How Casting Spells Should Really Work

Warning: extreme MTG nerdery ahead.

Starting with the Magic Origins update bulletin, I’ve observed the rules manager/team trying to work out the process of casting a spell, trying to allow what should be legal (using Bestow when you can’t cast creatures), disallow what shouldn’t be legal (casting Squee out of Ixalan’s Binding), and not twist themselves into a pretzel of incoherent nonsense in the process… which is what happened with the latest update to 601.3e.  Well, actually it started in the previous update with Mystic Forge rulings, but they doubled down on that mistake here and made it really bad.

Assuming (as always) no other relevant cards/effects, and speaking normatively throughout, if you have a Mystic Forge out and a Deathmist Raptor on top, you shouldn’t be able to cast it.  You can’t normally cast the top card of your library.  It’s not an artifact or colorless nonland card, so Mystic Forge shouldn’t let you cast it.  Done.  Full stop.  Nothing lets you cast that object, so you can’t take the game action of starting to cast it.  This shouldn’t even be a question.***

With the new update, Cascade on a 3 mana spell, which reads “… until you exile a nonland card whose converted mana cost is less than this spell’s converted mana cost” still (correctly) doesn’t recognize Beck//Call as a card with CMC<3 because it’s CMC 2+6=8…. while at the same time, Kari Zev’s Expertise, which says “You may cast a card with converted mana cost 2 or less from your hand…” somehow DOESN’T even recognize that it’s an 8 CMC card.  And Brazen Borrower’s Petty Theft can be cast from the graveyard using Wrenn and Six’s emblem. What in the actual fuck.  Eli- C’mon man.

In the explanation, Eli said “if you’re allowed to cast a spell with a certain mana cost or color…” and I don’t know if that’s just a typo or an actual misunderstanding, but Kari Zev’s Expertise says CARD, not SPELL, and it has to SAY card and MEAN card for anything to make any sense, and the failure to properly separate thoughts about cards and thoughts about spells seems to be at the root of all of the issues.


The process of casting a spell can be reduced to

  1. Casting an object
  2. as a spell with certain characteristics/targets/etc
  3. at a particular time

The tricky part is that the properties of the spell are not known at the start of casting, and the allowable times (and other things) depend on those properties.  Furthermore, since the gamestate changes between 1 and 2 when the object goes on the stack, what’s allowed and prohibited can change in the middle of casting (e.g. Squee and Ixalan’s Binding).  None of this is a problem, or even particularly complicated, but the algorithm to process it all has to be correct or it can spit out some fantastic levels of nonsense.


 

Let’s start with part 1, casting an object.  Their original attempt was that you can literally cast anything- a card from your hand, from your opponent’s library, whatever, and it would be dealt with later if it was illegal.  This was.. not a good idea, although it had the seed of one.  Since we don’t know what the spell is going to look like at this point, ignoring spell prohibitions here is correct, but something has to control what cards can be cast.  As it turns out, the correct answer to that is the same as the answer to why you can’t just put your opponent’s entire hand into his graveyard whenever you feel like it- you can only take legal game actions, and that isn’t one.  The path they went down instead, trying to determine what objects can be put on the stack based on what their spells might end up looking like, is effectively a category error.  The beginning of casting is about CARDS, not SPELLS.

There are *no other prohibitions* on what objects can be cast.  The set of legal objects should be defined *constructively*- cards in your hand by rule and whatever other objects effects allow you to cast (cards in the graveyard with Flashback, cards CMC<=2 in hand while resolving Kari Zev’s Expertise, copies from Isochron Scepter, etc).  Combined with the rules on timing and priority, which needs a small (obvious) rewrite to 117.1 to comply, we get the proper result

“A player with priority can cast a castable card” with the set of castable cards defined constructively as above.  There’s no way to cast an object at a random time because the only times casting something is an allowed game action are when you have priority or when something is telling you to during its resolution.  There’s no way to cast a random object because it’s never a legal game action.  This *does* allow you to start casting a sorcery during your opponent’s turn, or even start casting a land from hand, but this is actually fine and no different than starting to cast a spell you can’t choose targets for, which has been CompRules-legal the whole time.  The rest of 117 needs to be updated to “normally”, etc. to fit with this paradigm.

There are no cards that prohibit casting *cards*, so there are no prohibitions to worry about here (there are a couple that prohibit playing lands based on characteristics, but playing lands uses a special action and they don’t change characteristics in the middle of that action, so it’s not a problem).  All affirmative prohibitions (e.g. “Players can’t play creature spells”) affect *spells*, not *cards*, so there’s nothing else to worry about here.


 

As far as dealing with the spell aspect, once the object is on the stack, there’s no reason at all to intervene before the legality check in 601.2e as long as the previous steps can be followed (immediate rewind if you can’t choose legal targets, etc). No other objects move, so no more information can be leaked, and 601.3 is basically completely wrong/useless as it currently exists.  The only necessary checks in 601.2e are

  1. Land spells are illegal to cast (regardless of other types)
  2. If the spell was cast while the player had priority, the casting is illegal unless it is an instant, has flash, or was cast during the player’s main phase when the stack was empty.  (“as though it had flash” gets by this, obviously)
  3. If an effect prohibits the spell’s casting, considering the spell’s properties at this time, the casting is illegal.

That’s it… almost.  Squee is still escaping from Ixalan’s Binding.  A Grafdigger’s Cage variant that functioned while in the graveyard could (conceivably) be successfully cast from the graveyard.  The fix for that is trivial though.  The set of spell prohibitions is locked in before the object is put on the stack, so prohibitions that depend somehow on the to-be-cast object itself still apply, and then that set of prohibitions is what’s checked in 601.2e.  Squee is stuck.  So is my cage.

And that’s it for real.  To summarize:

  1. The player picks a castable CARD (object), either a card in hand or an object an effect allows to be cast, completely ignoring any properties the resulting spell may or may not have, any spell-casting prohibitions, and the possibility/impossibility of completing announcement successfully.
  2. The set of spell-casting prohibitions is locked in.
  3. The object is put on the stack
  4. Announcement proceeds up to 601.2e (if possible, rewind if not)
  5. Legality vs. the prohibitions in step 2 is checked using the spell’s current properties
  6. If legal, proceed to 601.2f. If illegal, rewind.

 


***If you’re wondering how to verify a morph was properly cast with Mystic Forge, sure, it’s ugly, but verifying ANY morph was ever properly cast in any way is ugly, but we survived multiple formats with morph and manifest legal and played at the same time, and this is nothing in comparison.

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