I’ve noticed a different behavior in Flex with respect to how we detect cold start compared to Windows Consumption. When testing with Windows Consumption, I see the X-MS-COLDSTART header in the request, but in Flex, this header isn’t set.
I typically add a tag if the request is a cold start, but since the header is missing, request.IsColdStart() returns false. This also means we will miss this log as coldStartData will be null.

The header is windows only and is set by MinIYarp on the worker.

Proposal: Set the header during specialization.