How do I find out why my AppSail service shows 'Execution failed. Please check the startup command or port.'?

This error means Catalyst AppSail found no process listening on your app’s port, so the instance never started serving. The reason is usually in your service’s application logs: open your project in the Catalyst console, go to DevOps → Logs, set Log Type to Application and select your AppSail service. You can also open Serverless → AppSail → your service → Instances → History and click an instance’s logs icon.

Then check these, in order:

1. Your code listens on the port in the X_ZOHO_CATALYST_LISTEN_PORT environment variable, on all interfaces (0.0.0.0), not on a hard-coded port or localhost.
2. The startup command in Configuration is the command that starts your server locally. AppSail runs it without a shell, so a command that uses the variable must be wrapped, for example sh -c 'python3 -m http.server ${X_ZOHO_CATALYST_LISTEN_PORT}'.
3. Your app starts listening within 10 seconds. Open the port first and load large files or models afterwards.
4. Every dependency and environment variable your app needs at startup is in the deployment.

Refer to the AppSail Configurations help page for details.