OOC:
TheGenerator says:
... player asks "Can I use ...
If the fiction they described shows them using a Move then the move is valid. If a few Moves fit --as they do here, or as Defy Danger always does-- then the player should be able to pick the one that suits their best Stats (or preferred outcomes... it is all about the outcomes, since they are in the fiction and that comes first), it is the natural thing for anyone to use their strengths out of habit.
They could always revert down to Defy Danger to get to use their preferred Stats, don't make them do that and take away the extra control the other Moves give. Also, change the Stats whenever you think another one would be better. Attacking with DEX is very common (but can be a problem, so beware).
If there is a Move that fits better it should be discussed. Often the GM will call for a Basic Move (that is what they have in front of them) and the players SHOULD speak up and point out that they have a more specialised Class Move that does it better, but which the GM did not know/remember they took.
I encourage all the players to call out when they think a Move applies. But this can get disruptive at some tables.
TheGenerator says:
... didn't really think about the mechanics ...
It was definitely a GM Move, and you only get to make GM Moves at the times the rules say. Though you can always justify it as a Golden Opportunity if you have to.
TheGenerator says:
... an action that the weapon was probably not designed for ...
Might be. But it is the PLAYER'S weapon. Would they not know better what it can take?
TheGenerator says:
... the attack failed + the weapon got damaged, I'd feel like that may be too hard ...
That attack does not need to 'fail', the rules don't actually say what happens on a 6-. But most would assume they don't hit (they don't get the listed options from the 7-9 or 10+), AND that the GM makes a Move. Those are the rules. Not hitting is not a GM Move, nothing never happens. Take Away their Stuff is a GM Move, and could make sense after failing to hit a foe.
TheGenerator says:
... fighter and "Without your weapon, you are useless ...
It is worse than that. They can always get another weapon. But this is their SIGNATURE WEAPON. It is part of the playbook and more than 'just another item'.
TheGenerator says:
... need repairs in the next town ...
Depending on the Fighter's other moves, or who else is in the party (Immolator?) they may be able to repair it sooner, and the damage may not even influence the mechanics of play while waiting for repair, but that 'next town' may be a whole adventure away. Very dangerous move.
You can always reverse your decision. Either completely OOC "Sorry, I did not think about how bad that would be" or in the story "You stare at your broken weapon and then at the fire, 'how hard can it be'? You stoke the fire and heat the weapon, borrow a hammer from your sleeping dwarf friend reshape the blade. You can take the Blacksmith Advanced Move for now, and choose to pay for it with XP when the time comes, or have it be a one-time thing."