Initial Troubleshooting Step for 400 Bad Request Error Suddenly to Azure App Service

To resolve this issue you 1st note down time and got to azure App Service diagnostic tools . Follow the below step

🔧 Accessing Logs via Kudu Debug Console

1. Go to App Service in the Azure Portal.
2. Click on Advanced Tools → Go → Debug Console (Kudu).
3. In the Debug Console, navigate to:
   Home > site > wwwroot > LogFiles (or Logs, depending on your app structure).
4. Locate the latest log file (e.g., eventlog.xml, application.log, or custom log).
5. Click to download or edit/view the log file directly.

Then search the 400 and go to last of the file