That makes sense. Thanks :)

Are there rules against taking this playbook other than via the custom move at the beginning? (For example, can you switch to it, or start as it?).

What is ?-harm.

Also, I think they do things knowingly to avoid Move, as I feel it : they poisonned calmly and with a great plan a NPC, and so there were no Seize by Force of Go Aggro. All I could do is say "ok, it works" because nothing in the rules seems to prevent it.

Take my advice with a grain of salt, because I haven't played the game, just read the book.

But the rules do have something to prevent that. Whenever the players look at you to say something, you make a move. Even if the game didn't have moves for players, the MC moves are sufficient to resolve conflicts.

You have a ton of options. Since you know what they are planning to do, you can frame to any point in the plan. (Don't forget to say what honesty demands though, if you think of a complication that they should be able to foresee then probably you want to "tell them possible consequences and ask"). There is no move that says "Ok, it works." (The closest is "offer an opportunity, with or without a cost") If you want to say that it works, then it should be part of another move. Like, if the poisoning was at a banquet maybe you say.. "At the banquet, he's tearing into his food when suddenly he starts choking. He grabs at his throat panicked, and then looks over at you. You know he knows it was you, what do you do?"

You don't have to address their plan directly, if you don't want to. You can "announce off-screen badness", which is entirely unrelated. Or if the plan starts later, you can have some other event occur in between. You can also have the guy they are planning to poison burst into the room. Or maybe they hear scratches at the door, like someone is leaning against it. If its the sort of plan that requires coordination between multiple PC's, you could frame to the middle and offer an opportunity with the cost of abandoning their friends.

As long as you are following the principles, and doing moves, you can decide something failed or was only partially effective. If that seems harsh, its might be a hard move, so only do it if they offered you an opportunity first.

Most of the your moves are bad for the players, in some way, so probably they will want to roll when they can. I'm not suggesting you should try to like.. bully them into rolling with the MC moves (Doing so would probably involve ignoring the rules about hard moves, and being a fan), just saying that even if you apply them fairly, its probably in their best interest to roll. But thats up to them. (Well mostly, obviously you can make moves that will require them to act under fire.)