Just a note in case anyone is worried I’m adding a mage to every encounter, I very rarely use counterspell against my players; it’s one of the spells I consider to have high “fun-ruining” potential.
I’m struggling a bit to decide on how to handle this interaction in a way that feels fair. From my understanding RAW, a character doesn’t know what spell is being cast. I think you can use your reaction to make an arcana check to discern it, but of course then you can’t counterspell it. For enemy spellcasters I generally describe what’s being cast, instead of naming the spell right away, but it can slow combat down, and is a bit one-sided since when a player casts a spell they lead with “I cast X”. This leads to an imbalance where I’m aware of what’s needed to counterspell something while the players are not, and can cause some awkwardness trying to decide how to play around that without metagaming.
I can think of a few different ways to handle this, each with its own drawbacks, but I’m curious to hear what y’all do at your tables!
The Pathfinder 2 way.
PF2 has the general Counteract mechanic:
(critical success and failures being when you roll ten or more higher or lower than the DC)
You expend a spell slot containing the same spell and roll to counter the cast spell, with varying possible degrees of success.
You can later specialize it in various interesting ways with feats, for example one that lets you spend a prepared spell from merely the same school, or with a spell that is especially thematically appropriate, or redirect the countered spell, or just eat it.
It’s a whole mechanic that you can build around, it’s exciting when it happens, you feel like you’ve earned it. It’s a reward for clever play as opposed to a button that you push.