Azure Functions App returning 401 Unauthorized for no reason

Question

I have my azure functions app working fine in general. I enabled function level authorization and already providing x-functions-key with the correct key in the header.
So azure function is consumed by an Android app, in the appcenter logs, i am seeing some logs have

"StatusCode": 401,
"ReasonPhrase": "Unauthorized",

This makes no sense. I have fully logged request and request is just fine when i re-execute it using postman or similar. In the Azure portal under functions availability all green and in the connected Application Insights nothing is reported. Why am i getting this response?

EDIT:

Beside that error there are a few others in the Appcenter diagnostics. 503 and 404 response codes there are also. When i try to find those requests in the Azure portal I have no trace neither in Azure Functions monitoring nor in Application insights. What kind of issue is this?

Answer 1

Change the AuthorizationLevel to Anonymous.

public static async Task<IActionResult> Run(
                [HttpTrigger(**AuthorizationLevel.Anonymous**, "get", "post", Route = null)] HttpRequest req,
                ILogger log){

}

Answer 2

@Emil Alipiev 401 error shouldn't be observed until and unless the authentication is not passed correctly. As you have confirmed that you are passing the x-functions-key correctly and the same request works from the postman as per your observation. I would like to take a closer look offline at how you are calling the function app.

For 404 error there might be some calls where the request URL was not correct when called from a different application. 503 error we need to look into more details at the function app end as there could be different reasons for 503 (Service Unavailable) errors.