game-ai

SKILL.md

Game AI: decisions, steering, and pathfinding

Build believable NPC behavior from three separable layers: decide (what to
do), steer (how to move there), and path (how to route around the map).
Keep them decoupled — a behavior tree picks a target, the pathfinder produces
waypoints, steering follows them. This skill teaches the engine-neutral
algorithms; bind them to your engine via the related skills below.

When to use

Use when implementing enemy/NPC logic: patrols, chase/flee, guard states,
group movement, or "find a path to the player".

Use to choose between an FSM (few clear states), a behavior tree (many
reactive behaviors with priorities), or steering (smooth local movement).

Use when integrating pathfinding: A* on a grid/graph, or driving an engine
navmesh agent.

When not to use: for the engine's concrete navmesh/agent API and baking,
use unity-navmesh, unreal-behavior-trees, or Godot's NavigationAgent2D/3D
(see that engine skill). For movement/collision feel, use physics-tuning. For
spawning waves along lanes, see the tower-defense genre skill.

Core workflow

Pick the decision model by complexity. 2–5 states with obvious
transitions → FSM. Many behaviors, priorities, interruption, reuse → behavior
tree. Continuous "how strongly do I want each option" → utility scoring.

Separate decision from motion. The decision layer outputs an intent
(target position, action). Steering or pathfinding turns intent into motion.

Path on the right graph. Grid tiles, waypoint graph, or a baked navmesh.
Fewer nodes = faster A*. Prefer the engine's navmesh for 3D; A* on a grid for
tile games.

Steer along the path, not straight to the goal — follow the next waypoint,
advancing when close, so agents round corners.

Recompute paths sparingly. Pathfind on a timer or when the goal moves a
tile, not every frame. Cache the path; only the waypoint index advances.

Verify by observation. Watch the agent: does it reach the goal, get stuck
on corners, oscillate between states? Draw the path and current state on
screen while tuning.

Patterns

1. Finite state machine (one state object, explicit transitions)

# Each state is a small object with enter/update/exit. The machine owns "current".
class_name State
func enter(agent): pass
func update(agent, dt) -> State: return null   # return a new state to transition
func exit(agent): pass

# --- Chase state: returns Patrol when the player escapes sight range ---
class Chase extends State:
    func update(agent, dt) -> State:
        if not agent.can_see(agent.target):
            return Patrol.new()                 # transition by returning next state
        agent.move_toward(agent.target.position, dt)
        return null                             # null = stay in this state

# --- Driver: call once per frame ---
func tick(dt):
    var next = current.update(self, dt)
    if next != null:
        current.exit(self); next.enter(self); current = next

Keep transition logic inside states (or in a table), never as a growing pile
of if flags. One state owns one behavior; that is what keeps an FSM readable.

2. Behavior tree tick (composite nodes return a status)

# A node's tick() returns SUCCESS, FAILURE, or RUNNING (still working this frame).
enum Status { SUCCESS, FAILURE, RUNNING }

# Sequence: run children in order; stop at the first non-SUCCESS (logical AND).
func sequence_tick(children, agent, dt) -> int:
    for child in children:
        var s = child.tick(agent, dt)
        if s != Status.SUCCESS:
            return s                 # FAILURE or RUNNING short-circuits the sequence
    return Status.SUCCESS

# Selector: try children until one succeeds or is RUNNING (logical OR / fallback).
func selector_tick(children, agent, dt) -> int:
    for child in children:
        var s = child.tick(agent, dt)
        if s != Status.FAILURE:
            return s                 # SUCCESS or RUNNING stops the search
    return Status.FAILURE

A guard AI reads top-down: Selector[ Sequence[CanSeePlayer?, Chase], Patrol ]
— chase if visible, otherwise patrol. See references/behavior-trees.md for
leaf nodes, decorators (Inverter, Cooldown), and a blackboard.

3. Steering: seek and arrive (smooth, frame-rate independent)

# Seek: accelerate toward a target at full speed. Steering = desired - current.
func seek(pos, vel, target, max_speed, max_force) -> Vector2:
    var desired = (target - pos).normalized() * max_speed
    return (desired - vel).limit_length(max_force)   # a force, not a teleport

# Arrive: like seek, but ramp speed down inside slow_radius so it stops cleanly.
func arrive(pos, vel, target, max_speed, max_force, slow_radius) -> Vector2:
    var offset = target - pos
    var dist = offset.length()
    if dist < 0.001: return -vel                      # already there: kill drift
    var ramped = max_speed * min(dist / slow_radius, 1.0)
    var desired = offset / dist * ramped
    return (desired - vel).limit_length(max_force)

# Per frame: vel += steering * dt; pos += vel * dt   (always scale by dt)

4. A* heuristic must not overestimate (or paths stop being shortest)

# Match the heuristic to the movement. An ADMISSIBLE heuristic (never larger
# than the true remaining cost) keeps A* optimal.
def heuristic(a, b):
    dx, dy = abs(a.x - b.x), abs(a.y - b.y)
    # return dx + dy             # Manhattan: 4-direction grids (no diagonals)
    return (dx + dy) + (1.414 - 2) * min(dx, dy)   # octile: 8-direction grids
# f(n) = g(n) + h(n): g = cost from start, h = heuristic to goal.
# Overestimating h is faster but no longer guarantees the shortest path.

The full A* loop (priority queue, came_from reconstruction, grid + waypoint
graphs) is in references/pathfinding.md.

Pitfalls

Pathfinding every frame tanks the frame rate. Recompute on a timer or only
when the target moves to a new tile; follow the cached waypoints in between.

Steering straight to the goal instead of to the next waypoint makes agents
hug walls and corners. Follow the path; advance the waypoint when within radius.

Inadmissible A* heuristic (e.g. Euclidean distance scaled up, or Manhattan
on a diagonal grid) returns fast but non-shortest paths. Pick the heuristic
that matches your allowed moves.

Behavior tree leaves that never return RUNNING for multi-frame actions
(walking, playing an animation) cause the tree to restart the action every
tick. Return RUNNING until the action completes.

FSM transition spaghetti: scattering if state == ... checks everywhere
recreates the mess an FSM exists to prevent. Keep transitions in the state.

No line-of-sight or stuck check → agents grind into walls forever. Add a
timeout that forces a repath or a state change.

References

references/pathfinding.md — complete A* (priority queue, reconstruction),
grid vs waypoint graphs, when to defer to an engine navmesh.

references/behavior-trees.md — node taxonomy, leaf/decorator implementations,
blackboard, and FSM-vs-BT selection.

Related skills

unity-navmesh, unreal-behavior-trees — concrete engine AI/navigation APIs.

physics-tuning — movement, collision response, and agent radius.

procedural-gen — generating the graph/level the AI navigates.

tower-defense, fps-shooter — genres that compose this skill.