Infinite loop

The famous infinite loop that allowed Yugi Muto to defeat "Slifer the Sky Dragon"

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. Previously, Upper Deck Entertainment rulings ruled that the player could not play a card if it would voluntarily cause an infinite loop.

The North and Latin American TCG Policy 2.1 amended this policy, allowing a player to voluntarily activate a card or effect, or place a monster on the field in a way that would cause an infinite loop. When an infinite loop is caused, regardless of whether it was done voluntarily or involuntarily, it is first determined as to whether the loop would cause a victory condition; for example, if an infinite loop in which a Duelist draws 2 cards per iteration will eventually end in fulfilling a victory condition for one of the Duelists. If the infinite loop would not fulfill a victory condition, then each action in one iteration of the loop is performed a single time, and then the card ruled to be the source of the infinite loop is automatically sent to the Graveyard by game mechanics. If multiple cards are causing the loop, the most recently activated is sent to the Graveyard by game mechanics. Any effects that activate as a result of these card(s) being sent to the Graveyard proceed normally.

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 Summons "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.

Old Ruling: The player is not allowed to Summon "X-Head Cannon".

New Ruling: The player is allowed to Summon "X-Head Cannon". As "Pole Position" is the card that is the primary cause of the loop, it will be sent to the Graveyard by game mechanics after the above sequence has been performed once. "X-Head Cannon", the monster with the highest ATK after "Pole Position" has left the field, will then be destroyed by the effect of "Pole Position".

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.

Old Ruling: Since the infinite loop was caused by the unavoidable normal draw, "Pole Position" is sent to the Graveyard by game mechanics. Due to a special ruling, no monster is destroyed by the effect of "Pole Position".

New Ruling: As "Pole Position" is the card that is the primary cause of the loop, it will be sent to the Graveyard by game mechanics after the above sequence has been performed once. "Muka Muka", the monster with the highest ATK after "Pole Position" has left the field, will then be destroyed by the effect of "Pole Position".

Under the old UDE rulings for "Pole Position", only playing a card could be disallowed. If "Banner of Courage" is the card causing the ATK fluctuations, the turn player may still voluntarily enter their Battle Phase and send "Pole Position" to the Graveyard 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 activates "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.

Old Ruling: Player 1 is not allowed to activate "Imperial Order".

New Ruling: After Player 1 activates "Imperial Order" and causes the loop, whichever card is identified as the cause will be sent to the Graveyard by game mechanics (no official ruling has been issued as to which is the cause).

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.

Old and New Rulings: Since this loop was caused by the natural end of the turn, 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 sent to the Graveyard 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.

Old Ruling: Player 2 is not allowed to activate "Royal Decree".

New Ruling: After Player 2 activates "Royal Decree", whichever card is identified as the cause of the loop will be sent to the Graveyard by game mechanics.

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.

Old and New Rulings: Since the loop cannot be avoided, whichever card is identified as the cause will be sent to the Graveyard by game mechanics (no official ruling has been issued as to which is the cause).

Cyber Blader

Originally, the "3 monsters" effect of "Cyber Blader" could negate Continuous Effects, leading to the possibility of infinite loops. This was addressed in the OCG with the erratum issued in Dimension Box Limited Edition, but persisted for several more years in the TCG until the same erratum was issued in Speed Duel GX: Duel Academy Box.

Voluntary

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

Originally, if Player 2 Summoned "Cyber Blader", the following would have occurred:

• 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.

Old Ruling: Player 2 was not allowed to Summon "Cyber Blader".

New Ruling: Whichever card was identified as the cause would have been sent to the Graveyard by game mechanics.

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".

Under the original effect, during Player 1's Standby Phase, the following would have occurred:

• The mandatory effect of "Jam Breeding Machine" Special 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" 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.

Old and New Rulings: Since the loop could not be avoided, whichever card was identified as the cause would have been sent to the Graveyard by game mechanics (no official ruling was issued, but it would have likely been either "Snatch Steal" or "Cyber Blader").

In video games

Certain video games may implement their own rulings for infinite loops, or fail to recognize them leading to a crash or a softlock.

For example, Yu-Gi-Oh! Ultimate Masters: World Championship Tournament 2006 does the following:

• Both of the above "Cyber Blader" examples softlock the game. The monster equipped with "Snatch Steal" will display a never-ending animation of repeatedly changing sides, forcing the player to reset or turn off the game.
• In at least the two voluntary "Amplifier" examples, the game will automatically destroy "Imperial Order" or "Royal Decree" after several loops in order to end the loop.
• Interestingly, the infinite loop detection function appears to be built into (at least) the programming of "Royal Decree". For example, if a "Cyber Blader" loop occurs while "Royal Decree" is on the field, the game will destroy "Royal Decree" after several loops, even though it has no effect on the loop and the loop continues afterward as a softlock.

In Yu-Gi-Oh! Master Duel, if an infinite loop occurs (even if it has a net change), the game will generally stop it automatically somewhere around the 10th loop for those consisting of activations, or the 16th step if it is caused by never-ending Continuous Effects. In the latter case, one of the cards involved in the loop will also be destroyed.

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 sent to the Graveyard 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".

In the anime

The anime shows some infinite loops with net change used to end a duel.

• 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 episode 64 of GX, Syrus Truesdale uses "Decoyroid" to force Tyranno Hassleberry's "Ultimate Tyranno" to attack it. After that card is destroyed by battle, it's revived with the effects of "Ambulanceroid" and "Rescueroid". The net change in this loop is provided by "Cyber Summon Blaster" which inflicts 300 damage to Hassleberry every time "Decoyroid" is re-Summoned. The loop is maintained due to "Ultimate Tyranno's" effect that forces it to attack every monster once each, if able.

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.