How to find and stop dev servers Claude Code, Cursor and Codex left running

By techynAR · Updated 28 October 2026

Short answerAI coding agents like Claude Code, Cursor and Codex often start a dev server in the background and lose track of it, then start another one on the next free port. Run lsof -nP -iTCP -sTCP:LISTEN to list every server still running, check which project each one belongs to, and stop the leftovers with kill <PID>. Portmole shows the same list in your menu bar, by project name, with one click to close each server.

Why this keeps happening

When you ask an agent to “run the app” or “check it in the browser”, it usually starts npm run dev (or pnpm dev, uvicorn, rails s…) as a background process so it can keep working. That process outlives the step that started it. A few turns later the agent has forgotten it, runs the command again, finds port 3000 taken and quietly moves to 3001. Do that across a few sessions and projects and your Mac is running five copies of the same app.

• The new server lands on a different port, so the page you have open is an old copy and your changes seem to do nothing.
• Each leftover keeps using memory and CPU, and file watchers keep rebuilding in the background.
• Eventually a project refuses to start with EADDRINUSE: address already in use.

Step 1: list everything that is still running

lsof -nP -iTCP -sTCP:LISTEN

Every line is a process waiting for connections. Dev servers usually show up as node, bun, python, ruby or deno on ports like 3000, 3001, 5173, 8000 or 8080. To check a run of ports at once, give lsof a range:

lsof -nP -iTCP:3000-3010 -sTCP:LISTEN

Step 2: work out which project each one belongs to

Five lines that all say node are not much help. These two commands tell you what a PID actually is. The first shows the full command and how long it has been running, the second shows the folder it was started from:

ps -o pid,ppid,etime,command -p 12345

lsof -a -p 12345 -d cwd

A PPID of 1 means whatever started the server (a terminal, an editor or an agent session) has already exited and the server is an orphan. Those are almost always safe to stop.

Step 3: stop the leftovers

kill 12345

Plain kill sends SIGTERM so the server can shut down cleanly. Use kill -9 only if it is still listening a few seconds later. To stop everything on ports 3000 to 3010 in one go:

lsof -t -iTCP:3000-3010 -sTCP:LISTEN | xargs kill

Check the list from step 1 first. A database such as Postgres on 5432 or a Docker container should usually stay running.

Stop it happening again

Give your agent a rule in its instructions file (CLAUDE.md, AGENTS.md or .cursor/rules) so it reuses the server instead of starting a new one:

Before starting a dev server, check whether one is already running:
lsof -nP -iTCP:3000 -sTCP:LISTEN
If it is, reuse it. Never start a second server on another port.
When you are done with a server you started, stop it.

Then make the port strict, so a leftover shows up as an error straight away instead of a silent move to 3001. In Vite, run vite --strictPort or set server.strictPort: true. For other frameworks, pin the port explicitly (for example next dev -p 3000) so you always know where the app should be.

See it all without the terminal

Portmole is a free Mac menu bar app made for exactly this. It lists every server by the page's title (“Acme Dashboard”, not “node”), shows the command that started it and its project folder, and closes it with one click. Servers your agents start show up as they appear, and the little mole hops each time one does.