Ue Module Build System

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

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

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

Loading Phases

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