Hi there!

A 502 "Application failed to respond" error usually means Railway's proxy can't reach your application. For NestJS, this typically happens when your app isn't listening on the right host and port combination.

Make sure your NestJS app is configured to listen on 0.0.0.0 and uses the PORT environment variable that Railway provides. In your main.ts, update your bootstrap function to grab the port from the environment and bind to the correct host:

const port = process.env.PORT || 3000;

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  // ... your other configuration
  await app.listen(port, "0.0.0.0");
}
bootstrap();

The key thing here is that the host needs to be 0.0.0.0 (not localhost or 127.0.0.1), and the port should come from the environment variable. Once you've made this change, redeploy and the error should clear up.

If you've already got this set up correctly, double-check your service's Settings → Networking section to make sure the target port isn't misconfigured, and take a look at your deploy logs to confirm the application is starting without errors.

I have updated the code as per your comments; however, I am still encountering a 502 error (“Application failed to respond”). Please review the attached screenshot for reference. - https://prnt.sc/lE0sydzv436O

Additionally, I have shared another screenshot showing a workspace error on Railway after the build deployment. Kindly check and let me know your thoughts. - https://prnt.sc/egxEqQMeyU-P

this isn’t a nestjs port issue anymore. your screenshots show a workspace/monorepo misconfiguration

railway is building the whole repo, but your backend is in a subfolder, so the server never actually starts → proxy returns 502

so follow this :

1. go to your backend service → settings

2. set root directory to the backend folder (the one with package.json + nest app)

   • OR explicitly set a start command, e.g.
     npm run start (from backend folder)
     or npm --workspace backend run start

3. redeploy

4. confirm logs show nest application successfully started

also double-check settings → networking → target port matches process.env.PORT.

when the app actually starts, the 502 will disappear.

I have already configured the backend as the root for the builder deployment. Please refer to the attached screenshot for confirmation.

I have also attached the package.json file from the root folder. Please review it here.

Here is my backend NestJS package.json file. Please review it.

Here is my frontend NextJS package.json file. Please review it.