Unreal Behavior Trees

Unreal Behavior Trees

Author NPC decision-making in UE5 with Behavior Trees driven by a Blackboard: structure the

tree with composites, gate branches with decorators, keep state current with services, and run

it from an AIController. Targets UE 5.4+.

When to use

• Use when building enemy/NPC AI: creating a BT_/BB_ asset pair, structuring

Selector/Sequence branches, adding decorators (conditions) and services (periodic updates),

writing custom BTTask/BTService nodes, or wiring an AIController to run the tree.

• Use when the project has Behavior Tree (BT_) and Blackboard (BB_) assets and an

AAIController.

When *not* to use: the *concept* of AI (FSM vs BT vs steering, cross-engine) →

game-ai. Pure navigation/pathing math is engine navmesh (BT's MoveTo uses it). Simple

one-off logic may be cheaper as a small state machine than a full tree.

Core workflow

• Create the pair: a Blackboard (BB_) holds typed keys (the AI's memory: TargetActor,

LastKnownLocation, bIsInvestigating); a Behavior Tree (BT_) references that Blackboard.

• Possess and run. An AAIController possesses the pawn and calls RunBehaviorTree(BT),

which also initializes the referenced Blackboard.

• Structure with composites. Selector runs children left→right until one *succeeds*

(priority/fallback: "attack, else chase, else patrol"). Sequence runs children until one

*fails* (do-all: "move to cover → reload → peek"). Simple Parallel runs one main task

alongside a secondary.

• Gate branches with Decorators that read Blackboard keys (e.g. "Has Target?" guards the

combat branch). Set Observer Aborts so the tree re-evaluates when the key changes.

• Keep the Blackboard current with Services attached to a branch — they tick periodically

(e.g. update TargetActor via a sight check) only while that branch is active.

• Do work in Tasks, which return Succeeded, Failed, or InProgress (latent tasks like

MoveTo finish later).

• Verify with the Behavior Tree debugger during PIE — it highlights the running node and

shows live Blackboard values, so you see exactly which branch executes.

Patterns

1. AIController that runs the tree (C++)

void AEnemyAIController::OnPossess(APawn* InPawn)
{
    Super::OnPossess(InPawn);
    if (BehaviorTree)                 // UPROPERTY(EditAnywhere) TObjectPtr<UBehaviorTree>
        RunBehaviorTree(BehaviorTree); // initializes & uses the Blackboard the BT references
}

2. A priority tree (node structure)

ROOT
└── Selector (try combat, else investigate, else patrol)
    ├── Sequence            [Decorator: Blackboard 'TargetActor' Is Set, Observer Aborts: Both]
    │     ├── Task: MoveTo (TargetActor)          // latent: returns InProgress then Succeeded
    │     └── Task: Attack
    ├── Sequence            [Decorator: 'LastKnownLocation' Is Set]
    │     ├── Task: MoveTo (LastKnownLocation)
    │     └── Task: Wait (3s) + clear key
    └── Task: Patrol (BTTask_FindPatrolPoint -> MoveTo)

Observer Aborts: Both makes the combat branch interrupt patrol the instant TargetActor is

set, and bail out when it's cleared — this is what makes the AI feel reactive.

3. Updating the Blackboard from code (e.g. on seeing the player)

void AEnemyAIController::SetTarget(AActor* Target)
{
    if (UBlackboardComponent* BB = GetBlackboardComponent())
        BB->SetValueAsObject(TEXT("TargetActor"), Target);   // key name must match the BB asset
}
// Clear with BB->ClearValue(TEXT("TargetActor")); to drop back to a lower-priority branch.

Pitfalls

• AI never starts — the pawn isn't possessed (set the Pawn's *Auto Possess AI* to "Placed

in World or Spawned" and assign the AIController), or RunBehaviorTree was never called.

• MoveTo instantly fails — no NavMesh in the level (add a Nav Mesh Bounds Volume), or the

target is off the navmesh.

• Branch doesn't react to changes — the gating Decorator's Observer Aborts is set to

None; set it to Self/Lower Priority/Both so the tree re-evaluates when the key changes.

• A task hangs the tree — a custom task returned InProgress and never calls

FinishLatentTask. Always complete latent tasks.

• Blackboard key typos — SetValueAsObject("Taget", ...) silently does nothing; match the

key name and type exactly, or use a cached FBlackboardKeySelector.

• Sequence vs Selector confusion — Sequence = AND (stops on first failure); Selector = OR

(stops on first success). Swapping them inverts the behaviour.

References

• For a custom C++ UBTTaskNode (instant and latent ExecuteTask returning EBTNodeResult,

with a FBlackboardKeySelector), read references/custom-bttask.md.

• Primary docs: "Behavior Trees in Unreal Engine"

(https://dev.epicgames.com/documentation/en-us/unreal-engine/behavior-trees-in-unreal-engine).

Related skills

• game-ai — engine-agnostic AI design (FSM, BT, steering, pathfinding choices).
• unreal-cpp-gameplay — the AIController and pawn classes in C++.
• fps-shooter / tower-defense — genres that compose enemy AI.