Infinite loop

The famous infinite loop that allowed Yugi 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. There are also controlled loops, which are similar to infinite loops but are instead created by a player intentionally repeating a set of actions or making the same decisions rather than 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

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 'stolen' 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 '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.
• 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 'stolen' 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 '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.
• 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

Infinite loops with net change have been seen a number of times in the anime:

• 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 takes control of it by "Brain Control" so that while it regenerates, it does it on Yami's side, 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. This is arguably the most famous infinite loop to be seen in the anime.
• 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 ∞.

Controlled loops

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

Some loops that require only a few cards (fewer than 4) to pull off have caused cards to become Forbidden or Limited on the current Forbidden/Limited lists, as otherwise their presence would be detrimental to gameplay.

Quillbolt Loop

Main article: Quillbolt Loop

This uses the effect "Quillbolt Hedgehog" to Special Summon itself by having a Tuner on the field along with a monster effect that Tributes a monster for damage such as "Cannon Soldier". "Quillbolt" can be Tributed again and again and continuously revived, as long as "Imperial Iron Wall" prevents "Quillbolt Hedgehog" from being banished by its own effect.

A far more consistent variation of this loop uses "Plaguespreader Zombie". Summon both "Quillbolt" and "Plaguespreader" to the field, and Xyz Summon "Ghostrick Socuteboss", then activate its effect. Then, Summon "Ghostrick Angel of Mischief" by using "Socuteboss as the Xyz Material", then detach an Xyz Material (not "Socuteboss)" to add a generic "Ghostrick" Spell/Trap to your hand. Place this Spell/Trap back on top of your Deck to revive "Plaguespreader", then revive "Quillbolt" with its effect. This returns to the start of the loop.

Flint Lock Loop

Main article: Flint Lock Loop

This uses "Flint" and two "Flint Locks" to pass the "Flint" back and forth. The Spell Card "Morale Boost" is used to gain 1000 Life Points every time an Equip Card is equipped. Because no Equip Card leaves the field, the effect to lose 1000 Life Point is never reached. This allows a player to gain an unlimited number of Life Points while the opponent may be powerless to stop it.

Gearfried Loop

If a player controls "Gearfried the Iron Knight", then it is possible to activate "Butterfly Dagger - Elma" an unlimited number of times per turn, by equipping it to "Gearfried", whose effect will destroy it, at which point "Elma's" effect will return it to the hand to be equipped again. This can easily be combined with "Royal Magical Library" to draw an unlimited number of cards in one turn (causing an easy Exodia FTK, etc.). Another option is to combine it with "Spell Absorption" to gain an arbitrarily high number of Life Points; adding "Fire Princess" gives an unlimited amount of damage to inflict in one turn on top of that. This loop is the reason why "Butterfly Dagger - Elma" is currently Forbidden on the Forbidden/Limited List.

Gate Material Loop

By combining "Fusion Gate" and "Chain Material", it is possible to Summon or activate nearly any card an unlimited number of times per turn, through the correct (and very risky) setup. These loops generally revolve around Fusion Summoning several Fusion Monsters by banishing Fusion Material Monsters from the Main Deck, followed by using said monsters to activate effects or Summon other monsters, then finally Fusion Summoning "Elemental HERO Electrum" to return the originally banished monsters back to the Deck to be banished again. For an example, see Elemental HERO Gustav Max OTK, an OTK that involves Summoning "Superdreadnought Rail Cannon Gustav Max" at least four times in one turn. For further and more detailed examples of possible setups, see this external link.

Primal Seed Loop

If a card is active that banishes Spell Cards that would go to the Graveyard (such as "Macro Cosmos" or "Banisher of the Light"), and there is a "Black Luster Soldier - Envoy of the Beginning" or "Chaos Emperor Dragon - Envoy of the End" on the field, then any player with two copies of "Primal Seed" and a Normal or Quick-Play Spell Card in their hand can activate that Spell Card an unlimited number of times per turn, by repeatedly using one copy of "Primal Seed" to bring back the other copy along with that Spell Card. This allows for an OTK with a card as weak as "Sparks", as well as numerous other possibilities such as arbitrarily high Life Points or ATK. This loop is why "Primal Seed" is currently Limited in the OCG; it was Limited in the TCG as well until 1 April 2014.

Wicked Worm Beast Loop

If Wicked Worm Beast is special summoned in the End Phase of the turn it is automatically returned to the hand. Therefore, if a player has 7 or more cards in their hand in the End Phase of the turn and Dark World Grimoire active, they can discard Wicked Worm Beast, Special Summon it with Grimoire, and return it to the hand again. Although this has limited value, if the opponent activated Maxx "C", they could deck out. Similar strategies can occur using DNA Surgery and Cyber Summon Blaster or Soul-Absorbing Bone Tower, the latter of which works with Zombie World. This loop is more easily achieved using Silent Wobbe with a card that takes control of it.

In the anime

• In episode 64 of Yu-Gi-Oh! GX, Syrus used the mandatory effect of Hassleberry's "Ultimate Tyranno" against him, forcing it to attack and destroy "Decoyroid" over and over while the combined effects of "Ambulanceroid" and "Rescueroid" continuously Summoned it back to the field. This allowed Syrus's "Cyber Summon Blaster" to continuously damage Hassleberry until he lost the Duel.
• In episode 98 of Yu-Gi-Oh! ZEXAL, Yuma Tsukumo and Astral fused into Dark ZEXAL against Vector, with the latter, who was blinded by rage, taking control. He equipped "Dark ZEXAL Weapon - Chimera Clad" to "Chaos Number 39: Utopia Ray V", granting it immunity by battle destruction and the ability to attack once again with doubled ATK whenever Dark ZEXAL took battle damage from attacks involving it. He then proceed to attack the higher-ATK "Number 104: Masquerade", taking the first damage and triggering the first ATK boost. However, Vector activated "Extra Hundred" in response to the second attack, so that "Masquerade" always had 100 more ATK than "Utopia Ray V". Astral didn't realize the danger, and continued to attack again and again, losing 100 LP each time, until he was down to 600. At that point, Yuma managed to cancel the fusion and promptly ended the turn, preventing the 500 damage of "Sargasso the Different Dimension Battlefield" from reducing their LP to 0. Had he not managed to, Astral would probably have continued the loop until he lost.
• In episode 99 of Yu-Gi-Oh! ARC-V, Declan Akaba activated the effect of "D/D/D Wisdom King Chaos Apocalypse" against Jean-Michel Roger, which lets him Special Summon it from his hand by destroying two face-up Spell/Trap Cards he controls, those cards being "D/D Savant Nikola" and "D/D Savant Thomas". Since "Nikola" was destroyed while it was in the Pendulum Zone, its effect let Declan return a "D/D/D" monster, "Chaos Apocalypse", on his field to his hand, and then place up to two face-up "D/D" Pendulum Monsters from his Extra Deck ("Nikola" and "Thomas") to his Pendulum Zone. Declan followed up with "Dark Contract with the Abyss Pendulum", inflicted damage equal to the DEF of a "D/D" Pendulum Monster that was returned to his hand. As the DEF of "Chaos Apocalypse" is 2000, this was enough to wipe out the rest of Jean's LP. Attempting to bypass his loss, Jean re-entered the Duel and took the 2000 LP intrusion penalty. Declan revealed that he would simply use the effect of "Chaos Apocalypse" to Special Summon it again, triggering the effect of "Nikola", and in turn triggering the effect of "Abyss Pendulum", which would deplete Jean's LP again. Realizing that Declan would repeat this loop each time he attempted to re-enter the Duel, Jean forfeited and fled.