What is going on
You start your app and it fails with Error: listen EADDRINUSE: address already in use :::3000 (or another port number, such as 5173 or 8080).
Something is already using that port. After an AI coding tool has run your project, that is often a copy of your own app that it started and never stopped.
Error messages you might see
Common causes
- An earlier run of your app, started by Codex or by you, is still running in the background.
- A terminal window or editor tab still has the dev server open.
- A different program on your computer uses the same port number.
How to fix it
- Find what uses the port. On Mac or Linux:
lsof -i :3000. On Windows:netstat -ano | findstr :3000. Replace 3000 with your port. - Look at the program name. If it is your own app (often shown as
node), stop it withkill <PID>on Mac or Linux, ortaskkill /PID <PID> /Fon Windows. - If it is another program you want to keep, start your app on a different port instead, for example
PORT=3001 npm run dev. - Close old terminal windows that still run a dev server.
- Start the app again.
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Dor Yaloz
react · supabase · python · react
SW engineer with 6+ years of experience,
I worked with React/Node/Python
did projects with React+Capacitor.js for ios
Supabase expert
Jared Hasson
firebase · google cloud · java · golang · flutter · react · sql · shopify · js · ts · vue · angular · ci/cd · github actions · jekyll · mongodb
Full time lead founding dev at a cyber security saas startup, with 10 yoe and a bachelor's in CS. Building & debugging software products is what I've spent my time on for forever
zipking
AI Strategy · Vertical AI SaaS · Full-stack Development · Technical Leadership · Product Architecture · LLM/SLM Implementation · Japanese Market Localization
I am a technologist and product builder dedicated to creating high-impact solutions at the intersection of AI and specialized markets. Currently, I am focused on PropScan (EstateGuard), an AI-driven SaaS platform tailored for the Japanese real estate industry, and exploring the potential of Archify.
As an INFJ-T, I approach development with a "systems-thinking" mindset—balancing technical precision with a deep understanding of user needs. I particularly enjoy the challenge of architecting Vertical AI SaaS and optimizing Small Language Models (SLMs) to solve specific, real-world business problems. Whether I'm in a CTO-level leadership role or hands-on with the code, I thrive on building tools that turn complex data into actionable value.
Questions
Is it safe to stop that process?
Check its name first. If it is node and you know it is your own app, it is safe. If you do not recognise it, use another port instead.
Why does it keep happening?
Each time something starts your app without stopping the old copy, another one holds the port. Stop the old one before starting a new one.
