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skills/quodsoler/unreal-engine-skills/ue-ai-navigation

ue-ai-navigation

SKILL.md

UE AI and Navigation

You are an expert in Unreal Engine's AI and navigation systems.

Context

Read .agents/ue-project-context.md for project AI plugins, subsystem configs, enabled modules (AIModule, NavigationSystem, GameplayStateTreeModule, SmartObjectsModule), and existing AI frameworks.

Information Gathering

Before implementing, clarify: AI complexity, navigation needs (ground/fly/swim, dynamic obstacles, streaming), perception senses required, Behavior Tree vs. State Tree preference, multiplayer authority model, and agent count for budget planning.

AI Architecture

APawn
  └── AAIController (server-only in multiplayer)
        ├── UBehaviorTreeComponent  (UBrainComponent subclass)
        │     └── UBehaviorTree asset → UBlackboardData
        ├── UBlackboardComponent    (AI knowledge store)
        ├── UAIPerceptionComponent  (sight, hearing, damage)
        └── UPathFollowingComponent (NavMesh path execution)

Build.cs modules: AIModule, NavigationSystem, GameplayTasks

AIController

// MyAIController.h
UCLASS()
class AMyAIController : public AAIController
{
    GENERATED_BODY()
public:
    AMyAIController();
    UPROPERTY(EditDefaultsOnly, Category = AI)
    TObjectPtr<UBehaviorTree> BehaviorTreeAsset;
protected:
    virtual void OnPossess(APawn* InPawn) override;
    UFUNCTION()
    void OnTargetPerceptionUpdated(AActor* Actor, FAIStimulus Stimulus);
};

// MyAIController.cpp
AMyAIController::AMyAIController()
{
    bStartAILogicOnPossess = true;
    bStopAILogicOnUnposses = true;
    // PerceptionComponent declared in AAIController; configure senses here or in BP defaults
}

void AMyAIController::OnPossess(APawn* InPawn)
{
    Super::OnPossess(InPawn);
    if (BehaviorTreeAsset)
        RunBehaviorTree(BehaviorTreeAsset); // calls UseBlackboard internally
    if (UAIPerceptionComponent* PC = GetAIPerceptionComponent())
        PC->OnTargetPerceptionUpdated.AddDynamic(this, &AMyAIController::OnTargetPerceptionUpdated);
}

Key AAIController API

// Navigation
EPathFollowingRequestResult::Type MoveToActor(AActor* Goal, float AcceptanceRadius = -1,
    bool bStopOnOverlap = true, bool bUsePathfinding = true, bool bCanStrafe = true,
    TSubclassOf<UNavigationQueryFilter> FilterClass = {}, bool bAllowPartialPath = true);

EPathFollowingRequestResult::Type MoveToLocation(const FVector& Dest, float AcceptanceRadius = -1,
    bool bStopOnOverlap = true, bool bUsePathfinding = true,
    bool bProjectDestinationToNavigation = false, bool bCanStrafe = true,
    TSubclassOf<UNavigationQueryFilter> FilterClass = {}, bool bAllowPartialPath = true);

void StopMovement();
bool HasPartialPath() const;
EPathFollowingStatus::Type GetMoveStatus() const;

// Focus
void SetFocus(AActor* NewFocus, EAIFocusPriority::Type Priority = EAIFocusPriority::Gameplay);
void SetFocalPoint(FVector NewFocus, EAIFocusPriority::Type Priority = EAIFocusPriority::Gameplay);
void ClearFocus(EAIFocusPriority::Type Priority);

// Brain / Blackboard
bool RunBehaviorTree(UBehaviorTree* BTAsset);
bool UseBlackboard(UBlackboardData* BlackboardAsset, UBlackboardComponent*& BlackboardComponent);
UBlackboardComponent* GetBlackboardComponent();

// Team (IGenericTeamAgentInterface)
void SetGenericTeamId(const FGenericTeamId& NewTeamID);

// Delegate: FAIMoveCompletedSignature ReceiveMoveCompleted (RequestID, Result)

On Pawn: AIControllerClass = AMyAIController::StaticClass(); AutoPossessAI = EAutoPossessAI::PlacedInWorldOrSpawned;

Blackboard

Type Get Set
Object GetValueAsObject SetValueAsObject
Vector GetValueAsVector SetValueAsVector
Bool GetValueAsBool SetValueAsBool
Float GetValueAsFloat SetValueAsFloat
Int GetValueAsInt SetValueAsInt
Enum GetValueAsEnum SetValueAsEnum
Name GetValueAsName SetValueAsName
Rotator GetValueAsRotator SetValueAsRotator
String GetValueAsString SetValueAsString
Class GetValueAsClass SetValueAsClass 
UBlackboardComponent* BB = GetBlackboardComponent();
BB->SetValueAsObject(TEXT("TargetActor"), SomeActor);
BB->SetValueAsVector(TEXT("LastKnownLocation"), Location);
BB->ClearValue(TEXT("TargetActor"));
bool bSet = BB->IsVectorValueSet(TEXT("PatrolLocation"));

// Observer (called when key changes)
FBlackboard::FKey KeyID = BB->GetKeyID(TEXT("TargetActor"));
FDelegateHandle H = BB->RegisterObserver(KeyID, this,
    FOnBlackboardChangeNotification::CreateUObject(this, &AMyAIController::OnBBKeyChanged));
BB->UnregisterObserver(KeyID, H);

// High-perf cached accessor (avoids repeated name lookups):
FBBKeyCachedAccessor<UBlackboardKeyType_Bool> BBInCombat;
// Init: BBInCombat = FBBKeyCachedAccessor<...>(*BBComp, KeyID);
// Use: bool b = BBInCombat.Get(); BBInCombat.SetValue(*BB, true);

Mark keys Instance Synced to share values across all AI using the same UBlackboardData (squad-wide alerts via UAISystem propagation).

Behavior Tree Nodes

Custom Task

UCLASS()
class UMyBTTask_Attack : public UBTTaskNode
{
    GENERATED_BODY()
public:
    UMyBTTask_Attack() { NodeName = TEXT("Attack"); INIT_TASK_NODE_NOTIFY_FLAGS(); }
    UPROPERTY(EditAnywhere) FBlackboardKeySelector TargetKey;
protected:
    virtual EBTNodeResult::Type ExecuteTask(UBehaviorTreeComponent& OwnerComp, uint8* NodeMemory) override;
    virtual void TickTask(UBehaviorTreeComponent& OwnerComp, uint8* NodeMemory, float DeltaSeconds) override;
    virtual EBTNodeResult::Type AbortTask(UBehaviorTreeComponent& OwnerComp, uint8* NodeMemory) override;
};

EBTNodeResult::Type UMyBTTask_Attack::ExecuteTask(UBehaviorTreeComponent& OwnerComp, uint8* NodeMemory)
{
    AActor* Target = Cast<AActor>(
        OwnerComp.GetBlackboardComponent()->GetValueAsObject(TargetKey.SelectedKeyName));
    if (!IsValid(Target)) return EBTNodeResult::Failed;
    // Start async work → return InProgress; call FinishLatentTask() when done
    return EBTNodeResult::InProgress;
}

void UMyBTTask_Attack::TickTask(UBehaviorTreeComponent& OwnerComp, uint8* NodeMemory, float DeltaSeconds)
{
    FinishLatentTask(OwnerComp, EBTNodeResult::Succeeded); // or Failed
}

EBTNodeResult::Type UMyBTTask_Attack::AbortTask(UBehaviorTreeComponent& OwnerComp, uint8* NodeMemory)
{
    return EBTNodeResult::Aborted; // cleanup; or InProgress + FinishLatentAbort()
}

Custom Decorator

UCLASS()
class UMyBTDecorator_CanSee : public UBTDecorator
{
    GENERATED_BODY()
public:
    UMyBTDecorator_CanSee()
    {
        INIT_DECORATOR_NODE_NOTIFY_FLAGS();
        bAllowAbortLowerPri = true; bAllowAbortChildNodes = true;
        FlowAbortMode = EBTFlowAbortMode::Both;
    }
    UPROPERTY(EditAnywhere) FBlackboardKeySelector TargetKey;
protected:
    virtual bool CalculateRawConditionValue(
        UBehaviorTreeComponent& OwnerComp, uint8* NodeMemory) const override
    {
        AActor* Target = Cast<AActor>(
            OwnerComp.GetBlackboardComponent()->GetValueAsObject(TargetKey.SelectedKeyName));
        return IsValid(Target) && OwnerComp.GetAIOwner()->LineOfSightTo(Target);
    }
};

Custom Service

UCLASS()
class UMyBTService_UpdateTarget : public UBTService
{
    GENERATED_BODY()
public:
    UMyBTService_UpdateTarget() { Interval = 0.5f; RandomDeviation = 0.1f; INIT_SERVICE_NODE_NOTIFY_FLAGS(); }
protected:
    virtual void TickNode(UBehaviorTreeComponent& OwnerComp, uint8* NodeMemory, float DeltaSeconds) override
    {
        TArray<AActor*> Hostiles;
        OwnerComp.GetAIOwner()->GetAIPerceptionComponent()->GetPerceivedHostileActors(Hostiles);
        AActor* Best = nullptr; float BestDist = FLT_MAX;
        FVector MyLoc = OwnerComp.GetAIOwner()->GetPawn()->GetActorLocation();
        for (AActor* A : Hostiles)
        {
            float D = FVector::Dist(MyLoc, A->GetActorLocation());
            if (D < BestDist) { BestDist = D; Best = A; }
        }
        OwnerComp.GetBlackboardComponent()->SetValueAsObject(TEXT("TargetActor"), Best);
    }
};

Built-in Nodes (reference)

Tasks: BTTask_MoveTo, BTTask_MoveDirectlyToward, BTTask_Wait, BTTask_WaitBlackboardTime, BTTask_RunEQSQuery, BTTask_PlayAnimation, BTTask_MakeNoise, BTTask_RotateToFaceBBEntry, BTTask_RunBehavior, BTTask_RunBehaviorDynamic, BTTask_FinishWithResult

Decorators: BTDecorator_Blackboard, BTDecorator_CompareBBEntries, BTDecorator_Cooldown, BTDecorator_TagCooldown, BTDecorator_Loop, BTDecorator_TimeLimit, BTDecorator_DoesPathExist, BTDecorator_IsAtLocation, BTDecorator_CheckGameplayTagsOnActor, BTDecorator_ForceSuccess

Composites: Selector (first success), Sequence (first failure), SimpleParallel (main task + background subtree; main task drives completion). UE does not ship a general-purpose Parallel node — SimpleParallel is the built-in option. For true parallel execution of N branches, implement a custom UBTCompositeNode or chain multiple SimpleParallel nodes.

AI Perception

// In AIController constructor:
#include "Perception/AISenseConfig_Sight.h"
#include "Perception/AISenseConfig_Hearing.h"
#include "Perception/AISenseConfig_Damage.h"

UAIPerceptionComponent* AIP = CreateDefaultSubobject<UAIPerceptionComponent>(TEXT("AIPerception"));
SetPerceptionComponent(*AIP);

UAISenseConfig_Sight* Sight = CreateDefaultSubobject<UAISenseConfig_Sight>(TEXT("Sight"));
Sight->SightRadius = 2000.f;
Sight->LoseSightRadius = 2500.f;
Sight->PeripheralVisionAngleDegrees = 60.f;
Sight->AutoSuccessRangeFromLastSeenLocation = 400.f;
Sight->DetectionByAffiliation.bDetectEnemies = true;
AIP->ConfigureSense(*Sight);
AIP->SetDominantSense(Sight->GetSenseImplementation());

UAISenseConfig_Hearing* Hearing = CreateDefaultSubobject<UAISenseConfig_Hearing>(TEXT("Hearing"));
Hearing->HearingRange = 3000.f;
Hearing->DetectionByAffiliation.bDetectEnemies = true;
Hearing->DetectionByAffiliation.bDetectNeutrals = true;
AIP->ConfigureSense(*Hearing);

// Perception handler:
void AMyAIController::OnTargetPerceptionUpdated(AActor* Actor, FAIStimulus Stimulus)
{
    UBlackboardComponent* BB = GetBlackboardComponent();
    if (Stimulus.WasSuccessfullySensed())
    {
        BB->SetValueAsObject(TEXT("TargetActor"), Actor);
        BB->SetValueAsVector(TEXT("LastKnownLocation"), Stimulus.StimulusLocation);
    }
    else
    {
        // Lost target — keep last known location for investigation
        BB->SetValueAsVector(TEXT("LastKnownLocation"), Stimulus.StimulusLocation);
    }
}

// Report noise manually (e.g., gunshot):
UAISense_Hearing::ReportNoiseEvent(GetWorld(), Location, Loudness, Instigator, MaxRange, Tag);

// Report damage:
UAISense_Damage::ReportDamageEvent(GetWorld(), DamagedActor, Instigator, Amount, EventLoc, HitLoc);

// On perceived actors — add UAIPerceptionStimuliSourceComponent:
#include "Perception/AIPerceptionStimuliSourceComponent.h"
UAIPerceptionStimuliSourceComponent* Source =
    CreateDefaultSubobject<UAIPerceptionStimuliSourceComponent>(TEXT("StimuliSource"));
Source->bAutoRegister = true;
Source->RegisterForSense(TSubclassOf<UAISense>(UAISense_Sight::StaticClass()));
Source->RegisterForSense(TSubclassOf<UAISense>(UAISense_Hearing::StaticClass()));

// Query perception state:
UAIPerceptionComponent* PC = GetAIPerceptionComponent();
TArray<AActor*> Visible; PC->GetCurrentlyPerceivedActors(UAISense_Sight::StaticClass(), Visible);
TArray<AActor*> Hostiles; PC->GetPerceivedHostileActors(Hostiles);
bool bCanSee = PC->HasActiveStimulus(*Target, UAISense::GetSenseID<UAISense_Sight>());
PC->ForgetActor(Target);
PC->ForgetAll();
// Per-actor delegate (shown above):
// OnTargetPerceptionUpdated — fires once per actor whose perception state changed
// Batch delegate — fires once per frame with all updated actors:
// OnPerceptionUpdated — signature: void(const TArray<AActor*>& UpdatedActors)
// Also: OnTargetPerceptionForgotten, OnTargetPerceptionInfoUpdated

Additional sense configs: UAISenseConfig_Touch (fires on physical contact with perceived actor), UAISense_Team (propagates enemy awareness across teammates via IGenericTeamAgentInterface), UAISense_Prediction (predicts future location — call UAISense_Prediction::RequestPawnPredictionEvent).

Navigation System

#include "NavigationSystem.h"
UNavigationSystemV1* NavSys = UNavigationSystemV1::GetCurrent(GetWorld());

// Random reachable point
FNavLocation ResultLoc;
bool bOk = NavSys->GetRandomReachablePointInRadius(Origin, Radius, ResultLoc);

// Project onto NavMesh
FNavLocation Projected;
NavSys->ProjectPointToNavigation(WorldLoc, Projected, FVector(500, 500, 500));

// Sync path query
FPathFindingQuery Query; Query.StartLocation = Start; Query.EndLocation = End;
FPathFindingResult Result = NavSys->FindPathSync(Query);
if (Result.IsSuccessful()) { TArray<FNavPathPoint>& Pts = Result.Path->GetPathPoints(); }

// Async path query
FNavAgentProperties NavAgent = GetPawn()->GetNavAgentPropertiesRef();
NavSys->FindPathAsync(NavAgent, Query,
    FNavPathQueryDelegate::CreateUObject(this, &AMyAI::OnPathFound));

// Dynamic obstacle on actor:
#include "NavModifierComponent.h"
UNavModifierComponent* Mod = CreateDefaultSubobject<UNavModifierComponent>(TEXT("NavMod"));
Mod->AreaClass = UNavArea_Obstacle::StaticClass();

// Custom nav filter (prefer certain areas):
UCLASS() class UMyNavFilter : public UNavigationQueryFilter { ... };
MoveToActor(Target, -1.f, true, true, true, UMyNavFilter::StaticClass());

// Runtime NavMesh rebuild (after procedural generation):
NavSys->Build();

// Access RecastNavMesh for agent config (mirrors Project Settings > Navigation)
ARecastNavMesh* RNM = Cast<ARecastNavMesh>(NavSys->GetDefaultNavDataInstance());
// Key properties: AgentRadius, AgentHeight, AgentMaxStepHeight, AgentMaxSlope
// ANavMeshBoundsVolume must exist in level — without it, no tiles generate.
// For streaming/open-world: use UNavigationInvokerComponent on AI pawns instead.

Off-mesh links: Use ANavLinkProxy in-level, or implement INavLinkCustomInterface for traversal callbacks.

EQS (Environment Query System)

#include "EnvironmentQuery/EnvQueryManager.h"

UPROPERTY(EditDefaultsOnly) TObjectPtr<UEnvQuery> FindCoverQuery;

void AMyAIController::RunCoverQuery()
{
    FEnvQueryRequest Request(FindCoverQuery, this);
    Request.Execute(EEnvQueryRunMode::SingleResult,
        FQueryFinishedSignature::CreateUObject(this, &AMyAIController::OnCoverDone));
}

void AMyAIController::OnCoverDone(TSharedPtr<FEnvQueryResult> Result)
{
    if (Result.IsValid() && Result->IsSuccessful())
        GetBlackboardComponent()->SetValueAsVector(TEXT("CoverLocation"),
            Result->GetItemAsLocation(0));
}

Run modes: SingleResult (cheapest), RandomBest5Pct, RandomBest25Pct, AllMatching

BTTask_RunEQSQuery: set EQSRequest.QueryTemplate, BlackboardKey, RunMode. Async lifecycle managed via FBTEnvQueryTaskMemory.RequestID.

Custom context (resolve BB actor for use in tests):

UCLASS()
class UEnvQueryContext_Enemy : public UEnvQueryContext
{
    GENERATED_BODY()
    virtual void ProvideContext(FEnvQueryInstance& QI, FEnvQueryContextData& CD) const override
    {
        AAIController* C = Cast<AAIController>(Cast<APawn>(QI.Owner.Get())->GetController());
        AActor* Enemy = Cast<AActor>(C->GetBlackboardComponent()->GetValueAsObject(TEXT("TargetActor")));
        if (IsValid(Enemy)) UEnvQueryItemType_Actor::SetContextHelper(CD, Enemy);
    }
};

See references/eqs-reference.md for generator and test configurations.

State Trees (UE 5.1+)

Use State Trees for simpler state machines, designer-friendly workflows, and Smart Object integration. Use Behavior Trees for complex reactive combat with priority-based interrupts.

// Build.cs: "GameplayStateTreeModule"
#include "Components/StateTreeComponent.h"

UCLASS()
class AMyNPC : public ACharacter
{
    GENERATED_BODY()
    UPROPERTY(VisibleAnywhere) TObjectPtr<UStateTreeComponent> StateTreeComp;
};
AMyNPC::AMyNPC() { StateTreeComp = CreateDefaultSubobject<UStateTreeComponent>(TEXT("StateTree")); }
void AMyNPC::BeginPlay() { Super::BeginPlay(); StateTreeComp->StartLogic(); }

State Tree tasks use FStateTreeTaskBase + FInstanceDataType. Override EnterState, Tick, ExitState.

State Tree evaluators (FStateTreeEvaluatorBase) run persistently across all active states — use them for shared context data (nearest enemy, threat level) that multiple states read, analogous to BT services.

Smart Objects

Add USmartObjectComponent to world actors (set SmartObjectDefinition). AI interacts via USmartObjectSubsystem:

// Build.cs: "SmartObjectsModule"
#include "SmartObjectSubsystem.h"
USmartObjectSubsystem* SOS = USmartObjectSubsystem::GetCurrent(GetWorld());
FSmartObjectRequestFilter Filter;
FSmartObjectRequest Req(FBox(Origin, Origin).ExpandBy(500.f), Filter);
FSmartObjectRequestResult Res = SOS->FindSmartObject(Req);
if (Res.IsValid())
{
    FSmartObjectClaimHandle Handle = SOS->MarkSlotAsClaimed(Res.SlotHandle, ESmartObjectClaimPriority::Normal);
    // ... use slot, then:
    SOS->MarkSlotAsFree(Handle);
}

Common Mistakes

Polling instead of event-driven: Do not check per-frame in TickTask if a BTDecorator_Blackboard observer abort or WaitForMessage achieves the same result.

Overcomplicated BTs: Flatten needless nesting. Use services for periodic knowledge updates at Interval >= 0.5s. Gate expensive EQS with BTDecorator_Cooldown.

NavMesh gaps: Always place ANavMeshBoundsVolume. For streaming/procedural levels, call NavSys->Build() after generation or use UNavigationInvokerComponent on AI pawns.

Server vs client: AAIController only exists on the server. Replicate AI state via Pawn replicated properties, not through the controller. BT Blackboard is not replicated.

Large worlds: Use hierarchical NavMesh (RecastNavMesh actor settings). Set EQS max parallel queries per frame in Project Settings > AI > EQS.

Edge Cases

  • Flying/swimming: Set FNavAgentProperties::bCanFly/bCanSwim; assign a UNavArea subclass with matching SupportedAgents flags. Custom UNavArea subclasses define traversal cost and area flags — create one per movement domain (e.g., UNavArea_Water with high cost for ground agents, zero for swimmers). Apply via NavModifierVolumes or NavMesh generation settings.
  • Dedicated server: Sight perception uses collision traces, not rendering — works fine. Guard non-server code with HasAuthority().
  • AI with GAS: BT tasks request ability activation; abilities manage their own latent logic.

Related Skills

  • ue-actor-component-architecture — component setup patterns for AI
  • ue-gameplay-framework — GameMode AI spawning, controller/pawn relationships
  • ue-cpp-foundations — delegates, subsystems, UObject patterns
  • ue-gameplay-abilities — GAS + AI integration

Reference Files

  • references/behavior-tree-patterns.md — patrol, combat, flee, investigate, squad BT patterns
  • references/eqs-reference.md — generator and test configurations for spatial queries
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