Final Fantasy X auto-abilities

Auto-Abilities are special abilities that appear in Final Fantasy X. Unlike Command Abilities, Auto-Abilities automatically take effect and can be assigned to weapons and armor, similar to Support Abilities in previous games.

All pieces of equipment can have a maximum of one to four Auto-Abilities. Most equipment have Auto-Abilities already assigned to them when acquired, however, once the player reaches Guadosalam, they are able to customize Auto Abilities onto equipment as long as they have the required number of items and as long as the equipment has at least one empty slot. Once an Auto Ability is added to a piece of equipment, it cannot be removed.

Brotherhood, Celestial Weapons, and non-empty-slotted equipment cannot be customized.

On occasion, customizing equipment may change the name and appearance of that equipment. For example, adding the Firestrike Auto-Ability to the Variable Steel will change its name to Flametongue.

Below is the list of Auto-Abilities that can be customized to weapons and armor.

Aeon Auto-Abilities
Aeons also have "equipment", though it is not generally visible in-game. Their weapons initially contain an empty slot, followed by Piercing, followed by -, and finally another empty slot. Bahamut, Anima and the Magus Sisters have Break Damage Limit instead of the first empty slot, and Valefor does not have Piercing. Once the respective Celestial Weapons are upgraded with at least the crests, the first empty slot changes to Break Damage Limit. Their armors do not change; they always contain Sensor, a blank slot, Break HP Limit and Break MP Limit; the elemental aeons have their respective element Eater instead of the blank slot.

Dummied Auto-Ability
One other oddity is that an ability exists in the code (though it is unobtainable without a cheat device or save editor) called Curse Ward.

Mechanics
Some equipment abilities do not work exactly the way that would be expected. Any ability that increases a stat by a percentage, in the event of multiples of these equipped at once, the percentages are added together before applying the effect. For example, equipping HP+30% and HP+20% would result in a 50% boost to HP, not a 56% boost as may be expected.

Additionally, apart from HP and MP, these abilities do not affect the stat itself, but rather, they affect damage. For example, Strength+20% does not increase a unit's Strength stat by 20%, but rather boosts any physical damage dealt by 20%. This difference is important for two reasons; because a 20% increase to the stat does not necessarily correspond to a 20% increase in damage as the formula is not linear, and secondly because "attacks which are determined by Strength" and "attacks which deal physical damage" (and likewise for Magic, and for defensive stats), while often going hand in hand, are two different things. For example, Fenrir's attack Fangs of Chaos is considered physical, but is not affected by Strength or Defense; in this case, Defense+20% would still reduce the damage. All Overdrives are considered special damage rather than physical or magical, and as such, Strength+20% has no impact on their damage.

Status abilities work in a similar way. Status-touch abilities increase the chance of infliction by 50%, while Status-strike abilities increase it by 100%. If both are together, which can be achieved by customizing the -touch first and then the -strike, the weapon will have a 150% chance of inflicting the status. This can make a difference against enemies with partial resistance to the status. This further combines with the chance that an attack carries; for example, if one has Darktouch and Darkstrike equipped and uses Dark Attack, the result is a 250% chance of inflicting Darkness.

Status resist abilities provide a 50% resistance, which is subtracted (not multiplied) from the chance. For example, an attack with a 80% chance of inflicting Silence, used on a character with Silence Resist, would have a 30% chance of working (not 40%). Status proof abilities do not give a 100% resistance, but rather "perfect" resistance—no matter how high the infliction chance is, status proofs (and Ribbon) will always protect the unit, unless the attack is one of the few that bypass protection. For the most part, immunity-bypassing statuses cannot be prevented, but there is one exception: protection-bypassing Slow will still be prevented by Auto-Haste (or active SOS Haste).

There are attacks that always inflict a status unless the target is perfectly immune. Aside from some enemy attacks, the status Buster skills, and all status-inflicting items, except Farplane Wind, carry this property.

Abilities that modify AP or gil gain only work if the user is on the frontline at the end of the battle.

Trivia

 * It is possible in two cases to obtain an equipment ability on the abnormal type of equipment. When first arriving in the Calm Lands, it is possible to purchase an armor for Tidus that has Magic Counter, an ability usually only found on weapons. The optional boss Geosgaeno in Baaj Temple can sometimes drop a weapon with No Encounters, an ability usually only found on armors. In both cases, these abilities function exactly as they normally would. Indeed, through a cheat device or a save game editor, it is possible to put any ability on any type of equipment, regardless of what type it normally needs to be on; and all such abilities function as normal.

Auto-Ability (FFX)