I think we're going to stick with multiclass level being determined from the level where you first gained a move from that class.

Thank you for considering my position on this. If you do decide to keep this mechanic, please consider putting something in the move text that appears on the character sheets, so the importance of taking casting multiclasses as early as possible is obvious, and not hidden in a rulebook that is otherwise rarely necessary to look at for character creation and advancement.  As it is, the only reference to levels could suggest that the move acts as though you are one lower level, which is not the case for spellcasting.

Er, that's not what was said. Never was said that the GM should fiat. What was said was that if you spend years in game practising at magic, the GM should make Dungeon World feel real and give you the benefit of what you've done. Just because you haven't spent an advance on it you can't car spells  regardless of the fact that you've been studying for years? That seems odd to me.

That is literally DM fiat. The most clear and text book example I could think of. Anything that happens in a game because the DM simply decided it should (or to permit it), and not on game rules, is DM fiat.

Now, DM fiat is not a bad thing, or something that shouldn't be done at the table.  Quite the contrary.  Judicious use of it is an essential part of DMing.  And Dungeon World's greatest strength is it's clear, and well defined understanding of where DM fiat should be applied, and where the players are given mechanical agency through Moves.

My point was rather that DM fiat, as cited in an argument by the poster I was responding to, does not in any way demonstrate that any mechanic is "good" or even "not bad".  If a group decides to ignore a game mechanic, and come up with something else instead, how can that possibly demonstrate that the game mechanic is good?  Any and all mechanics in any RPG ever can be ignored if the group wants to.

Just because you don't like the mechanic doesn't mean it is bad, nor a "trap".

What's your point?  That it's just my opinion that the mechanic is bad, and a trap?  Well, yes, it is.  Obviously.  All any of us could possibly say on the topic is an opinion.  Saying that it's just my opinion is meaningless.  Attack my arguments if you think I'm wrong.

I won't speak for others here, but I really don't think you've given any compelling arguments that the mechanic is bad, just that the mechanic doesn't do something you want it to, and are unhappy that the response from others isn't to agree with you, but rather to say "if you don't like it, don't run it that way."

No, I'm dissatisfied that the response from others isn't to actually disagree with my arguments, but raise points that are entirely tangential to them.  The existence of DM fiat is irrelevant to what I'm saying.

So, please, tell my why my argument that the mechanic is a trap for new players and roleplayers is unsound.  Tell my why it's necessary for this one mechanic to determine the fiction, when the whole game is based on fiction determining fiction, not mechanic.

But telling me "if you don't like it, don't run it that way" is meaningless.  Of course I don't have to run it that way.  That is always true, of any mechanic.  It doesn't need to be stated, and in no way undermines any arguments I or anyone else could ever make about game design.

Understand that I'm not posting out of concern as a player.  I am that optimizing, system mastering, powergamer that won't fall into the trap of not taking multiclass dabbling in casting at level 2.  As a player, the mechanic's flaws don't apply to me.  As a DM, I don't care that much either.  To me, the flaw is manifest, and so is the solution.  The rest of the game is good enough that patching this flaw won't put me off of it.

Rather, I'm posting on a discussion forum, for an in-beta game, suggesting how I think it's design could be improved.  In that context, telling me "if you don't like it, don't run it that way" is a non-sequitor.

Anyway, the difference between the multiclass moves and gaining the moves via descriptive, fiction first, positioning is exactly that: one way you get to prescriptively say, "I've learned how to cast spells," and the other way you say, "I'm going to practice magic junk every day with the wizard," and through a lot of plot and description and probably more than a few moves you eventually learn how to do magic. Maybe you get the Cast A Spell move with associated stuff, maybe you get a custom move related to hedge magic. Depends on the fiction. It's not demonstrably folk ignoring the multiclass rules, it's folk approaching the same goal differently. That's not a bad thing.

It's folk approaching the same goal differently, by ignoring the multiclass rules and using fiat instead.  Now, again, at the table, that's OK.  If people want to ignore mechanics and do something else, more power to them.  'tis the wonderful nature of RPGs.

But the point of mechanics is to give players clear elements where they can do things without having to ask for permission from the DM.  This gives the players a greater sense of agency in the world, and greater sense of accomplishment when they accomplish their characters goals not simply because the DM said they could, but because they said the could.

So, as game design, it still means that new players and roleplayers are being punished for their lack of system mastery.  It means that I, the optimizer, have greater player agency because I can use the system mechanics to build the character I want, in an effective way.  And of course, I have just as much access to DM fiat as anyone else.

Whereas the new players and roleplayers that didn't take Dabbler at level 2 need to either accept a permanently gimped character (if they actually use the game mechanics), or appeal to DM fiat to have the character they want.  They have fewer/less-good options, as players, than I do, for no good reason I've seen.  I don't think that's fair.

(edit: removed reference to Oberoni crap. After reading more posts on it than I care to ever again, it appears nobody knows what it actually means and it's just a catchphrase to not actually mention any formal fallacy.  Will now never mention Oberoni again, and if you want to discuss formal fallacies, actually discuss formal fallacies. Otherwise I will assume you are using a bunk appeal to authority, which while not a formal fallacy still leads to a weaker argument when your sources stuck are uncited)

I said exactly what I meant, so what anyone else thinks Oberoni fallacy means is irrelevant.  If you think I was ambiguous, or incorrect in what I said, then say so.

I did not cite Oberoni as an appeal to authority, or to imply it is a formal fallacy.  I would have thought "in D&D circles" would have made that manifestly obvious.  I cited it only as, well, a citation.  I was appropriating someone else's thoughts (the poster named Oberoni), and to not cite him would be plagiarism.  This being an internet forum, the readers of which having ready access to search engines, I did not think a formal "source" would be necessary.

There isn't a right or wrong way to do this, just different things to say about the world we're creating.

Maybe not, but there are good and bad mechanics. And the mechanic you've settled on is a bad one that punishes, severely, lack of system mastery, or building characters organically through roleplay. And it's particularly glaring in a system that, otherwise, is so well designed to avoid those sorts of traps.

If your level is one lower than your current level then you can never cast a spell, get to 10th level, and suddenly be a 9th level cast. That's fine, but it doesn't quite fit the world—why did the wizard spend 9 levels working on spellcasting to be that awesome, where you spent 9 levels stabbing things and suddenly become a master caster?

But you're not a master caster.  A master caster is, as you point out, a Wizard with 10 levels of casting, and 9 Wizard advanced moves. The Fighter who takes Dabbler at level 10 "suddenly" gains more casting potency than a fighter at level 2, sure, but he's not just 9 levels a better Fighter; he's also 9 levels a more awesome character.

Furthermore, why must the Fighter spend 9 levels only stabbing things?  Perhaps the FIghter was also studying spellbooks all this time, and it only just "clicked" at level 10?  It's never feasible for mechanics to enforce that all possible roleplayings a player might dream up using them will make sense.  After all, I could roleplay a Wizard that does nothing but stab things with a dagger all day, and never looks at his spellbook, yet somehow gains more spell levels, and can "suddenly" cast 9th level spells at high level when he finally looks at his spellbook again. All we can do provide mechanics that can be roleplayed coherently.

This is nothing new to your game; in fact, this principle is the very core of the game. The mechanics do not tell us what is and is not possible in the fiction. The fiction does that. The mechanics provide support, and give us a fair way to resolve tasks and conflicts that do not have an obvious resolution.

So, can a 10th level Fighter suddenly become a 9th level caster?  Unless it's mechanically broken for that to happen (and it isn't), the mechanics should be silent on it.  Let the fiction determine whether it can happen. Let the DM challenge the player to justify it. Maybe the Fighter's had his father's spellbook on him the whole time.  Maybe the evil wizard he just slew transferred a fragment of his dark soul into his. Or maybe the DM and player can simply compromise and say that those caster levels will need to be gained gradually over the course of some passage of in-universe time. Perhaps add that as a piece of advice for the DM.

This is ultimately the same as the Fighter attacking a giant iron golem with a dagger.  Is it encoded into the mechanics of Hack and Slash that a dagger is useless against a 20' magically-animated golem made of 6in thick iron?  No. The fundamental mechanics of the game cite common sense to tell us that. Unless, of course, the Fighter's player can justify it to the DM.