Azure App Service is a service used to create and deploy scalable, mission-critical web apps.
When an App Service runtime or OS reaches its retirement date, Azure doesn't switch your app off. Existing apps keep their configured version; it just disappears from the portal for new apps. What usually breaks an app afterward is a side effect of the retirement:
- On Linux, an image based on an EOL OS (for example Debian 11 Bullseye, which reached end of life on Aug 31, 2026) can no longer install packages at startup.
- On Windows, about 60 days after EOL the runtime is installed on demand ("JIT"), and that install can fail.
- Check which stack the app is actually running. Run
az webapp config show -g <rg> -n <app> --query "{linux:linuxFxVersion, net:netFrameworkVersion}". This tells you which retirement applies to you. ✔ To verify: the output shows the runtime/version, for examplePYTHON|3.11. - Read the startup log. Go to Diagnose and solve problems → "Web App Down", or check the Log stream / Kudu
LogFiles. On Linux, look forbullseyeandRelease file … is not valid yet/expired, orapt-get404s. On Windows, look for a JIT/runtime install error. ✔ To verify: you can quote the exact error line. - Move to a supported version. Update the runtime in Configuration → General settings (or run
az webapp config set --linux-fx-version "<STACK>|<supported version>"), redeploy, and test in a staging slot before swapping. ✔ To verify:az webapp list-runtimes --os linux(or--os windows) lists the version you picked, and the site returns 200.
Temporary workaround, not a permanent fix: if you're blocked on Linux by Bullseye package installs, point APT at archive.debian.org or bake the dependencies into a custom container. On Windows, if logs show "Not enough disk space" during a runtime install, scale the plan up one tier and back down to land on a healthy worker.
References:
- Stack Overflow: Changing the target framework in an Azure web app — https://stackoverflow.com/questions/56995412/changing-the-target-framework-in-an-azure-web-app (how to change
linuxFxVersionand use slots) - Stack Overflow: Issue with azure web app runtime stack .NET 6 — https://stackoverflow.com/questions/79620904/issue-with-azure-web-app-runtime-stack-net-6 (EOL versions disappear from list-runtimes)