[dmorgan81]

The Magic tournament rules cover this:

“Some loops are sustained by choices rather than actions. In these cases, the rules above may be applied, with the player making a different choice rather than ceasing to take an action. The game moves to the point where the player makes that choice. If the choice involves hidden information, a judge may be needed to determine whether any choice is available that will not continue the loop.”

Basically if a player has open information, like an activated ability, that could end the loop that player is not allowed to not use it to keep the loop going indefinitely. If that player instead has hidden information, i.e. a card in hand, that could end the loop any player can call a judge to confirm that and force the player to end the loop.

Note this doesn’t extend past cards in hand, though. If a player has some way to search their deck for a card that could end the loop, they are not forced to search and then play that card. At that level it moves from a player intentionally delaying the game with the resources at hand or in play to a judge dictating a player’s actions.

[greiskul]

One nitpick, even if you have have an action that stops the loop, you are not necessarily required to take it, and can choose the draw instead.

719.5. 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.

The rule about choices in mandatory action comes into play for cases like an Oblivion Ring loop. If there is another valid target for the oblivion ring, you must choose it instead of forcing a draw. You can't say that you always choose the opponents Oblivion ring.

[caf]

There was recently a ruling/clarification on exactly this kind of loop that has become relevant in competitive Pioneer format: https://mtgrocks.com/mtg-ruling-may-cause-problematic-turn-t...

[thom]

Yeah, I’ve come round to this because the base case is a zero mana activated ability that you never stop activating. That ought to give you a loss on time the moment your opponent calls a judge and you refuse to stop. More complex loops where you nevertheless choose to continue and can’t make a convincing case you’re progressing should be the same. The recent ruling just short circuits that discussion for one specific combo so I suppose I’m fine. For online play we have clocks and they should fix MTGO’s stack depth.

I’m still not convinced uncontrollable loops should be forcibly avoided. That seems unnatural and I’m not really against draws per se. I guess I’d just feel very angry in chess if I were forced to play a worse move and lose when a repetition leading to a draw was available.

[thom]

Don’t like that at all! I suppose some chess tournaments have “no draws before move X” but it’s a feeble rule that players easily overcome with repetitions etc. Forcing someone to take an action that they might deem worse for their chances seems wrong.

[dmorgan81]

It’s mostly for time purposes. Nobody wants to wait another hour for the round to end because someone is trying to draw their game, which means those players potentially have to play yet another game. In reality it doesn’t happen that often.

In your games with your friends feel free to ignore this rule. If you made a deck that managed to pull it off I would think it was cool.

[greiskul]

The rule is not precisely that, it is only if the mandatory actions of the loop offer you the choice, that you must eventually choose one that stops the loop. Classic infinite loop example is 3 oblivion rings exiling each other. If there is another valid target, you must choose it.