The Homekit controller component supports hvac modes of Heat, Cool, and Heat_Cool, but the use of Heat/Cool mode is severely limited since there is no support for specifying heat/cool thresholds.
Per a discussion with @jc2k here his intent is to create a mock thermostat to validate how an actual iOS client handles the various min/max ranges returned by Homekit to see if this capability can be added.
This feature request is simply to track the effort and to provide any assistance in testing.
The corresponding GitHub ticket for this is #25218.
I’ve now made the mock and confirmed what the iOS client does. It uses the target temperature characteristic in heat and cool mode and it uses the 2 thresholds in heat_cool mode. This is a bit challenging on the HA side - sometimes it supports temperature ranges and sometimes it doesn’t, which creates scope for glitchy mode transitions…
Thanks for the update! I was also following #25218 so knew that you were making some progress with understanding the iOS client behavior. Thanks so much for all your effort in improving this component!