# “Port 3000 is already in use”

**What you see:** Starting the app fails with `EADDRINUSE` or says port 3000 is in use.

**Why it happens:** Another copy of the app (or another app) is already running on that port.

## How to fix it

1. Look for another terminal running `npm run dev` and stop it (Ctrl + C).
2. Or ask your agent to find and stop the process using port 3000.

## Message for your agent

```text
My app says port 3000 is already in use. Find what's using it, tell me what it is, and stop it if it's an old copy of my app.
```

**How to prevent it:** Stop the app when you finish a session.

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Source: [Agentic Makers](https://agenticmakers.app/en/help/port-3000-in-use) · License: CC BY 4.0