What’s running on port 4321 on my Mac?

Check what’s on port 4321

lsof -nP -iTCP:4321 -sTCP:LISTEN

It’s a node process. Two Astro projects look identical in that list, so check the working folder and whether it’s dev or preview:

ps -o pid,etime,command -p <PID>
lsof -a -p <PID> -d cwd -Fn | tail -1

With Stat Space: statspace port 4321.

Free it

Stop it with kill <PID>. Astro doesn’t fail when 4321 is taken: it moves to 4322, 4323 and so on. That keeps your new server running but can leave an old one behind for days. To check the whole range at once:

lsof -nP -iTCP:4321-4330 -sTCP:LISTEN

Change the port

From the command line:

astro dev --port 4400
astro preview --port 4400

Or once, in astro.config.mjs:

export default defineConfig({
  server: { port: 4400 },
});

server can also be a function that receives { command }, which lets you give dev and preview different ports so they never collide.

Who can reach it

By default Astro binds to localhost, so only your Mac can connect. --host or server: { host: true } listens on all addresses, and Astro prints a Network URL. That’s handy for testing on a phone, but it also serves your drafts, unpublished pages and any server endpoints to everyone on the network. See dev server exposed to the network.

Seeing it live

Stat Space shows 4321 as an Astro server with its project folder, the launcher that started it and its uptime. With several Astro sites running, each row names its own folder, and --host sessions get flagged.