Infinite loop

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An infinite loop is a never-ending cycle of cards trying to resolve.

There are three distinct types of infinite loops: loops with No Net Change, loops with Net Change, and controlled loops.

Loops with No Net Change

These occur when a series of mandatory actions repeat after each other in a cycle, and there is no net change occurring each time it repeats.

The rules of the game state that a player may not voluntarily activate any card if its activation would cause such a loop.

If such a loop is caused by something uncontrollable, such as a player drawing a card during the Draw Phase or a mandatory effect, the card ruled to be the source of the infinite loop is automatically destroyed by game mechanics.

Example #1

  • "Pole Position", "Luminous Spark", and "Gemini Elf" are face-up on the field.
  • A player attempts to Summon "X-Head Cannon". The following would occur:
    • "X-Head Cannon" would be Summoned with 1800 ATK.
    • Since "X-Head Cannon" is a LIGHT monster, "Luminous Spark" increases its ATK by 500, to 2300.
    • "X-Head Cannon" is now the monster on the field with the highest ATK, so "Pole Position" makes it unaffected by Spell Cards.
    • "X-Head Cannon" is now unaffected by "Luminous Spark", reducing its ATK back to the original 1800.
    • "X-Head Cannon" is now no longer the monster on the field with the highest ATK, so it is no longer unaffected by Spell Cards.
    • "Luminous Spark" now increases the ATK of "X-Head Cannon" by 500, to 2300.
    • "X-Head Cannon" is now the monster on the field with the highest ATK, so "Pole Position" makes it unaffected by Spell Cards.
    • The above sequence would repeat infinitely, with the ATK of "X-Head Cannon" never being able to resolve.
  • Therefore, it is an illegal move for either player to Summon "X-Head Cannon".

The mechanics of "Pole Position" naturally lend itself to causing such infinite loops; see Card Rulings:Pole Position for further examples.

Example #2

  • Player 1 controls two monsters, and activates "Snatch Steal" to take control of one of Player 2's monsters. Player 1 now controls three monsters.
  • Player 2 attempts to Summon "Cyber Blader". The following would occur:
    • Since Player 1 controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The 'stolen' monster returns to Player 2's control.
    • Since Player 1 now controls two monsters, the effect of "Cyber Blader" instead doubles its ATK. "Snatch Steal"'s effect is no longer negated, so Player 1 gains control of the 'stolen' monster.
    • Since Player 1 now controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The 'stolen' monster returns to Player 2's control.
    • This sequence repeats infinitely, and the controller of the 'stolen' monster cannot be resolved.
  • Therefore, it is an illegal move for Player 2 to Summon "Cyber Blader".
  • If Player 2 had already controlled "Cyber Blader" before Player 1 activated "Snatch Steal", then Player 1 activating "Snatch Steal" would be an illegal move.

Example #3

  • Player 1 controls "Jam Breeding Machine" and one monster. Player 1 activates "Snatch Steal" to take control of one of Player 2's monsters. Player 1 now controls two monsters.
  • Player 2 controls "Cyber Blader".
  • During Player 1's Standby Phase, "Jam Breeding Machine"'s mandatory effect Summons a "Slime Token" to Player 1's field.
  • Now the situation is the same as in Example #2, causing an infinite loop. However, it was created by a mandatory uncontrollable effect rather than a voluntary action. In this case, the card that started the loop, "Jam Breeding Machine", would be automatically destroyed by game mechanics before it could Summon the Token.

Loops with Net Change

Unlike loops with no net change, these loops cause a change to occur each time the cycle repeats. In these cases, the loop will continue until either the Duel ends or such a change is no longer occuring, and then the card that began the loop will be automatically destroyed by game mechanics.

Example #1

  • Player 1 controls a face-down "All-Out Attacks" and a face-down "Morphing Jar #2" and "Hiro's Shadow Scout" that were Special Summoned. Player 1 has no monsters left in their Deck.
  • Player 1 activates "All-Out Attacks". The following will occur:
    • Player 1's face-down monsters will be flipped, causing both their effects to activate.
    • The effect of "Hiro's Shadow Scout" will make the opponent draw 3 cards.
    • The effect of "Morphing Jar #2" will return both flipped monsters back to the Deck, and Player 1 will then reveal cards from the top of his/her Deck until he/she reveals two monsters. Since the only two monsters in his/her Deck are the two that were just returned, they will be the ones revealed.
    • The two revealed monsters will be Special Summoned face-down to Player 1's field.
    • Player 1's "All-Out Attacks" will then again flip the two monsters face-up, causing both their effects to activate.
    • The effect of "Hiro's Shadow Scout" will make the opponent draw 3 cards.
    • "Morphing Jar #2" will continue to return the two monsters to the Deck and reset them face-down onto the field, where "All-Out Attacks" will flip them.
    • This uncontrollable loop repeats until Player 2 has drawn almost his/her whole Deck and only has two or fewer cards left.
    • At this point, the effect of "Hiro's Shadow Scout" will force Player 2 to draw a card(s) when they have none in their Deck, causing Player 2 to lose the Duel.
  • If "Hiro's Shadow Scout" were to be replaced with "Needle Worm", each iteration of the loop would make Player 2 mill 5 cards from their Deck, and the loop would continue until Player 2 had no cards left to mill. At this point, any net change stops occuring, and the card that began the loop ("All-Out Attacks") would be destroyed.

The above loop is the basis of the All-Out Attacks OTK. More examples and methods can be found on that page.

Infinite loops with net change are notably seen a number of times in the anime; perhaps the most famous one occurred in [67] of Yu-Gi-Oh! where Yami Yugi, facing off against a "Slifer the Sky Dragon" with over 10,000 ATK, was able to force Strings to draw infinitely from his Deck, causing him to Deck Out and lose.

Controlled loops

These are not truly 'infinite' loops, but rather a series of voluntary actions that can be repeated as many times as a player wants. Since there is no uncontrollable element to these loops and a player can choose to stop them at any time, these loops are legal to use.

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