Artificial intelligence
Artificial intelligence, abbreviated as AI, in the Super Smash Bros. series refers to the intelligence of any computer-controlled (CPU in Brawl and later, CP in 64 and Melee) 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.
Difficulty levels
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 3 for Smash 64, Brawl, and SSB4, and 1 for Melee. 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, while the level 9 almost always will. Lower level CPUs are also unlikely to shield or dodge an attack; in Melee, they roll only for shifting directions, and in Brawl, they randomly and occasionally use their shield as their almost only form of defending. On the other hand, higher level ones almost always defend from attack, generally having good reflexes or dodging any attack when not in lag - this is most pronounced in Brawl, where Level 9 CPUs can dodge almost any attack with one-frame reaction, resulting in disproportionate perfect shielding capabilities. Higher level ones also vary more between attacks: low-leveled ones just move around the foe and randomly input an attack, usually a ground attack or special attack, while high-leveled ones are by far more aggressive and rely more on stronger attacks such as aerials or grabs. Their 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 Smash 64, while high-level ones in Brawl are capable of properly combining or alternating between different recovery techniques.
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 Smash 64 and Melee, these are set to a pre-determinated level (levels 3 and 1 respectively), while in Brawl, 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 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 SSB64 and Melee, it is somewhat unfavorable to fight CPUs on Training mode. Players instead tend to train against them on normal Versus modes.
Certain 1-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 behaviour they would normally have.
The amiibo figures to be released with Super Smash Bros. for Wii U 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 labelled with "FP", and can gain experience to level up after battle, capping at Level 50. It has been stated that battling amiibo against each other will yield more experience than battling a human or regular CPU. Additionally, the amiibo will be able change tactics after repeated battles (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 throws).
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 train 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 Kirby player persistently used Stone on a CPU, it would be hit by the attack most of the time, without defending against or punishing it, and do no more than occasionally dodge it, even at level 9. However, a skilled or even a remotely competent human player would easily avoid the attack by rolling, sidestepping, shield-grabbing or just walking away, and then punish Kirby with a grab or strong attack, such as a charged smash. Another example is CPUs not fighting off edge-guarders in any of the Smash games, which allows players to KO them with basic edgeguarding techniques, while experienced players would prevent this at all costs.
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. Another camp maintains what level is best is dependent on what a player is trying to practice.
