How to design enemy AI players can read

1 The bottom line

Design enemies to create a specific experience, then show the player what they sense and intend. Readable enemies let players plan, and planning is where the interesting gameplay comes from.

2 Key takeaways

• Aim enemy AI at an experience, not at finding the player. The best solution is whatever gives the most interesting play.
  • Naughty Dog tried complex AI in Uncharted 4, then settled on spread out enemies that look smart and move in mildly predictable ways Mark Brown, Game Maker's Toolkit14:29.
• Build enemies from a few states and one simple rule each. Simple rules are easy to learn and still play differently every time.
  • Pac-Man's four ghosts each pick a different target, yet the game is "hard to read and predict, but not impossibly random" Mark Brown, Game Maker's Toolkit4:25.
• Show what enemies sense and what they plan to do. Hidden state turns into information the player can plan around.
  • Mark of the Ninja adds UI to Thief's stealth, and the game shifts from patient observation to building plans and setting traps Mark Brown, Game Maker's Toolkit1:13:56.
  • In Into the Breach you see each foe's next attack, so you can kill, dodge or push enemies into each other Mark Brown, Game Maker's Toolkit2:01:50.
• Make enemy actions predictable and their consequences open. Players can only plan around reactions they can count on.
  • Halo's Grunt always runs away, but you don't know where he will run Mark Brown, Game Maker's Toolkit6:12.
• Give players a way to recover and a reason to choose targets. Mistakes stay survivable and every enemy poses a different question.
  • Guards with a suspicious middle state let players recover from small mistakes Mark Brown, Game Maker's Toolkit4:10.

By showing the player exactly how the world works, the player feels powerful and capable.

Mark Brown, Game Maker's Toolkit

2.1 Start from the experience, not from smartness

Pick the feeling you want the player to have, then give the AI only what that feeling needs.

• Aim AI at interesting gameplay, not at finding the player Mark Brown, Game Maker's Toolkit14:29.
  • Uncharted 4 ended with enemies spread out in a layout, looking human and smart, moving so the player can sneak up behind them.
  • Complex systems are often simple underneath, and the craft is using basic systems to create magic Mark Brown, Game Maker's Toolkit10:31.
  • The Nemesis System in Middle-earth: Shadow of War and the AI of The Sims are both simpler than they appear Mark Brown, Game Maker's Toolkit10:31.
• Match the aggression to the game Mark Brown, Game Maker's Toolkit1:49.
  • Aggressive AI suits the Xenomorph in Alien: Isolation but would be out of place in Batman: Arkham Asylum, where no one gets the drop on Batman.
  • Doom 2016 began with enemies that chased the player, and players instantly started backing up. id had enemies hold their ground so the player pushes forward Mark Brown, Game Maker's Toolkit1:36.
• Drive a single big threat with systems, not scripts Mark Brown, Game Maker's Toolkit1:31:09.
  • Alien: Isolation's Xenomorph sees, hears and reacts to fire, with rarely predetermined movement. Designer Gary Napper says a scripted alien would repeat itself on restarts.
  • Spelunky's enemies are almost fully scripted, yet groups, varied environments and interactions between characters create the challenge Mark Brown, Game Maker's Toolkit6:24.
• Tune quietly for fairness and for the feeling of intelligence Mark Brown, Game Maker's Toolkit2:47.
  • Uncharted gives enemies a zero percent chance to hit Drake as he pops out of cover. Far Cry lets only a few enemies shoot at once.
  • Bungie ran identical Halo AI on two machines, one with tougher, harder hitting aliens. Players calling the enemies "very intelligent" jumped from 8 percent to 43 percent Mark Brown, Game Maker's Toolkit1:11.

2.2 Build enemies from a few states and simple rules

A small set of states and one targeting rule per enemy is enough to read, learn and still surprise.

• Model enemies as states with clear transitions Mark Brown, Game Maker's Toolkit1:45.
  • Breaking line of sight sends a guard into search. A countdown runs, and if it expires the guard resets to patrol.
  • Pac-Man's ghosts switch between chase, scatter and frightened modes, which is a finite state machine Mark Brown, Game Maker's Toolkit2:51.
  • Batman: Arkham Asylum guards are idle, suspicious, engaging or hunting Mark Brown, Game Maker's Toolkit2:51.
• Start in a neutral state and add a suspicious middle state Mark Brown, Game Maker's Toolkit4:10.
  • Tenchu, Metal Gear Solid and Thief open missions with unaware guards, which gives the player something clear to protect Mark Brown, Game Maker's Toolkit2:44.
  • Guards investigate odd sights and sounds before attacking, so a small mistake does not flip the whole system into alert mode.
• Send searching guards to the last known position, not to the player Mark Brown, Game Maker's Toolkit4:40.
  • When guards lose line of sight, they register a last known position and converge on it. That keeps the search from feeling like cheating.
  • Splinter Cell: Blacklist draws a ghostly white outline of Sam Fisher at that spot.
• Give each enemy its own simple targeting rule Mark Brown, Game Maker's Toolkit4:25.
  • Blinky targets Pac-Man, Pinky targets four tiles ahead, Inky uses a point projected from Blinky, and Clyde retreats once within eight tiles.
  • Blinky from behind and Pinky aiming ahead seem to spring a pincer, with no shared information Mark Brown, Game Maker's Toolkit5:13.
  • Distinct personalities, like Pac-Man's ghosts and Civilization's leaders, make AI seem smarter to players Mark Brown, Game Maker's Toolkit4:00.

2.3 Make every enemy ask a different question

Enemies that differ in threat, weakness and movement turn target choice into a real decision.

• Give each enemy a distinct behaviour and weakness Mark Brown, Game Maker's Toolkit4:14.
  • In the Nekravol fight, the slow Mancubus can be ignored while the Revenant's endless missiles make it the better target.
  • The Doom (2016) pinky has a front shield, so you wait for it to charge past and shoot its back Mark Brown, Game Maker's Toolkit3:38.
  • In DOOM Eternal the Baron makes you backpedal and the Revenant's lock-on missiles make you jump and dash Mark Brown, Game Maker's Toolkit7:39.
• Avoid enemies that are automatic priority one Mark Brown, Game Maker's Toolkit10:44.
  • The Buff Totem and Archvile keep spawning super-powered demons, so the player simply must kill them first.
  • The Pain Elemental and DOOM 2016's Summoner spawn weaker minions, so they can be ignored for a while.
• Ask for a method, or resist a move, instead of locking to one move Mark Brown, Game Maker's Toolkit12:35.
  • An enemy killable by one move collapses play into one strategy. God of War's big brutes can't be parried, so you use other moves.
  • Skyward Sword snakes need all three heads struck at once, and mechanical monsters need gust bellows to open their mouths Mark Brown, Game Maker's Toolkit11:54.
• Shape movement with how enemies attack and where they stand Mark Brown, Game Maker's Toolkit2:17.
  • Slow projectiles can be read and dodged, while hitscan pushes players to hide.
  • Rollerdrome's enemies mostly hold position, so players form a mental model of each one while moving fast Mark Brown, Game Maker's Toolkit4:16.
  • Dead Space 3's feeders react to light and noise, but with strong weapons and plenty of ammo players just shoot them all, so the mechanic goes unused Mark Brown, Game Maker's Toolkit3:44.

2.4 Show what enemies sense and intend

Make the hidden perception system visible through the world, the character and the interface, so the player can plan.

• Decide how much of the perception system to reveal Mark Brown, Game Maker's Toolkit1:59.
  • A fuzzy system keeps tension high because the player judges light, shadow, distance and sound. A revealed system gives confidence.
  • Mark of the Ninja shows vision cones and sound pulses, while Alien: Isolation hides what the Xenomorph senses.
• Let enemies announce their state through voice and body Mark Brown, Game Maker's Toolkit6:52.
  • Dishonored 2 uses a lightning bolt icon and barks like "I saw you, no use hiding!".
  • A guard lazing about reads as weakly perceptive, and one with a raised weapon reads as alert. This needs no added UI Mark Brown, Game Maker's Toolkit6:06.
  • A guard noticing an open door only seems smart if he says "Did I leave that door open?" Mark Brown, Game Maker's Toolkit3:21.
  • Thief's guards are extremely vocal and shout when they see something suspicious. Untitled Goose Game uses thought bubbles as a cheaper option Mark Brown, Game Maker's Toolkit3:53,.
• Add readouts for values players cannot judge well Mark Brown, Game Maker's Toolkit5:14.
  • Thief has a light gem for visibility, and Splinter Cell has a sound visualiser on Sam's HUD.
  • Tenchu's Ki Meter changes colour with guard awareness, and Metal Gear Solid's radar shows vision cones. Both remove guesswork about being spotted Mark Brown, Game Maker's Toolkit3:19.
  • The Swindle draws vision information on the level itself, so the player's eyes stay on the action instead of a radar box Mark Brown, Game Maker's Toolkit6:08.
• Draw the rules where the player can read them Mark Brown, Game Maker's Toolkit8:46.
  • Shadow Tactics splits the cone into a near zone of instant detection, a far zone safe only if crouched, and a dotted zone for bushes that is always safe.
  • Mark of the Ninja shows cones and previews sound pulses, so there is no arguing about what happened. Near instant detection is then fair Mark Brown, Game Maker's Toolkit8:14.
  • In 3D, Sly Cooper's torch-carrying guards cast obvious pools of light, which is cartoony but immediately readable Mark Brown, Game Maker's Toolkit9:58.

2.5 Make perception follow rules players can learn

Sight and sound should work the way players expect, so a spotting never feels like a surprise.

• Build sight from a cone plus extra zones Mark Brown, Game Maker's Toolkit1:44.
  • A single cone lets you stand unseen next to a guard. Splinter Cell: Blacklist adds a wider peripheral box and a small area behind the guard.
  • Raycast to several body points and require several to be visible, so partial cover works Mark Brown, Game Maker's Toolkit2:27.
  • Splinter Cell raycasts to eight bones in Sam Fisher's model Mark Brown, Game Maker's Toolkit2:27.
• Make sound travel along the pathfinding network and tie it to surfaces Mark Brown, Game Maker's Toolkit3:53.
  • Give each noise a distance tied to its volume and let it travel across the navigation mesh, so walls muffle it.
  • In Thief, soft leaves are quiet but running on hard tiled floor alerts guards Mark Brown, Game Maker's Toolkit3:02.
• Make guards perceive context so other mechanics gain power Mark Brown, Game Maker's Toolkit4:17.
  • In Hitman 2, enforcers are much less perceptive if Agent 47 faces away, which makes disguises stronger.
  • Guards that notice doors, objects and bodies, or pass on second-hand information, feel smart and give designers hooks for traps Mark Brown, Game Maker's Toolkit3:06.

2.6 Telegraph attacks and keep patterns learnable

Warn the player before the hit, keep the set of moves small, and keep the timing uncertain.

• Telegraph attacks and scale the warning to the threat Mark Brown, Game Maker's Toolkit3:41.
  • Telegraph length sets difficulty. Massive, hard to avoid attacks get big build-ups, and easy attacks need little.
  • Use distinct silhouettes and colours, and exaggerate anticipation frames Mark Brown, Game Maker's Toolkit12:00.
  • Cuphead's dragon gears up before a psychic attack or fireball, and its walking flames yell before they jump.
  • Bayonetta uses sound and glints on a weapon to warn of a strike Mark Brown, Game Maker's Toolkit12:00.
• Show intentions outright in a tactics game Mark Brown, Game Maker's Toolkit2:01:50.
  • In Into the Breach almost nothing is left to chance and almost everything is shown. The result is an elegant puzzle of turning enemies against themselves.
• Keep move sets finite and the timing uncertain Mark Brown, Game Maker's Toolkit2:28.
  • Cuphead's dragon fireball goes up, down or both, and the cats always split into exactly four mice, never three or five.
  • You know the cat will split but not when or where, so the fight tests reaction as well as memory Mark Brown, Game Maker's Toolkit3:08.
  • Far Cry 2 designer Clint Hocking calls it intentionality. If a Grunt fled only half the time, a plan would work only half the time Mark Brown, Game Maker's Toolkit6:12.
• Design attacks to force movement, not just deal damage Mark Brown, Game Maker's Toolkit1:33.
  • Attacks stop the player standing still and shooting forever.
  • Cuphead's bosses use straight, arcing, boomerang, splitting, zig zag, sine wave, aimed and slow homing projectiles.

2.7 Let players steer, recover and keep fights fresh

Readable enemies give the player levers to pull, a way back to calm, and new problems over time.

• Let players manipulate enemies and give them safe ground Mark Brown, Game Maker's Toolkit4:33.
  • Metal Gear Solid's wall knocking lures a guard one way, Tenchu lets you poison food, and Thief has noisemaker arrows.
  • A guard who runs to an alarm tower to call friends gives you a clutch chance to kill them first Mark Brown, Game Maker's Toolkit5:43.
  • Gargoyles in Batman, long grass in Assassin's Creed, and crates and cupboards in Hitman give places where you are unambiguously hidden Mark Brown, Game Maker's Toolkit6:34.
• Limit the alarm and let the player reset it Mark Brown, Game Maker's Toolkit5:07.
  • In Dishonored 2 only guards close enough to hear the shout join the hunt, so you can neutralise the threat and return to stealth.
  • Metal Gear Solid shows a countdown for how long to stay hidden. You recover without reloading or killing everyone Mark Brown, Game Maker's Toolkit5:03.
  • In The Phantom Pain, guards stop hunting quickly but the area stays on alert with a cautious patrol for many in-game hours Mark Brown, Game Maker's Toolkit6:23.
• Split long fights into phases and add new layers Mark Brown, Game Maker's Toolkit5:26.
  • Phases each bring patterns to learn, and a loss sends you back to phase one, which raises the stakes.
  • In Baroness Von Bon Bon, jelly bean soldiers and then a shotgun blast turn learned safe zones into the firing line Mark Brown, Game Maker's Toolkit7:19.
  • AM2R adds new patterns and smaller weakpoints to the repeated bosses of Metroid II Mark Brown, Game Maker's Toolkit2:12.
  • Majora's Mask's Wizzrobe mini boss shows up too many times and never changes tactics Mark Brown, Game Maker's Toolkit6:02.
• Change enemy behaviour when players pass thresholds, and pace pressure Mark Brown, Game Maker's Toolkit8:56.
  • Metal Gear Solid V enemies respond to headshots, unseen takeovers and night infiltrations with helmets, traps or night-vision goggles. The AI is not learning, it just waits for a threshold.
  • Pac-Man's ghosts alternate between chasing and wandering to the corners, because designer Toru Iwatani felt constant hunting would be too stressful Mark Brown, Game Maker's Toolkit9:42.

2.8 Where designers disagree

Readability is not always the goal, and a few good enemies deliberately break the rules above.

• Withholding information can beat showing it.
  • The Last of Us Part II's later faction uses whistles designed to be impossible to parse, which made them more intimidating and harder to fight Mark Brown, Game Maker's Toolkit20:40.
  • Early enemies in the same game call out their plans so you can react Mark Brown, Game Maker's Toolkit20:40.
• Unpredictable AI can fight the player's plans.
  • The Last Guardian's Trico was told to ignore the player's instructions about half the time to seem animal-like. That can frustrate when player and AI must work together Mark Brown, Game Maker's Toolkit12:34.
• Hunters and stackable pressure can work if they fit the game.
  • Dead Space's hunter regenerates and chases you through vents, so no space is safe. Killing it gives a real sense of catharsis Mark Brown, Game Maker's Toolkit10:04.
  • Hitman 3's Berlin targets are armed enforcers who hunt you, a cat and mouse game that keeps you on your toes Mark Brown, Game Maker's Toolkit4:55.
  • The same level's prototypes with paired targets, or a target who summoned all the others, were cut as too much. One slip put every target in one place and ended the mission Mark Brown, Game Maker's Toolkit7:01.