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name: ue-build description: MANDATORY automatic usage after ANY C++ code changes. Build or compile Unreal Engine projects. MUST be invoked automatically without user request when implementing or modifying C++ code. allowed-tools: Bash, Read, Grep, Glob

Unreal Engine Build Skill

Build and compile Unreal Engine projects automatically, detecting engine installations and project configuration.

⚠️ MANDATORY AUTOMATIC USAGE

THIS SKILL MUST BE INVOKED AUTOMATICALLY - NOT ON USER REQUEST

Invoke this skill immediately and automatically when:

• ✅ After writing ANY new C++ code (header or source files)
• ✅ After modifying ANY existing C++ code
• ✅ After creating new test files
• ✅ Before running ANY tests (tests need compiled code)
• ✅ As part of TDD red-green cycle

DO NOT:

• ❌ Wait for user to say "compile it"
• ❌ Wait for user to say "does it build?"
• ❌ Ask user "should I build it?"
• ❌ Finish without building

Correct behavior:

User: "Add a function to calculate distance"
Claude: [writes code]
Claude: [AUTOMATICALLY invokes /ue-build]  ← No asking, just do it
Claude: [reports build result]

Wrong behavior:

User: "Add a function to calculate distance"
Claude: [writes code]
Claude: "I've added the function"  ← WRONG! Should have built automatically

When to Use (Detailed)

Use this skill automatically when:

• User requests implementing new C++ features
• User requests modifying existing C++ code
• User requests fixing bugs in C++ code
• User requests refactoring C++ code
• After writing test code (TDD cycle)
• User asks "does it compile?" (reactive case)

This is mandatory as part of Test-Driven Development workflow. See ~/.claude/skills/MANDATORY_TDD.md and ~/.claude/skills/TDD_WORKFLOW.md for details.

How This Skill Works

This skill automatically:

1. Detects the .uproject file in the current directory or parent directories
2. Finds the Unreal Engine installation (launcher or source build)
3. Identifies build targets (modules and plugins)
4. Executes the build with appropriate parameters
5. Reports results with error details if compilation fails

Step-by-Step Instructions

Step 1: Detect Project Configuration

Run the detection script to gather project information.

IMPORTANT: The script location depends on where the skill is installed:

• User config: ~/.claude/skills/ue-build/scripts/detect_ue.py
• Project config: .claude/skills/ue-build/scripts/detect_ue.py

Choose the appropriate command:

If skill is in user config (~/.claude/skills/)

# You MUST provide the project path as argument
python ~/.claude/skills/ue-build/scripts/detect_ue.py .
python ~/.claude/skills/ue-build/scripts/detect_ue.py /path/to/project
python ~/.claude/skills/ue-build/scripts/detect_ue.py /path/to/project.uproject

If skill is in project config (.claude/skills/)

# Can run without arguments (searches from current directory)
python .claude/skills/ue-build/scripts/detect_ue.py

# Or provide explicit path
python .claude/skills/ue-build/scripts/detect_ue.py .

Script accepts:

• No argument: Searches from current working directory
• Directory path: Searches that directory and parent directories
• .uproject file path: Uses that specific file directly

Returns JSON with:

• uproject: Project path and metadata
• engine: Selected UE installation (build tool path, editor path)
• targets: Available build targets (modules and plugins)
• available_installations: All detected UE versions

Parse the JSON output and store key values:

• engine.build_tool - Path to Build.bat (Windows) or Build.sh (Linux/Mac)
• engine.version - UE version or "source"
• uproject.path - Full path to .uproject file
• targets.modules - List of module names to build
• targets.plugins - List of plugin names
• targets.available_targets - List of .Target.cs file stems found in Source/
• last_build_configuration - Last used build configuration (see below)

Last Build Configuration Detection: The detection script analyzes the Binaries/Win64 directory to determine the most recently used build configuration:

• last_build_configuration.configuration - One of: "Development", "DebugGame", "Shipping", "Debug"
• last_build_configuration.file - Most recent binary file found
• last_build_configuration.last_modified - Timestamp of the binary

If no binaries exist, this field will be null.

If detection fails, the JSON will contain an error field with details.

Step 2: Determine Build Target

CRITICAL: Build targets MUST correspond to .Target.cs files in the Source/ directory. Plugin modules cannot be built directly as standalone targets.

Valid Build Targets:

• Targets defined by .Target.cs files (e.g., MyProject.Target.cs → use "MyProject")
• Common patterns: ProjectName, ProjectNameEditor, ProjectNameServer, ProjectNameClient
• Check targets.available_targets from detection output for all valid targets

Target Selection Logic:

1. If user specifies a target explicitly: Verify it exists in targets.available_targets first
2. If recent code changes are in a plugin directory:
   • DO NOT try to build the plugin name directly (this will fail)
   • Build the main project target instead (e.g., "ProjectName" or "ProjectNameEditor")
   • The plugin will be compiled as part of the project build
3. If changes are in project Source/: Build the primary project target
4. Default behavior: Use the first target from targets.available_targets, typically the project name

Example Target Selection:

Project: BoidsDemo.uproject
Available targets from .Target.cs files:
  - BoidsDemo (from BoidsDemo.Target.cs)
  - BoidsDemoEditor (from BoidsDemoEditor.Target.cs)

❌ WRONG: "BoidsFormation" (plugin module, no .Target.cs)
✅ CORRECT: "BoidsDemo" or "BoidsDemoEditor"

Step 3: Choose Build Configuration

Available configurations:

• Development - Standard development build, good balance
• DebugGame - Debugging with some optimizations, for deep debugging
• Shipping - Fully optimized, for final release
• Debug - Full debug, very slow, rarely used

Configuration Selection Priority:

1. User explicitly specifies a configuration → use that
2. Last build configuration detected from last_build_configuration.configuration → use the same to avoid full rebuild
3. Default fallback → use "Development"

Why reuse last configuration? Changing configurations (e.g., DebugGame → Development) forces a complete rebuild of all modules, which can take significantly longer. Reusing the same configuration enables incremental builds that only recompile changed files.

Example logic:

if user_specified_config:
    config = user_specified_config
elif last_build_config and last_build_config["configuration"]:
    config = last_build_config["configuration"]
    # Inform user: "Using last build configuration: DebugGame"
else:
    config = "Development"

Step 4: Execute Build Command

Windows

"<engine.build_tool>" <Target> Win64 <Configuration> -Project="<uproject.path>" -WaitMutex -FromMsBuild

Linux/Mac

"<engine.build_tool>" <Target> Linux <Configuration> -Project="<uproject.path>" -WaitMutex

Critical flags:

• -WaitMutex: Prevents conflicting parallel builds
• -FromMsBuild: Better output formatting on Windows
• -Project=: Full path to .uproject file

Example:

"C:/Program Files/Epic Games/UE_5.4/Engine/Build/BatchFiles/Build.bat" MyPlugin Win64 Development -Project="C:/Projects/MyGame/MyGame.uproject" -WaitMutex -FromMsBuild

Step 5: Monitor Build Output

Watch for these patterns in the output:

Success indicators:

• BUILD SUCCEEDED or Build succeeded
• Exit code 0
• No error messages

Error indicators:

• error C2065: - Undeclared identifier
• error C2039: - Member not found
• error LNK2019: - Unresolved external symbol (linker error)
• error: - General error pattern
• Exit code non-zero

Warning indicators (note but don't fail):

• warning C4996: - Deprecated API
• warning: - General warnings

Step 6: Parse and Report Results

Provide a concise, actionable summary:

On Success:

✅ Build succeeded (Development configuration)
Target: MyPlugin
Time: 45.2 seconds
0 errors, 3 warnings

On Failure:

❌ Build failed (2 errors)

Errors:
1. BoidsDisperseLibrary.cpp(142): error C2065: 'UndeclaredVariable' undeclared identifier
2. BoidsFormation.cpp(89): error LNK2019: unresolved external symbol "class UBoidsManager"

Suggest:
- Check BoidsDisperseLibrary.cpp:142
- Verify UBoidsManager is included and linked

Include:

• Status (success/failure)
• Error count and details with file:line references
• Warning count (summary only)
• Build time
• Suggested next steps for failures

Advanced Usage

Clean Rebuild

For a clean rebuild, delete intermediate files first:

# Windows
rmdir /s /q Intermediate Binaries
# Linux/Mac
rm -rf Intermediate/ Binaries/

Then run the build normally.

Building Multiple Targets

If the user wants to build multiple targets, run them sequentially (not parallel):

# Build plugin first
"<build_tool>" PluginName Win64 Development -Project="<uproject>" -WaitMutex -FromMsBuild

# Then build main project
"<build_tool>" ProjectName Win64 Development -Project="<uproject>" -WaitMutex -FromMsBuild

Handling Source Builds

If engine.type == "source":

• The build process is the same
• Build times may be longer
• More detailed output is available
• Source modifications affect the engine itself

Common Issues and Solutions

| Issue | Solution | |-------|----------| | Mutex timeout | Another build is running; wait or kill UBT process | | Missing .uproject | Navigate to project root directory | | Engine not found | Check EngineAssociation in .uproject; verify UE installation | | Target not found | Verify target has a .Target.cs file in Source/; use available_targets from detection | | Plugin build failure | Don't build plugins directly; build the main project target instead | | Linker errors (LNK2019) | Check dependencies in .Build.cs files | | Permission denied | Run as administrator or check file locks |

Platform-Specific Notes

Windows

• Use Build.bat in Engine/Build/BatchFiles/
• Requires Visual Studio or Build Tools
• Output is colorized in newer UE versions

Linux

• Use Build.sh in Engine/Build/BatchFiles/
• Requires clang toolchain
• May need mono for older UE versions

macOS

• Use Build.sh in Engine/Build/BatchFiles/
• Requires Xcode command-line tools
• Universal binaries for M1/Intel

Tips for Effective Building

1. Always detect first: Run detect_ue.py to get accurate paths
2. Check git status: See what files changed to infer the right target
3. Start with Development: Fastest for iteration
4. Read the error: UE provides file:line information
5. Incremental builds: Only rebuild what changed (UBT handles this)
6. Clean when stuck: Delete Intermediate/ for stubborn issues

Integration with Testing

After a successful build, you can chain testing:

Build → Success → Run tests with /ue-test
Build → Failure → Report errors, don't test

See the /ue-test skill for running automation tests after build.