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Application failed to respond

Exact error

Application failed to respond

What it means

Replit's proxy/health check could not reach your app process on the expected port. The server is not listening where Deployments expect, or it crashed before binding.

Likely causes

• Server bound to localhost/127.0.0.1 instead of 0.0.0.0.
• Hard-coded port instead of process.env.PORT (or PORT from Secrets).
• .replit port config (localPort/externalPort) does not match what the app listens on.
• Process exited or threw during startup before listen().

How to fix it

1. Bind to 0.0.0.0 and the PORT environment variable:

   // Node / Express
   const port = process.env.PORT || 3000;
   app.listen(port, '0.0.0.0', () => console.log('listening on', port));
   
   # Python
   # port = int(os.environ.get("PORT", 3000))
   # app.run(host="0.0.0.0", port=port)

2. Open Publishing → Logs (or Console) for crash stack traces before the health check fails.

3. If `.replit` defines [[ports]], set localPort to your listen port and externalPort to 80 for the public URL; otherwise remove stale port entries so auto-detection can work.

4. Confirm Preview works with Run before republishing; missing production Secrets often crash the process only after Publish.

Prevent this next time

• Commit package-lock.json or requirements.txt so every environment installs the same dependencies.
• Read the port from `process.env.PORT` and bind to 0.0.0.0 in hosted environments.

Guide: catch outages before your users do.

Tools that help:

• Better Stack: Uptime, cron/heartbeat monitoring and log alerts.

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Related errors

• Error: listen EADDRINUSE: address already in use
• Error: Cannot find module 'express'
• FUNCTION_INVOCATION_FAILED (500)

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