Answer by mikyll98 for How do I return JSON only from Azure Functions (dotnet5)

I think this might help someone, it's related to Felipe Augusto answer.

If you use WriteAsJsonAsync() you must not add the header Content-Type manually:

response.Headers.Add("Content-Type", "application/json; charset=utf-8");

That's because WriteAsJsonAsync() already does that, as stated in the documentation:

Write the specified value as JSON to the response body.
The response content-type will be set to application/json; charset=utf-8.

If you try adding the header, you'll get a k8s.Autorest.HttpOperationException:

Error: Cannot add value because header 'Content-Type' does not support multiple values.