dave2008
Legend
I considered it interrupting your second of breath in between strikes, pushing yourself beyond normal limits to react to something. I suppose making it free wouldn't hurt much, though Spells cast as a reaction should still cost, I think.
I thought about that too, and I determined reaction speed of 1 for counterspell would work, as it is = to a cantrip (1+0) and faster than all other spells. Since, everything is declared up front I guess you would have to declare reactions up front as well.