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Effectively debugging web apps takes effort, especially when an HTTP request goes through multiple layers before reaching your web app. Follow the steps below to debug a specific app. You can choose to debug in an environment deployed to Upsun or with your app running locally but connected to deployed services. In either case, make sure to debug in a preview environment. For more general information, see how to troubleshoot development.

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1. Create a new environment

Start by creating a new environment completely isolated from production but with the same data for debugging: 
upsun branch debug-branch

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2. Get access

Access your app container via SSH:
upsun ssh

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3. Run your app in inspect mode

Stop the current process and restart it in inspect mode:
sv stop app
node --inspect <START_FILE>
Replace <START_FILE> with the file defined for your app’s start command. You get output something like this: 
Debugger listening on ws://127.0.0.1:9229/10701e5d-d627-4180-a967-d47a924c93c0
For help, see: https://nodejs.org/en/docs/inspector
Listening on port 8888

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4. (If debugging remotely) Forward the debugger port locally

In another terminal, create an SSH tunnel that forwards to the 9229 port: 
ssh -N -L 9229:localhost:9229 $(upsun ssh --pipe)

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5. Connect the debugger

You can now connect the debugger as if you were debugging a local application. See examples with some common tools:
Go to chrome://inspect. Find your running app under the Remote Target list. Click inspect to start the debugger.
Now when you load the site at your deployed URL (if debugging remote) or localhost (if debugging locally), the local debugger you’ve attached is called. Set breakpoints:
In the JavaScript files from your remote site: On the Run and Debug tab under Loaded Scripts find Attach: Remote Process > /app.

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Other issues

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pm2 process manager blocks other processes

If you’re using the pm2 process manager to start your app from a script, you might find it daemonizes itself and blocks other processes (such as backups) by constantly respawning. This may happen even if you use the --no-daemon flag. Instead of using a script, call pm2 start directly in your start command.
Last modified on March 11, 2026