when replying that way in any way, on text or actually just saying “cool”. my friends say cool a lot and I really don’t mind it and my partner says it too but some people have thought it was rude and I had a friend say “that’s what I say when I don’t give a damn”.
When “cool” is clearly an understatement about something negative or is in response to being stonewalled it is rude, otherwise it is chill.