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”.
You need to find ways to convey your intended tone of voice, that’s just the way it is. You could add a smiley. Even “Cool!” already looks a lot more enthusiastic than a dry “Cool.” with a full stop.