Artificial intelligence, abbreviated as AI, in the Super Smash Bros. series refers to the intelligence of any computer-controlled (CP in SSB and Melee, CPU from Brawl onward) character (called computer players in-game) in the single-player and Versus Modes. In Versus Mode, players can preset the AI of a computer player with a level setting. In Adventure Mode, All-Star Mode, and Classic Mode, players can still set it to some degree with the modes' difficulty setting.
Artificial intelligence levels range from 1 to 9, with 1 being the weakest and 9 being the strongest. The default CPU level in Versus matches is 1 for Melee and 3 in all other games, although rule settings in Ultimate allow players to select which level CPUs default to. In general, the level of an AI opponent determines how likely they are to follow through with a decision, as well as how fast they react, which results in the illusion of more skill - both a level 1 and a level 9 AI will decide to do something such as input an attack, but the level 1 will almost never do so, waiting for a long time before eventually doing it, while the level 9 almost always will instantly. Likewise, lower level CPUs are unlikely to shield or dodge an attack, using rolls simply to reposition themselves in some instances, and almost never using their shield at all, or at random times in Brawl. On the other hand, higher-level ones almost always defend from attack, generally having good reflexes or dodging any attack when not in lag - in all games, level 9 CPUs are able to perfectly land attacks against opponents when at the appropriate range from them, and in Brawl, Smash 4, and Ultimate, they can defend against almost any attack with one-frame reactions, resulting in disproportionate perfect shielding and dodging capabilities. Also due to this change in reflexes, low-level CPUs are prone to simply standing next to their opponent and using a weak attack such as a neutral attack or tilt, while high-level CPUs use aerial attacks, smash attacks and grabs more prominently.
The AI's recovery also improves as their level is increased: CPUs at low levels tend to recover in a simple and predictable pattern with their up specials, or may not use them at all in SSB, while high-level ones in Brawl, Smash 4, and Ultimate are capable of properly combining or alternating between different recovery techniques. For example, CPU Luigi in Smash 4 only uses his Super Jump Punch to recover at low levels (specifically Level 1 to 5), while at high levels (6-9) he also makes use of his Green Missile and Luigi Cyclone.
In Training mode, besides attacking, CPUs also have many other "modalities" that can be chosen, these being: Stand, Evade, Walk and Jump. When these are chosen, CPUs will act that way, allowing players to battle in different ways against them, and practice different types of techniques. In SSB and Melee, these are set to a predetermined level (levels 3 and 1 respectively), while in Brawl, Smash 4, and Ultimate, they can be chosen from 1 to 9 from the character selection screen as usual, which also improves their reflexes. For example, level 1 CPUs on Stand will just take on whatever attack is thrown on their way, and recover in a simple and gimpable pattern, while level 9 ones will almost always air dodge after tumbling (in Brawl) and will choose better recovery options. When set to Attack, CPUs will fight the player as on usual battles, but since the level cannot be selected in SSB and Melee, it is somewhat unfavorable to fight CPUs on Training mode. Players instead tend to train against them on normal Versus modes. CPUs set to Attack mode in Smash 4's Training mode are also much likelier to spam projectiles and other special moves than in other modes, using them even at close range from opponents.
Certain single-player stages and events in Melee give CPUs a level of 0, such as Event 19: Peach's Peril and Event 21: Ice Breaker, in which case they don't attack at all, but still follow whatever movement behavior they would normally have. Some also give them other specific behaviors, such as Event 3: Bomb-fest in Melee causing CPU Link and Samus to use their down special moves more often, and Event 31: The Ultimate Bodyguard in Brawl causing CPU Zelda to crouch and do nothing until the stage changes a second time. In Melee, these specific behaviors can be accessed via the Debug Menu.
In SSB, CPU levels appear on the character selection screen as a digit with two arrows on either side to adjust it up or down, appearing where the selected character's name would be displayed for a human. In Melee, the levels are instead adjusted with a red slider which can be selected with the A button and then moved with the control stick, which increases in number from left to right, above the selected CPU character's name. In Brawl, Smash 4, and Ultimate, only the level number to which the CPU is currently set is shown at first, now appearing where the option to choose a name would appear for human players; clicking the number displays a menu with the other levels, scrolled by moving the cursor up and down. In Smash 4 and Ultimate, the numbers appear with colors changing from indigo to red as the level increases, while in Brawl, all numbers appear in black but with an adjective to accompany each CPU level and describe its skill, namely Puny, Wimpy, Weak, Normal, Hardy, Strong, Burly, Mighty and Nasty in ascending order.
The amiibo figures released for Super Smash Bros. 4 and Super Smash Bros. Ultimate have stored data for a CPU fighter within them. The player may customize said character and train them by having them battle. The amiibo is labeled with "FP", and can gain experience to level up after the battle, capping at level 50. Battling amiibo against each other will yield more experience than battling a human or regular CPU. Additionally, amiibo will "learn" from repeated battles by changing its tactics in response to its opponents' behavior (e.g. if the amiibo battles an opponent that shields frequently, it will learn to use its own shield more effectively and counter shielding opponents with grabs).
Training against CPUs
Players commonly fight against CPUs to train when there aren't other players around to play. This is a practice that is criticized by some, as besides even the highest level CPUs being of a lower level of skill than a typical casual player, CPUs do things that human players never do (and will never do things that are common for human players to do), as well as having several exploitable flaws and being completely incapable of mind gaming, or even learning from their mistakes and changing their tactics accordingly.
Training against CPUs is said to not adequately build one's skills beyond basic offense and defense (such as the ability to read and bait opponents), and it is often argued that excessive CPU training may cause a player to actually become slightly worse by developing habits that only work against CPUs and will be heavily exploited by human players. For example, if a Marth player uses Counter every time a CPU gets close to attack, it would fall for it most of the time (especially higher-level CPUs), without trying to bait the counter or use grabs more often. However, a skilled or even a remotely competent human player would quickly catch onto the Marth player's strategy and exploit it by feigning an attack in the cases where Marth would commonly use Counter, and then punish him with a grab or delayed powerful attack such as a charged smash. Another example is CPUs not avoiding or fighting off edge-guarders effectively in the first three Smash games, which allows players to KO them with basic edgeguarding techniques, while experienced players would prevent this at all costs. Also in Smash 4 and Ultimate, higher-level CPU's (mostly levels 7-9) also have next to frame-perfect aerial dodging, only doing so when a player or CPU inputs the attack near them, or when items such as projectiles end up close enough to them. As a result, high-level CPUs in the air can only effectively be hit by rapid-hitting moves or through true combos. This can make practicing aerial combos infuriating, especially with characters who rely heavily on their aerial games, such as Marth, Peach, and Jigglypuff; this is further accentuated by their constant hold of the jump command while in the air (even when out of jumps), as they will, more often than not, phantom footstool the player, leaving the CPU in a favorable situation away from the player.
However, training against CPUs does have its merits. CPUs can be used to effectively practice combos and chain throws, as well as spacing, punishing and other tech skill related things. CPUs can be more effective than human players in this regard, as they're always available and won't refuse a match where a player just wants to practice the aforementioned things. Many top professional smashers have reported using CPUs to train, such as Nairo, Vinnie, and Mew2King.
There is, however, no consensus as to which level CPU is best to train against, with players mostly having their own personal opinion on what level is most effective. Some say level 1 CPUs are the best, as they won't have inhuman perfect shielding abilities and won't disrupt the player's training. Some say level 9 CPUs are the best, as they will actively try to fight the player and can punish mistakes the player makes. Some others say level 5 CPUs are the best for being a middle ground between the two extremes. Other players say Level 6-8 CPUs are the best for being challenging but not having inhuman reactions at the same time. Another camp maintains that what level is best is dependent on what a player is trying to practice.
Flaws in the AI
Main article: Flaws in artificial intelligence