NPC counterspell?

My instinct is that Countering is a thing that is announced before the player rolls their dice, but after they announce the casting. "I blast him with a magic missile!" *picks up dice* "Hold on, he's weaving some kind of counterspell to diffuse it, how do you overcome that?" And then let the player describe their struggle and trigger moves appropriately. Maybe it's a Defy before Casting, maybe it's understood that they Cast but on a 6- the consequence will be greater, maybe they take a -1 to the roll or something. A little discussion takes place beforehand and the standard is set.

One thing I like about DW (and AW) is that there seem like a lot of legit ways to handle a common trope like this.  I like that flexibility.  Also, there's a lot of delight in watching a skilled *W user do clever or novel things with the mechanics.

If I'm playing the wizard and I go to cast magic missile against some couterspelling sorcerer, I'm totally cool with that not triggering the cast a spell move and the GM narrating how the spell fizzles or is absorbed and then asking "what do you do?" I'm also cool with the spell triggering a custom move, say "when you try to overpower a counterspell, roll..." I'm also cool with going for the cast a spell move and then using the counterspell as color on a less than 10+ result. Any and all of those could be present in a cool game of DW.

The one thing I would not be cool with is triggering the Cast a Spell move, making my roll, and then being told my 10+ didn't really count. There's simply no good reason to trigger a move if you're not really triggering its results.