About battery draining, I did some specific tests and I can confirm that the issue is present and that it related to the zombies file transfers.
CristianoRiva
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Are you sure about the simulator?
My understanding is that the Simulator app doesn’t support the transferFile(_:metadata:) method at all.
I did not notice the battery draining issue. Did you try to apply the custom message workaround and to clean up zombies file transfers checking the progress?
@AngryS
about the workaround I am following the same approach suggested by @cfc and @JackAardal adding a specific custom message
https://stackoverflow.com/questions/78569322/missing-wcsessiondelegate-didfinish-callback-transferring-file-from-watchos-to-i