What is the process for determining if an infinite loop will be a draw?

Question

Related to this question, and inspired by this question, I realized that I don't know what the game play will actually look like when an infinite loop arises.

Imagine that Lethal Vapors and Solemnity are on the battlefield. Alice casts a creature with undying or persist, and it resolves. This should cause an infinite loop of triggers, because the creature will be destroyed when it enters the battlefield, and it will return to the battlefield when it dies.

Now, either player can easily stop the infinite loop, because Lethal Vapors has an ability that will end the loop, which can be activated by either player. But let's say both players think that the loss of turn that comes from activating that ability will be enough to cause them to lose the game. So what specifically happens now?

I would imagine that in a casual setting, both players would simply discuss "ok, this is an infinite loop. I'm not doing anything to stop it; are you doing anything to stop it? No? Ok, draw then".

But in a more formal setting (either a tournament, or a casual game where both players really want to follow every CR to the letter), whose responsibility is it to take what action and when? Alice has priority, so she passes it. Bob then passes priority, and Lethal Vapors' trigger resolves. Then Alice has priority, so she passes it. Bob then passes priority, so the undying trigger resolves. Is one of them responsible for using their priority to suggest a draw?

Are the rules for shortcuts relevant here? Although it seems like a shortcut isn't really involved, the rules for dealing with infinite loops are under the rules for taking shortcuts (section 720).

The only relevant rules I can see are these 3:

720.1b Occasionally the game gets into a state in which a set of actions could be repeated indefinitely (thus creating a “loop”). In that case, the shortcut rules can be used to determine how many times those actions are repeated without having to actually perform them, and how the loop is broken.

720.4. If a loop contains only mandatory actions, the game is a draw. (See rules 104.4b and 104.4f.)

720.5. No player can be forced to perform an action that would end a loop other than actions called for by objects involved in the loop.

So it says "the game is a draw", but when specifically does the game become a draw? Does one player have an obligation to propose a shortcut here? I don't actually see a legal shortcut that would work to cause a draw, however, as shortcuts for loops generally involve naming a finite number of times that you want the action to happen.

Answer 1

What you are describing is a loop of mandatory actions. Unless someone breaks it, the game ends in a draw.

720.4. If a loop contains only mandatory actions, the game is a draw. (See rules 104.4b and 104.4f.)

That said, noone is forced to break it even if they could.

720.5. No player can be forced to perform an action that would end a loop other than actions called for by objects involved in the loop.

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You seem to be asking how to get the game declared a draw. 104.4i allows the players to mutually agree to a draw.

104.4i In a tournament, all players in the game may agree to an intentional draw. See rule 100.6.

If the other player refuses to accept, you can't just keep passing, because that would be Slow Play.

That leaves the option of creating a shortcut. The active player should propose passing until the loop is broken (or until they get priority in the next phase/step for a loop that doesn't span phases/steps). If the opponent accepts the shortcut, the game is officially a draw because neither player ever gets priority again.

Answer 2

The example you named does not lead to a draw because it contains non-mandatory actions. It is a fragmented loop that has to be broken by the players, if possible.

Every time Lethal Vapors triggers, its ability goes on the stack and waits to resolve. After it has resolved, every player gets priority and thus has a chance to either pass or take an action.

116.3b The active player receives priority after a spell or ability (other than a mana ability) resolves.

That means there are non-mandatory actions in the loop, and therefore the game is not a draw.

720.4. If a loop contains only mandatory actions, the game is a draw. (See rules 104.4b and 104.4f.)

The case you describe is a fragmented loop:

720.3. Sometimes a loop can be fragmented, meaning that each player involved in the loop performs an independent action that results in the same game state being reached multiple times. If that happens, the active player (or, if the active player is not involved in the loop, the first player in turn order who is involved) must then make a different game choice so the loop does not continue.

Example: In a two-player game, the active player controls a creature with the ability “{0}: [This creature] gains flying,” the nonactive player controls a permanent with the ability “{0}: Target creature loses flying,” and nothing in the game cares how many times an ability has been activated. Say the active player activates his creature’s ability, it resolves, then the nonactive player activates her permanent’s ability targeting that creature, and it resolves. This returns the game to a game state it was at before. The active player must make a different game choice (in other words, anything other than activating that creature’s ability again). The creature doesn’t have flying. Note that the nonactive player could have prevented the fragmented loop simply by not activating her permanent’s ability, in which case the creature would have had flying. The nonactive player always has the final choice and is therefore able to determine whether the creature has flying.

In this case, breaking the loop would require one player to take an action other than passing priority.

"Passing priority" is a game choice:

720.1. When playing a game, players typically make use of mutually understood shortcuts rather than explicitly identifying each game choice (either taking an action or passing priority) a player makes.

Therefore, in your example, passing priority would become an illegal choice, and ending the loop, if possible, becomes mandatory. Activating Lethal Vapor's ability is one possibility.