Settings → General → AirDrop & Handoff, or use another port.

Linux. ss -ltnp 'sport = :3000' shows the listener and, in users:(("node",pid=1234,…)), its PID. Add sudo to see processes of other users. fuser -k -TERM 3000/tcp finds and stops it in one step; without -TERM, fuser -k sends SIGKILL.

Windows. netstat -ano | findstr :3000 lists connections on the port; the listener is the line that says LISTENING, and its PID is the last column. tasklist /FI "PID eq 1234" names it. In PowerShell, Get-NetTCPConnection -LocalPort 3000 -State Listen gives the OwningProcess, and Stop-Process -Id 1234 stops it. A dev server running inside WSL lives in the distro: run the Linux commands there.

Should I use kill -9?

Try a plain kill <PID> first. It sends SIGTERM, which lets the server close its connections and clean up. kill -9 (SIGKILL) can’t be refused, but nothing runs on the way out, and a parent such as npm or pnpm killed that way leaves its child, the real server, still listening. If the port is still taken after a kill, run the lookup again: the PID has probably changed to the child’s.

Why does the port keep getting taken?

Usually by your own dev server, started earlier and forgotten:

• in a terminal tab in another window, or a tab you closed while the server ran in the background;
• by a coding agent, in a shell you can’t see (see Claude Code started a second dev server);
• as a child process that outlived the parent you stopped (see Kill leftover dev servers).

Killing it fixes today. The next time, you look up the PID again.

How does localhost keep ports from colliding?

localhost is a free, open-source desktop app that runs your dev servers from one window. Each Service is your own command (pnpm dev, uv run fastapi dev, cargo run) with a Port, the TCP port it is expected to listen on, passed to the command as $PORT.

• It checks before it starts. Before starting a Service with a Port, localhost looks at who holds it. If something does, it says so, by name and PID (“Port 3000 is in use by node (PID 1234).”), and offers Stop it and start, Start on 3001 or Cancel.
• Or it moves, for that Run only. If you allow it, the Service starts on the next free port and gets it as $PORT; the configured Port doesn’t change. A Service that others depend on never moves, and neither does one whose port is held by something in the same Project.
• The Ports view shows every listener. Ports (⌘0, or Ctrl+0 on Linux and Windows) lists every process listening on a TCP port and whose it is: one of your Services, a process running inside one of your Projects’ folders, or nothing localhost knows. Select one and press ⌘. (Ctrl+.) to stop it; localhost asks first for anything it didn’t start.
• Stop means stopped. Stopping a Service ends its whole process tree, so no child is left holding the port.

How it decides a Service is up, and every rule for moving: Ports.

FAQ

How do I free port 3000 in one command on a Mac?

kill $(lsof -ti tcp:3000 -sTCP:LISTEN) stops whatever listens on port 3000: -t prints only the PID. Leave out -sTCP:LISTEN and it also matches clients connected to the port, such as your browser.

Can I just start my dev server on another port?

Yes: most read PORT or take a flag, such as next dev -p 3001 or vite --port 5174. It works for one server, but URLs, proxies and CORS settings that name the old port break until you change them too.

Does localhost change my command to use another port?

No. localhost never rewrites command lines. The port reaches the command only as the PORT variable ($PORT, or %PORT% in cmd), so the command has to read it, for example next dev -p $PORT. A moved port lasts for that Run only; the configured Port stays.