Unreal Engine 5 Gameplay skill for Codex

Done well means: logic sits in the right gameplay framework class, C++ owns systems and performance-sensitive code while Blueprints own content and tuning, every UObject pointer is visible to the garbage collector, the project compiles from the command line, and networked state is replicated from the server, not set on clients.

Before writing code

1. Read AGENTS.md, the .uproject (EngineAssociation gives the version, Modules and Plugins list what is enabled) and Source/<Module>/<Module>.Build.cs.
2. Note whether the game is multiplayer. That decides where every piece of state lives.
3. Find existing base classes (AMyCharacterBase, UMyGameInstance) and extend them rather than adding parallel hierarchies.
4. Blueprint assets (.uasset) are binary. Codex cannot edit them; changes that require reparenting a Blueprint or setting defaults must be listed for the user to do in the editor.

Which class owns it

Class	Exists on	Put here
AGameModeBase / AGameMode	Server only	Rules: spawning, win conditions, which pawn and controller classes
AGameStateBase	Server + all clients	Match state everyone sees: score, timer, phase
APlayerController	Server + owning client	Input mapping setup, UI ownership, player-intent RPCs
APlayerState	Server + all clients	Per-player public data: name, team, kills; survives pawn death
APawn / ACharacter	Server + relevant clients	Body: movement, health, abilities in play
UGameInstance / subsystems	Each process, whole session	Save data, platform services, cross-level state
UActorComponent	With its owner	Reusable behaviour: health, inventory, interaction

Prefer components and subsystems (UGameInstanceSubsystem, UWorldSubsystem) over growing Character classes and over global singletons.

Reflection macros

// HealthComponent.h
#pragma once
#include "CoreMinimal.h"
#include "Components/ActorComponent.h"
#include "HealthComponent.generated.h"   // must be the last include

DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams(FOnHealthChanged, float, NewHealth, float, Delta);

UCLASS(ClassGroup=(Gameplay), meta=(BlueprintSpawnableComponent))
class MYGAME_API UHealthComponent : public UActorComponent
{
    GENERATED_BODY()

public:
    UHealthComponent();

    UPROPERTY(EditDefaultsOnly, BlueprintReadOnly, Category="Health", meta=(ClampMin="1"))
    float MaxHealth = 100.f;

    UPROPERTY(BlueprintAssignable, Category="Health")
    FOnHealthChanged OnHealthChanged;

    UFUNCTION(BlueprintCallable, Category="Health")
    void ApplyDamage(float Amount);

    UFUNCTION(BlueprintPure, Category="Health")
    float GetHealth() const { return Health; }

protected:
    virtual void BeginPlay() override;
    virtual void GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutProps) const override;

    UPROPERTY(ReplicatedUsing=OnRep_Health, VisibleInstanceOnly, Category="Health")
    float Health = 0.f;

    UFUNCTION()
    void OnRep_Health(float OldHealth);
};

// HealthComponent.cpp
#include "HealthComponent.h"
#include "Net/UnrealNetwork.h"

UHealthComponent::UHealthComponent()
{
    PrimaryComponentTick.bCanEverTick = false;   // no Tick unless needed
    SetIsReplicatedByDefault(true);
}

void UHealthComponent::BeginPlay()
{
    Super::BeginPlay();
    if (GetOwner()->HasAuthority())
        Health = MaxHealth;
}

void UHealthComponent::ApplyDamage(float Amount)
{
    if (!GetOwner()->HasAuthority() || Amount <= 0.f) return;   // server decides
    const float Old = Health;
    Health = FMath::Clamp(Health - Amount, 0.f, MaxHealth);
    OnHealthChanged.Broadcast(Health, Health - Old);            // OnRep does not run on server
}

void UHealthComponent::OnRep_Health(float OldHealth)
{
    OnHealthChanged.Broadcast(Health, Health - OldHealth);
}

void UHealthComponent::GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutProps) const
{
    Super::GetLifetimeReplicatedProps(OutProps);
    DOREPLIFETIME(UHealthComponent, Health);
}

Specifier quick reference

Want	Specifier
Designers set default in Blueprint class only	EditDefaultsOnly
Per-instance in the level	EditAnywhere / EditInstanceOnly
Blueprint reads, C++ writes	BlueprintReadOnly
Blueprint implements, C++ calls	UFUNCTION(BlueprintImplementableEvent)
C++ default, Blueprint may override	UFUNCTION(BlueprintNativeEvent) + Foo_Implementation
Client asks server to act	UFUNCTION(Server, Reliable) (validate inputs in the body)
Cosmetic event to everyone	UFUNCTION(NetMulticast, Unreliable)

Pointers and garbage collection

• Member pointers to UObjects must be UPROPERTY() (use TObjectPtr<T> in UE5 headers) or the GC may destroy the object and leave a dangling pointer.
• Non-owning references that should not keep an object alive: TWeakObjectPtr<T>.
• Asset references that should not load eagerly: TSoftObjectPtr<T> / TSoftClassPtr<T>, loaded through the Asset Manager or StreamableManager.
• Never new or delete UObjects. Use NewObject<T>(Outer), CreateDefaultSubobject<T> (constructor only) and GetWorld()->SpawnActor<T>().
• Check with IsValid(Ptr); it also rejects objects pending kill.

Enhanced Input

void AMyCharacter::SetupPlayerInputComponent(UInputComponent* PlayerInputComponent)
{
    Super::SetupPlayerInputComponent(PlayerInputComponent);
    auto* Input = CastChecked<UEnhancedInputComponent>(PlayerInputComponent);
    Input->BindAction(MoveAction, ETriggerEvent::Triggered, this, &AMyCharacter::Move);
    Input->BindAction(JumpAction, ETriggerEvent::Started, this, &ACharacter::Jump);
}

void AMyPlayerController::BeginPlay()
{
    Super::BeginPlay();
    if (auto* Sub = ULocalPlayer::GetSubsystem<UEnhancedInputLocalPlayerSubsystem>(GetLocalPlayer()))
        Sub->AddMappingContext(DefaultMappingContext, /*Priority*/ 0);
}

MoveAction, JumpAction and DefaultMappingContext are EditDefaultsOnly UPROPERTYs assigned in the Blueprint subclass. Add "EnhancedInput" to PublicDependencyModuleNames.

C++ or Blueprint

Put in C++	Put in Blueprint
Systems used by many actors, base classes, components	Content variations, one-off level scripting
Per-frame math over many objects	Tuning values, timelines, VFX/SFX hookups
Replication, save data, anything needing code review and diffs	UI layout (UMG), animation graphs

Expose small, well-named BlueprintCallable functions rather than letting Blueprints reach into internals. Avoid Tick in Blueprints for anything that runs on many actors.

Build from the command line

# Windows: compile the editor target without opening the editor
"<UE>/Engine/Build/BatchFiles/Build.bat" MyGameEditor Win64 Development -Project="C:/Proj/MyGame.uproject" -WaitMutex
# macOS
"<UE>/Engine/Build/BatchFiles/Mac/Build.sh" MyGameEditor Mac Development -Project="/Proj/MyGame.uproject"
# Automation tests, headless
"<UE>/Engine/Binaries/Win64/UnrealEditor-Cmd.exe" "C:/Proj/MyGame.uproject" -ExecCmds="Automation RunTests MyGame; Quit" -unattended -nullrhi -nosplash -log

Packaging uses RunUAT BuildCookRun; it is slow and writes large outputs, so confirm with the user before running it.

Gotchas

• Changing a header with UPROPERTY/UFUNCTION/USTRUCT changes requires a full rebuild with the editor closed. Live Coding handles function bodies only and can corrupt Blueprints that depend on changed layouts.
• Constructor code runs for the Class Default Object at editor load. Do not access the world, other actors or game state there; use BeginPlay.
• OnRep_ functions do not fire on the server (or listen-server host). Call the shared logic explicitly there, as ApplyDamage does above.
• GameMode does not exist on clients. GetWorld()->GetAuthGameMode() returns null there.
• Missing module dependency gives linker errors (unresolved external symbol). Add the module (UMG, EnhancedInput, AIModule, NavigationSystem, GameplayTasks) to Build.cs.
• Renaming a C++ class referenced by Blueprints or maps needs a CoreRedirects entry in Config/DefaultEngine.ini, or the assets lose their parent class.

Verification

• Editor target builds from the command line with no new warnings
• Automation tests for new logic pass (IMPLEMENT_SIMPLE_AUTOMATION_TEST or Automation Spec)
• Play In Editor with 2 players, Net Mode “Play As Client”, for anything replicated
• Output Log has no LogScript or Accessed None errors during the test play
• Editor-only steps (Blueprint reparenting, asset defaults) are listed for the user

Hand-off summary

**Engine:** UE 5.x (EngineAssociation), module MyGame
**Added:** UHealthComponent (replicated, BlueprintAssignable OnHealthChanged)
**Build.cs:** + EnhancedInput, UMG
**Editor steps for you:** add HealthComponent to BP_PlayerCharacter; assign IA_Move, IA_Jump, IMC_Default
**Checks:** Build.sh MyGameEditor Mac Development passed; PIE 2 clients, damage replicates