Azure Function 4 broke suddenly with "Did not find functions with language [dotnet-isolated]" error

Question

I have been running an Azure Function on .NET7 (isolated) for a while, and all of a sudden, it stopped working for no apparent reason. I didn't deploy a new version, nor did I change the configuration. I can't connect to Log Stream to see what's wrong, but Resource Health / Function Executions and Errors tells me that the host initialization fails with HostInitializationException ("Did not find functions with language [dotnet-isolated]").

All the configuration is OK according to the Azure troubleshooter (and why wouldn't they be, the Function has been running for a good while), I checked LinuxFxVersion, tried setting FUNCTIONS_WORKER_PROCESS_COUNT from 5 to 10 to no avail.

Can anyone offer advice on what should I check to fix this problem?

Answer 1

I figured it out at the end. The problem was that the Function's client secret has expired silently in Microsoft Entra ID, and after renewing it and updating the Function configuration, things got back to normal.

The learning is that this error message can mean a lot of things, completely unrelated to what it says, and the golden rule with Azure Function troubleshooting is that you better double-triple check all the configuration/security settings before looking elsewhere.