Imported from JanVogelsang/UE-AgentFramework (UnrealEngine/skills/unreal-instructions/SKILL.md). Install upstream with npx skills add JanVogelsang/UE-AgentFramework --skill unreal-instructions. Copyright stays with the author.

Unreal Engine MCP Guide

1. Dual-MCP Architecture & Tool Routing

• unrealengine (Internal Editor, port 18777): Use for Blueprint/UMG/Level changes and asset manipulation (e.g., spawn_actor, inject_blueprint_nodes_t3d).
  • Constraint: ALWAYS use Unreal paths (/Game/...). NEVER use Windows paths or shell commands (rm, mv) for .uasset files.
  • UMG Tool Parameters: When using native tools to edit complex widget layouts (like set_widget_slot), be aware that layout parameters (anchors, offsets, alignment, Z-order) often must be passed inside a nested object (e.g., slot_properties), rather than at the top level of the tool arguments. Always check the tool schema structure carefully.
• cpp-ast-rag (External AST): The Python-based AST server. ALWAYS use its specialized tools (query_cpp_ast, search_vector_db, search_similar_blueprints) for C++ semantic lookups and documentation search instead of generic grep/file searches.
• Engine Class AST Fallback: The local project C++ AST (query_cpp_ast) indexes your project's custom C++ source code. For built-in Unreal Engine classes (e.g. UInstancedStaticMeshComponent, AActor, UCharacterMovementComponent), query_cpp_ast may not find a local declaration. If query_cpp_ast returns no results for an Engine class, immediately query Unreal Engine documentation via search_vector_db.
• Python API Documentation Lookup: The Unreal Engine Python API signatures (unreal.pyi) are indexed in the ChromaDB vector database. When writing Python scripts for Unreal Engine, ALWAYS use search_vector_db with queries like "unreal.SourceControl" or "take screenshot python" to retrieve exact class and method signatures before executing python scripts.
• Compilation: Use trigger_compile tool when Editor is open. NEVER run manual terminal builds (UBT/MSBuild) with an open Editor.
• Python Execution Safeguard: When running multi-line Python scripts, ALWAYS write the script to a temporary .py scratch file (e.g. scratch/script.py) using write_to_file before running it via python or execute_python_script. NEVER pass inline multi-line Python strings (python -c "...") inside terminal shell commands to prevent Windows PowerShell string escaping failures.

2. Local Environment & Workflows

Startup Requirement (Sandbox Pre-Authorization)

Launching the Unreal Editor spawns a long-lived GUI process outside the terminal sandbox. How to authorize this depends on your agent harness:

• If your harness provides an ask_permission tool (e.g. Antigravity): When this plugin first loads, proactively call ask_permission with Action: "unsandboxed" and Target: "C:/Program Files/Epic Games/UE_5.8/Engine/Binaries/Win64/UnrealEditor.exe" before any editor interaction is needed.
• If it does not (e.g. Claude Code, OpenAI Codex, Kilo Code): Skip this step entirely — do NOT search for or attempt to call ask_permission. Launching the editor goes through your harness's standard permission flow: the launch command itself will prompt for approval unless the installer already pre-approved it (e.g. via .claude/settings.json allow rules or your assistant's command allowlist).

Local Paths

• Unreal Engine Root: C:/Program Files/Epic Games/UE_5.8
• Unreal Editor Executable: C:/Program Files/Epic Games/UE_5.8/Engine/Binaries/Win64/UnrealEditor.exe
• Unreal Build Tool: C:/Program Files/Epic Games/UE_5.8/Engine/Build/BatchFiles/Build.bat
• Unreal Project File: C:/Users/janv1/Documents/Unreal Projects/AgentFrameworkTest/AgentFrameworkTest.uproject
• EOS DevAuthTool: C:/Program Files/EOS_DevAuthTool/EOS_DevAuthTool.exe
• Libclang Path: C:/Program Files (x86)/Microsoft Visual Studio/18/BuildTools/VC/Tools/Llvm/x64/bin/libclang.dll
• Launcher Script: C:/Users/janv1/Documents/Unreal Projects/UE-Antigravity/UnrealEngine/src/launch_editor.ps1

Developer Tool Workflows

1. EOS DevAuthTool

Only start this tool when testing multiplayer/online functionality or when explicitly requested. When required: powershell Invoke-CimMethod -ClassName Win32_Process -MethodName Create -Arguments @{ CommandLine = '"C:/Program Files/EOS_DevAuthTool/EOS_DevAuthTool.exe"' } Wait until the user has interacted with the opened EOS_DevAuthTool window (e.g. to log in, accept scopes, or configure credentials) before trying to use it for authentication.

2. Building the Project

To compile the C++ code and binaries for the editor (e.g. when editor is closed, to prevent out-of-date binaries preventing launch): powershell Start-Process -FilePath "C:/Program Files/Epic Games/UE_5.8/Engine/Build/BatchFiles/Build.bat" -ArgumentList "AgentFrameworkTestEditor", "Win64", "Development", '"C:/Users/janv1/Documents/Unreal Projects/AgentFrameworkTest/AgentFrameworkTest.uproject"', "-WaitMutex" -Wait -NoNewWindow

3. Launching the Unreal Editor

[!IMPORTANT] Windows Path Escaping Rule: Always use forward slashes (/) in path strings or use the launch_editor.ps1 helper script. Never include a trailing backslash inside double-quoted paths ("C:\Path"), as Windows parses " as an escaped quotation mark, corrupting command line arguments.

Recommended: Safe Launcher Script

powershell powershell -ExecutionPolicy Bypass -File "C:/Users/janv1/Documents/Unreal Projects/UE-Antigravity/UnrealEngine/src/launch_editor.ps1" -ProjectPath "C:/Users/janv1/Documents/Unreal Projects/AgentFrameworkTest/AgentFrameworkTest.uproject"

Direct Launch (Forward Slashes)

powershell Invoke-CimMethod -ClassName Win32_Process -MethodName Create -Arguments @{ CommandLine = '"C:/Program Files/Epic Games/UE_5.8/Engine/Binaries/Win64/UnrealEditor.exe" "C:/Users/janv1/Documents/Unreal Projects/AgentFrameworkTest/AgentFrameworkTest.uproject"' }

Multiplayer / EOS Launch

powershell Invoke-CimMethod -ClassName Win32_Process -MethodName Create -Arguments @{ CommandLine = '"C:/Program Files/Epic Games/UE_5.8/Engine/Binaries/Win64/UnrealEditor.exe" "C:/Users/janv1/Documents/Unreal Projects/AgentFrameworkTest/AgentFrameworkTest.uproject" -CustomConfig=EOS -AUTH_TYPE=developer -AUTH_LOGIN=localhost:8080 -AUTH_PASSWORD=TauDev' }

3. Coding Guardrails & Engine Invariants

Strict Pointer & Lifecycle Safety

• Data Asset Pointer Validation: Always verify IsValid() or check against nullptr before dereferencing Data Asset pointers (e.g., GunDataAsset->VisualizationAsset, Sprite->GetFName()). Unassigned or broken references in .uasset files will hard-crash the editor, particularly in Mass ECS and Niagara subsystems.
• Avoid Pointer Chaining (Early Init): Never chain pointer calls (e.g. GetMatchInstance()->GetMapGridSize()) without validating intermediate pointers. Subsystems or game states may be uninitialized during early lifecycle or standalone UI testing. Always provide fallback defaults when pointers are null.
• UI Dynamic Hover & Synthetic Events: Handlers receiving data asset pointers from UI events (e.g., OnUpgradeSlotHovered, AreUnitAttributesMatchingUpgradeRequirements) MUST verify the pointer before accessing properties. Slate synthetic mouse-move events frequently pass nullptr for unpopulated slots.

Config & INI Serialization Rules

• TMap Property Delimiters: Unreal's FMapProperty::ImportText_Internal fails to parse multi-line +MapKey= entries. Multi-entry TMaps in DefaultGame.ini must be formatted on a single line using parentheses pairs: MapProperty=((Key1,Val1),(Key2,Val2))

4. Skills Directory

Read the corresponding file with your harness's native file-reading tool (view_file in Antigravity, Read in Claude Code, or equivalent) when performing these tasks:

• blueprint-authoring: Modifying .uasset blueprints (nodes, variables, formatting).
• setup-replication: Network replication, RPCs, and RepNotify.
• add-component: Declaring/attaching UActorComponents in C++.
• setup-input: Enhanced Input IMCs, Actions, and bindings.
• niagara-authoring: Niagara VFX creation/modification.
• unreal-testing-sops: Automated UI, performance testing, and PIE SOPs.
• create-actor: Boilerplate for new Actor/Pawn C++ classes.
• create-interface: Blueprint and C++ interface creation.
• pie-verifier: Play-In-Editor (PIE) state and viewport checks.