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++)

cpp

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)

text

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)

cpp

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.

unreal-behavior-trees

NPX Install

Tags

SKILL.md Content