UE Module & Build System

You are an expert in Unreal Engine's module and build system. You understand Unreal Build Tool (UBT), ModuleRules, TargetRules, the .uproject manifest, plugin architecture, and the IWYU include discipline enforced by UE5.

Before Starting

Read .agents/ue-project-context.md if it exists — it provides module names, engine version, active plugins, and build targets that affect dependency and include configuration.

Ask which situation applies:

1. Configuring dependencies in an existing Build.cs
2. Creating a new module from scratch
3. Creating a new plugin
4. Resolving a build error (linker, include, or IWYU)
5. Setting up Target.cs for a new build target

---

Build.cs Anatomy

Every UE module has a ModuleName.Build.cs file next to its Public/ and Private/ directories.

// Source/MyModule/MyModule.Build.cs
using UnrealBuildTool;

public class MyModule : ModuleRules
{
    public MyModule(ReadOnlyTargetRules Target) : base(Target)
    {
        // PCH settings — use IWYU in UE5
        PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;
        // Enable strict IWYU (recommended for new modules in UE5)
        bEnforceIWYU = true;

        // Types accessible to modules that depend on MyModule
        PublicDependencyModuleNames.AddRange(new string[]
        {
            "Core",
            "CoreUObject",
            "Engine",
        });

        // Types used only internally (not re-exported in public headers)
        PrivateDependencyModuleNames.AddRange(new string[]
        {
            "Slate",
            "SlateCore",
        });

        // Load at runtime but don't link at compile time
        DynamicallyLoadedModuleNames.Add("OnlineSubsystem");
    }
}

Public vs Private Dependencies

Field	When to use
PublicDependencyModuleNames	A type from the dependency appears in your public headers
PrivateDependencyModuleNames	The dependency is consumed only in Private/ .cpp files

A common mistake: putting everything in PublicDependencyModuleNames. This bloats transitive include paths for every downstream module. Only promote to public when your public headers actually #include headers from that module.

Include Paths

// Expose extra paths to modules that depend on you
PublicIncludePaths.Add(Path.Combine(ModuleDirectory, "Public/Interfaces"));

// Expose extra paths only to this module's own source
PrivateIncludePaths.Add(Path.Combine(ModuleDirectory, "Private/Helpers"));

UBT automatically adds Public/ and Private/ — you rarely need to set these manually unless you have nested subdirectory headers you want to import without path prefixes.

API Export Macro

UBT generates MODULENAME_API from the module's directory name, uppercased. Any class, function, or variable that must be visible across DLL boundaries needs this macro:

// Public/MyClass.h
#pragma once
#include "CoreMinimal.h"

class MYMODULE_API FMyClass
{
public:
    void DoSomething();
};

// Standalone exported function
MYMODULE_API void MyFreeFunction();

Missing MYMODULE_API on a class that another module references causes "unresolved external symbol" linker errors.

PCH and IWYU

// UE5 recommended — each file includes exactly what it uses
PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;
bEnforceIWYU = true;

// Legacy — one monolithic PCH (avoid for new modules)
PCHUsage = PCHUsageMode.UseSharedPCHs;

With IWYU, every .cpp file includes its own .h first, then only what it directly uses:

// Private/MyClass.cpp
#include "MyClass.h"       // own header first
#include "Engine/Actor.h"  // only includes this file directly uses

Compiler Flags

// C++ exceptions — disable unless third-party code requires them
bEnableExceptions = false;

// Runtime type information — disable unless using dynamic_cast
bUseRTTI = false;

// Third-party static libraries shipped with the engine
AddEngineThirdPartyPrivateStaticDependencies(Target, "zlib", "OpenSSL");

---

Target.cs

Located at Source/ProjectName.Target.cs (and Source/ProjectNameEditor.Target.cs).

// Source/MyGame.Target.cs
using UnrealBuildTool;
using System.Collections.Generic;

public class MyGameTarget : TargetRules
{
    public MyGameTarget(TargetInfo Target) : base(Target)
    {
        Type = TargetType.Game;
        DefaultBuildSettings = BuildSettingsVersion.Latest;
        IncludeOrderVersion = EngineIncludeOrderVersion.Latest;

        // All game modules that UBT should compile
        ExtraModuleNames.AddRange(new string[] { "MyGame", "MyGameUtilities" });
    }
}

// Source/MyGameEditor.Target.cs
public class MyGameEditorTarget : TargetRules
{
    public MyGameEditorTarget(TargetInfo Target) : base(Target)
    {
        Type = TargetType.Editor;
        DefaultBuildSettings = BuildSettingsVersion.Latest;
        IncludeOrderVersion = EngineIncludeOrderVersion.Latest;
        ExtraModuleNames.AddRange(new string[] { "MyGame", "MyGameEditor" });
    }
}

Target Types

TargetType	Use for
Game	Standalone game executable
Editor	Editor build (includes editor-only modules)
Client	Networked client without server logic
Server	Dedicated server (no renderer)
Program	Standalone non-game tool

Build configurations: Debug (full symbols, no optimization), DebugGame (engine optimized, game debug), Development (default; balanced), Test (like shipping but with console/stats), Shipping (final release, strips all debug).

---

.uproject File

{
    "FileVersion": 3,
    "EngineAssociation": "5.4",
    "Category": "",
    "Description": "",
    "Modules": [
        {
            "Name": "MyGame",
            "Type": "Runtime",
            "LoadingPhase": "Default"
        },
        {
            "Name": "MyGameEditor",
            "Type": "Editor",
            "LoadingPhase": "Default"
        }
    ],
    "Plugins": [
        {
            "Name": "ModelingToolsEditorMode",
            "Enabled": true
        },
        {
            "Name": "MyPlugin",
            "Enabled": true
        }
    ]
}

Module Types

Type	When to use
Runtime	Core game logic, ships with game
RuntimeNoCommandlet	Runtime, excluded from commandlet processes
Editor	Editor-only — stripped from shipping builds
EditorNoCommandlet	Editor, excluded from commandlets
Developer	Tools usable in Editor and Development builds
DeveloperTool	Developer module, shows in editor UI
CookedOnly	Included only in cooked (packaged) builds
UncookedOnly	Included only in uncooked (development) builds
RuntimeAndProgram	Runtime module that also compiles into standalone Programs
EditorAndProgram	Editor module that also compiles into standalone Programs
Program	Standalone programs (UnrealHeaderTool etc.)

Loading Phases

Phase	Use for
EarliestPossible	First possible phase — core engine modules only
PostSplashScreen	After splash screen renders
PreEarlyLoadingScreen	Before the early loading screen
PreLoadingScreen	Before the main loading screen
PostConfigInit	Systems that must configure before engine starts
PreDefault	Modules that must be ready before Default modules
Default	Standard game modules (most common)
PostDefault	Modules that depend on Default modules being up
PostEngineInit	After full engine initialization
None	Not auto-loaded — requires FModuleManager::LoadModule()

---

Creating a New Module

Directory Structure

Source/
  MyModule/
    Public/
      MyModule.h          (optional module interface header)
      MyClass.h
    Private/
      MyModule.cpp        (module registration)
      MyClass.cpp
    MyModule.Build.cs

Module Interface

// Public/MyModule.h
#pragma once
#include "Modules/ModuleManager.h"

class IMyModule : public IModuleInterface
{
public:
    static IMyModule& Get()
    {
        return FModuleManager::LoadModuleChecked<IMyModule>("MyModule");
    }

    static bool IsAvailable()
    {
        return FModuleManager::Get().IsModuleLoaded("MyModule");
    }
};

// Private/MyModule.cpp
#include "MyModule.h"

class FMyModule : public IMyModule
{
public:
    virtual void StartupModule() override
    {
        // Initialize subsystems, register delegates, etc.
    }

    virtual void ShutdownModule() override
    {
        // Unregister delegates, clean up
    }
};

// Use IMPLEMENT_MODULE for non-gameplay modules
IMPLEMENT_MODULE(FMyModule, MyModule)

// Use IMPLEMENT_GAME_MODULE for modules containing UObject/gameplay code
// IMPLEMENT_GAME_MODULE(FMyModule, MyModule)

// Use IMPLEMENT_PRIMARY_GAME_MODULE for the main game module
// IMPLEMENT_PRIMARY_GAME_MODULE(FMyModule, MyGame, "MyGame")

The IMPLEMENT_MODULE macro (defined in Modules/ModuleManager.h) registers the module's initializer function. In DLL builds it registers a static FModuleInitializerEntry that maps the module name to its factory function. In monolithic builds it registers a static FStaticallyLinkedModuleRegistrant.

FDefaultModuleImpl and FDefaultGameModuleImpl are provided for modules that need no startup/shutdown logic — use them to avoid writing a class body.

Add to .uproject

{
    "Name": "MyModule",
    "Type": "Runtime",
    "LoadingPhase": "Default"
}

---

Creating a Plugin

Directory Structure

Plugins/
  MyPlugin/
    MyPlugin.uplugin
    Source/
      MyPlugin/
        Public/
          IMyPlugin.h
        Private/
          MyPlugin.cpp
        MyPlugin.Build.cs
      MyPluginEditor/           (optional editor-only module)
        Public/
        Private/
          MyPluginEditor.cpp
        MyPluginEditor.Build.cs
    Content/                    (optional, for content plugins)
    Resources/
      Icon128.png

.uplugin File

{
    "FileVersion": 3,
    "Version": 1,
    "VersionName": "1.0",
    "FriendlyName": "My Plugin",
    "Description": "Does something useful.",
    "Category": "Gameplay",
    "CreatedBy": "MyStudio",
    "CreatedByURL": "",
    "DocsURL": "",
    "MarketplaceURL": "",
    "CanContainContent": true,
    "IsBetaVersion": false,
    "IsExperimentalVersion": false,
    "Installed": false,
    "Modules": [
        {
            "Name": "MyPlugin",
            "Type": "Runtime",
            "LoadingPhase": "Default"
        },
        {
            "Name": "MyPluginEditor",
            "Type": "Editor",
            "LoadingPhase": "Default"
        }
    ]
}

Plugin with Runtime + Editor Modules

The runtime module must not include editor-only headers. Gate editor code:

// MyPlugin.Build.cs — runtime module
public class MyPlugin : ModuleRules
{
    public MyPlugin(ReadOnlyTargetRules Target) : base(Target)
    {
        PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;

        PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine" });

        // Editor-only dependency, only when compiling an editor build
        if (Target.bBuildEditor)
        {
            PrivateDependencyModuleNames.Add("UnrealEd");
        }
    }
}

// MyPluginEditor.Build.cs — editor module
public class MyPluginEditor : ModuleRules
{
    public MyPluginEditor(ReadOnlyTargetRules Target) : base(Target)
    {
        PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;

        PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine", "UnrealEd" });
        PrivateDependencyModuleNames.Add("MyPlugin");
    }
}

Engine vs Project Plugins

Engine plugins (Engine/Plugins/): available to all projects using that engine installation. Ship with Epic or marketplace installs. Project plugins (YourProject/Plugins/): project-specific, travel with the project. When both exist with the same name, the project plugin wins.

Content-only plugins: No Source/ directory, no Modules array in .uplugin. Contains only Content/ and Resources/. Set "CanContainContent": true. Used for distributing Blueprint libraries and asset packs without C++ compilation.

Edge case -- launcher vs source builds: Launcher (binary) installs only include pre-built engine module binaries. Adding a dependency on an engine module not in the pre-built set causes LNK1104. Source builds from GitHub expose all engine modules. Use Target.LinkType to conditionally include source-build-only dependencies.

---

Resolving Build Errors

Most build errors fall into a few categories: LNK2019 (missing dependency or missing MODULENAME_API), C1083 (missing dependency or wrong include path under IWYU), IWYU violations (include every header each file directly uses), and circular dependencies (extract a shared interface module or use DynamicallyLoadedModuleNames). Guard editor-only code in runtime modules with #if WITH_EDITOR and if (Target.bBuildEditor) in Build.cs. See references/common-build-errors.md for full error message lookup, causes, and fix patterns.

---

Common Anti-Patterns

• Putting everything in PublicDependencyModuleNames — inflates transitive include paths for every downstream module. Only promote to public when your public headers require it.
• Missing MODULENAME_API on exported symbols — compiles fine within the module but causes LNK2019 for any other module that tries to call it.
• Relying on transitive includes under IWYU — under bEnforceIWYU = true, each file must include every header it uses directly, even if another include would pull it in.
• Referencing editor modules from runtime modules without #if WITH_EDITOR — fails in Shipping/Server builds where editor modules are excluded.
• Wrong LoadingPhase — a module that registers asset types too late causes missing type errors at startup. Use PreDefault or PostConfigInit when needed.
• Not adding the module to .uproject — UBT will not compile a module that isn't listed in .uproject or a .uplugin.

---

Related Skills

• ue-cpp-foundations — UObject macros (UCLASS, UPROPERTY, UFUNCTION), reflection, and what must be in public headers for UHT to process

For detailed Build.cs field reference, see references/build-cs-reference.md. For error message lookup, see references/common-build-errors.md.