How to Fix Port Binding Error in AI-Built Apps
Your application fails to start because the required port is already in use or inaccessible. AI-generated configs often hardcode ports without fallback handling.
Error messages you might see
Error: listen EADDRINUSE: address already in use :::3000Error: listen EACCES: permission denied 0.0.0.0:80Why this happens in AI-generated code
Port already in use
A previous instance of the app or another service is still occupying the port.
Hardcoded port without env variable
AI code hardcodes port 3000 or 8080 instead of reading from PORT environment variable.
Privileged port without permissions
Binding to ports below 1024 requires root privileges that the process doesn't have.
How to fix it
Use environment variable for port
Read the port from `process.env.PORT` with a fallback: `const port = process.env.PORT || 3000`. Most hosting platforms set this automatically.
Kill the existing process
Run `lsof -ti:3000 | xargs kill -9` to free the port, or use a tool like `kill-port` to automate this.
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Frequently asked questions
How do I find what's using a port?
Run `lsof -i :3000` on macOS/Linux or `netstat -ano | findstr :3000` on Windows to see the process.
Why does my app work locally but not in Docker?
Ensure you bind to `0.0.0.0` not `localhost` inside Docker, and map the port correctly with `-p 3000:3000`.
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