Infinite loop

File:Infinite loop.gif
An **infinite loop** is a never-ending cycle of effects activating in response to each other, or a series of Continuous Effect(s) that never settle.

There are two distinct types of infinite loops: loops with no net change and loops with net change. Infinite loops contrast to controlled loops, which instead are maintained by a player intentionally repeating a set of actions or making the same decisions, rather than forced by game mechanics.

## 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 after each cycle, such that the game state is identical at the beginning and end of any cycle.

These types of infinite loops are illegal in gameplay, meaning 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 involuntary, such as a player conducting their normal draw or a mandatory effect, then the card ruled to be the source of the infinite loop is automatically destroyed by game mechanics.

### Pole Position

The mechanics of "Pole Position" naturally lend itself to causing such infinite loops when combined with Spell effects that directly or indirectly change a monster's ATK (see Card Rulings:Pole Position for further examples).

#### Voluntary

"Pole Position", "Luminous Spark", and "Gemini Elf" (1900 ATK) are face-up on the field.

If 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 its original ATK of 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 settling.

Therefore, it is an illegal move for either player to Summon "X-Head Cannon".

#### Involuntary

"Pole Position", "Opticlops" (1800 ATK), and "Muka Muka" are face-up on the field. "Muka Muka" is equipped with "Axe of Despair", and its controller has no cards in their hand (making "Muka Muka's" ATK 1600).

When "Muka Muka's" controller conducts their normal draw:

* "Muka Muka's" ATK becomes 1900.
* "Muka Muka" is now the monster on the field with the highest ATK, so "Pole Position" makes it unaffected by Spell Cards.
* "Muka Muka" is now unaffected by "Axe of Despair", reducing its ATK to 900.
* "Muka Muka" is now no longer the monster on the field with the highest ATK, so it is no longer unaffected by Spell Cards.
* "Axe of Despair" now increases the ATK of "Muka Muka" by 1000, to 1900.
* "Muka Muka" 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 "Muka Muka" never settling.

Since this loop was caused by a mandatory action, it cannot be prevented by restricting the player's actions. Since the loop cannot be avoided, "Pole Position" will be destroyed by game mechanics.

### Amplifier

The mechanics of "Amplifier" naturally lend itself to causing such infinite loops when combined with Trap effects on the field that change control of "Jinzo" or negate the effect of "Jinzo" or Trap Cards (see Card Rulings:Amplifier for the official examples).

#### Control

##### Voluntary

Player 1 controls a "Jinzo" which is owned by Player 2, but being controlled by Player 1 due to "Snatch Steal". "Jinzo" is equipped with "Amplifier".

If Player 1 attempts to activate "Imperial Order", the following would occur:

* "Imperial Order" negates the effect of "Snatch Steal" ("Amplifier" cannot be negated).
* Since "Snatch Steal" is now negated, control of "Jinzo" returns to its owner Player 2.
* Since Player 2 now controls "Jinzo" (which is equipped with "Amplifier"), it negates the effect of Player 1's "Imperial Order".
* Since "Imperial Order" is now negated, "Snatch Steal" is no longer negated, so Player 1 takes control of "Jinzo".
* Since Player 1 now controls "Jinzo" (which is equipped with "Amplifier"), it no longer negates the effect of Player 1's "Imperial Order".
* "Imperial Order" negates the effect of "Snatch Steal" ("Amplifier" cannot be negated).
* The above sequence would repeat infinitely, with the player who controls "Jinzo" never settling.

Therefore, it is an illegal move for Player 1 to activate "Imperial Order".

If Player 1 had already controlled "Imperial Order" before activating "Snatch Steal", then Player 1 activating "Snatch Steal" targeting "Jinzo" (equipped with "Amplifier") would be an illegal move.

##### Involuntary

Player 1 controls a "Jinzo" which is owned by Player 2, but being controlled by Player 1 due to "Snatch Steal". "Jinzo" is equipped with "Amplifier" and affected by "Effect Veiler". Player 1 also controls "The Emperor's Holiday".

At the end of the turn, the following would occur:

* "Effect Veiler" stops negating the effects of "Jinzo".
* "The Emperor's Holiday" negates the effect of "Snatch Steal" ("Amplifier" cannot be negated).
* Since "Snatch Steal" is now negated, control of "Jinzo" returns to its owner Player 2.
* Since Player 2 now controls "Jinzo" (which is equipped with "Amplifier"), it negates the effect of Player 1's "The Emperor's Holiday".
* Since "The Emperor's Holiday" is now negated, "Snatch Steal" is no longer negated, so Player 1 takes control of "Jinzo".
* Since Player 1 now controls "Jinzo" (which is equipped with "Amplifier"), it no longer negates the effect of Player 1's "The Emperor's Holiday".
* "The Emperor's Holiday" negates the effect of "Snatch Steal" ("Amplifier" cannot be negated).
* The above sequence would repeat infinitely, with the player who controls "Jinzo" never settling.

Since this loop was caused by a mandatory effect, it cannot be prevented by restricting the player's actions. Since the loop cannot be avoided, whichever card is identified as the cause will be destroyed by game mechanics (no official ruling has been issued as to which is the cause).

#### Negation

##### Voluntary

Player 1 controls "Skill Drain" and a "Jinzo" equipped with "Amplifier".

If Player 2 attempts to activate "Royal Decree", the following would occur:

* "Royal Decree" negates the effect of "Skill Drain".
* Since "Skill Drain" is no longer negating "Jinzo", "Jinzo" negates the effect of "Royal Decree".
* Since "Royal Decree" is no longer negating "Skill Drain", "Skill Drain" negates the effect of "Jinzo".
* Since "Jinzo" is no longer negating "Royal Decree", "Royal Decree" negates the effect of "Skill Drain".
* The above sequence would repeat infinitely, with which effect is active never settling.

Therefore, it is an illegal move for Player 1 to activate "Royal Decree".

If Player 2 had already controlled "Royal Decree" before Player 1 activated "Skill Drain", then Player 1 activating "Skill Drain" would be an illegal move.

##### Involuntary

Player 1 controls "Skill Drain". Player 2 controls a "Jinzo" which is owned by Player 1, but being controlled by Player 2 due to "Mind Control". "Jinzo" is equipped with "Amplifier". Player 2 also controls "Royal Decree".

During the End Phase, the following would occur:

* "Mind Control" stops applying, so control of "Jinzo" returns to Player 1.
* Since Player 1 now controls "Jinzo", "Amplifier" no longer prevents "Jinzo" from negating "Royal Decree", and prevents "Jinzo" from negating "Skill Drain".
* Since it is no longer being negated, "Skill Drain" negates the effect of "Jinzo".
* Since "Jinzo" is no longer negating "Royal Decree", "Royal Decree" negates the effect of "Skill Drain".
* Since "Skill Drain" is no longer negating "Jinzo", "Jinzo" negates the effect of "Royal Decree".
* Since "Royal Decree" is no longer negating "Skill Drain", "Skill Drain" negates the effect of "Jinzo".
* The above sequence would repeat infinitely, with which effect is active never settling.

Since this loop was caused by a mandatory effect, it cannot be prevented by restricting the player's actions. Since the loop cannot be avoided, whichever card is identified as the cause will be destroyed by game mechanics (no official ruling has been issued as to which is the cause).

### Cyber Blader

This loop only applies in the *TCG*. In the *OCG*, "Cyber Blader" can only negate activated effects.

#### Voluntary

Player 1 controls three monsters, one of which is owned by Player 2, but being controlled by Player 1 due to "Snatch Steal".

If 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 equipped monster returns to Player 2's control.
* Since Player 1 now controls two monsters, the effect of "Cyber Blader" instead doubles its ATK. The effect of "Snatch Steal" is no longer negated, so Player 1 gains control of the equipped monster.
* Since Player 1 now controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The equipped monster returns to Player 2's control.
* The above sequence would repeat infinitely, with the player who controls the monster equipped with "Snatch Steal" never settling.

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" (targeting a monster other than "Cyber Blader") would be an illegal move.

#### Involuntary

Player 1 controls two monsters, one of which is owned by Player 2, but being controlled by Player 1 due to "Snatch Steal". Player 1 also controls "Jam Breeding Machine". Player 2 controls "Cyber Blader".

During Player 1's Standby Phase, the following would occur:

* "Jam Breeding Machine's" mandatory effect Summons a "Slime Token" to Player 1's field.
* Since Player 1 controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The equipped monster returns to Player 2's control.
* Since Player 1 now controls two monsters, the effect of "Cyber Blader" instead doubles its ATK. The effect of "Snatch Steal" effect is no longer negated, so Player 1 gains control of the equipped monster.
* Since Player 1 now controls three monsters, the effect of "Cyber Blader" negates the effect of "Snatch Steal". The equipped monster returns to Player 2's control.
* The above sequence would repeat infinitely, with the player who controls the monster equipped with "Snatch Steal" never settling.

Since this loop was caused by a mandatory effect, it cannot be prevented by restricting the player's actions. Since the loop cannot be avoided, whichever card is identified as the cause will be destroyed by game mechanics (no official ruling has been issued, but it will likely be either "Snatch Steal" or "Cyber Blader").

## Loops with net change

Unlike loops with no net change, loops with net change cause a change to occur each time the cycle repeats. In these cases, the loop will continue until either the Duel ends or it ceases to have net change; if the latter, at this point the card ruled to have begun the loop will be automatically destroyed by game mechanics.

### All-Out Attacks

All-Out Attacks OTKs utilize a loop with net change to force the opponent to continuously thin their Deck (either by drawing cards or sending cards from the Deck to the Graveyard), eventually resulting in a Deck out.

#### End by the Duel ending

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.

If Player 1 activates "All-Out Attacks", the following will occur:

* Player 1's face-down monsters will be flipped face-up, 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 revealing two monsters. Since the only two monsters in Player 1's 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 again 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 infinite loop repeats until Player 2 has drawn close to their whole Deck, at which point the effect of "Hiro's Shadow Scout" will force Player 2 to draw when they do not have enough cards in their Deck. This will cause Player 2 to lose the Duel due to a Deck out.

#### End with no net change

Player 1 controls a face-down "All-Out Attacks" and a face-down "Morphing Jar #2" and "Needle Worm" that were Special Summoned. Player 1 has no monsters left in their Deck.

If 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 "Needle Worm" will make the opponent mill 5 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 revealing two monsters. Since the only two monsters in Player 1's 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 "Needle Worm" will again make the opponent mill 5 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 infinite loop repeats until Player 2 has milled their whole Deck, at which point the effect of "Needle Worm" ceases to cause the opponent to mill any cards. The loop now has no net change.

At this point, just as with infinite loops with no net change, the card that began the loop, "All-Out Attacks", would be destroyed by game mechanics. Once Player 1 ends their turn, Player 2 will be unable to draw a card during their Draw Phase and will lose the Duel.

### Red-Eyes Flare Metal Dragon

There is a very simple infinite loop involving "Red-Eyes Flare Metal Dragon" and "Beelze of the Diabolic Dragons" that ends with the end of the Duel.

If Player 1 controls "Red-Eyes Flare Metal Dragon" while Player 2 controls "Beelze of the Diabolic Dragons", and the effect of "Beelze of the Diabolic Dragons" is activated:

* Immediately after the effect of Player 2's "Beelze of the Diabolic Dragons" resolves, Player 1's "Red-Eyes Flare Metal Dragon" will inflict 500 damage to Player 2.
* Due to taking damage from an opponent's card effect, the mandatory effect of Player 2's "Beelze of the Diabolic Dragons" activates, and it gains ATK.
* Immediately after the effect of Player 2's "Beelze of the Diabolic Dragons" resolves, Player 1's "Red-Eyes Flare Metal Dragon" will inflict 500 damage to Player 2.
* This infinite loop repeats until Player 2 runs out of Life Points. This will cause Player 2 to lose the Duel.

## Special rulings to prevent infinite loops

Some cards have special rulings to prevent them from causing infinite loops.

### Destiny HERO - Departed

While "King Tiger Wanghu" is face-up on the field (and its effect is not negated), unless a card effect would make the ATK of "Destiny HERO - Departed" greater than 1400 when Special Summoned, its effect to Special Summon itself will not activate.

If this ruling were not in place, the following would happen during the Standby Phase:

* The mandatory effect of "Destiny HERO - Departed" activates, Special Summoning itself from the Graveyard to its owner's opponent's side of the field.
* The mandatory effect of "King Tiger Wanghu" activates, destroying "Destiny HERO - Departed".
* The mandatory effect of "Destiny HERO - Departed" activates, Special Summoning itself from the Graveyard to its owner's opponent's side of the field.
* The above sequence would repeat infinitely.

This ruling does not take net change into consideration, so "Destiny HERO - Departed" will not be Special Summoned even if "Cyber Summon Blaster" and "DNA Surgery" (declaring Machine) are active.

This ruling can also be extrapolated to cover "Destiny HERO - Departed" with a nonzero ATK of 2000 or less while the player controls "Slifer the Sky Dragon".

### Light and Darkness Dragon

The negation effect of "Light and Darkness Dragon" will only activate once while the effect of "Reverse Trap" is applying. In addition to preventing abuse, this also prevents infinite loops.

If this ruling were not in place, infinite loops would occur with many mandatory Trigger Effects. For example, the following would happen during the opponent's End Phase if that player controls "Judgment Dragon":

* The mandatory effect of "Judgment Dragon" activates to send cards from the Deck to the Graveyard.
* The mandatory effect of "Light and Darkness Dragon" is Chained, negating the effect of "Judgment Dragon" and increasing its own ATK and DEF by 500.
* The mandatory effect of "Judgment Dragon" activates to send cards from the Deck to the Graveyard.
* The mandatory effect of "Light and Darkness Dragon" is Chained, negating the effect of "Judgment Dragon" and increasing its own ATK and DEF by 500.
* The above sequence would repeat infinitely.

## In the anime

The anime shows two infinite loops with net change, both caused by Yami Yugi.

* In episode 67 of *Yu-Gi-Oh!*, Yami Yugi used the effects of Strings' "Slifer the Sky Dragon", "Revival Jam", and "Card of Safe Return" against him by attacking "Revival Jam", causing it to regenerate, then taking control of it by "Brain Control" so that while it regenerates, it does so on Yami's side of the field, forcing "Slifer" to attack and destroy "Revival Jam" over and over. This caused "Card of Safe Return" to make Strings continuously draw from his Deck until he got a Deck out and lost the Duel.
* In episode 182 of *Yu-Gi-Oh!*, Yami Yugi used the effects of "Legendary Knight Critias" (which had absorbed the effect of "Attack Guidance Barrier") and "Legendary Knight Hermos" to continuously redirect attacks between them while another effect of "Hermos" increased its ATK each time. By anime mechanics, this caused the ATK of "Hermos" to become ∞.

## In the manga

The manga shows an infinite loop with net change.

* In Duel 117 of *Yu-Gi-Oh! Duelist*, Dark Yugi used the effects of the Doll's "Slifer the Sky Dragon", "Revival Jam", and "Card of Safe Return" against him by attacking "Revival Jam", causing it to regenerate, then taking control of it by "Brain Control" so that while it regenerates, it does so on Yami's side of the field, forcing "Slifer" to attack and destroy "Revival Jam" over and over. This caused "Card of Safe Return" to make the Doll continuously draw from his Deck until he got a Deck out and lost the Duel.

## References

1. ↑ 1.0 1.1 UDE FAQ: Individual Card Rulings [P-R]
2. ↑ 2.0 2.1 UDE FAQ: Individual Card Rulings [A-C]
3. ↑ UDE FAQ: Individual Card Rulings [D-E]
4. ↑ UDE FAQ: Individual Card Rulings [L-O]