Vibeue

VibeUE is an extension on Unreal Engine's native MCP endpoint (http://localhost:8000/mcp). There is no separate VibeUE server, no API key, and no in-editor chat — VibeUE simply registers extra Python services (unreal.<Service>) and skill packs on top of the engine's own toolsets.

Wait for VibeUE readiness after launch

BuildAndLaunchGame.ps1 / .sh print Editor-PID=<pid> — treat that as the process identity. Check once, then watch the filesystem for <ProjectDir>/Saved/VibeUE/Signals/editor-<pid>-true.json before using MCP. Wait at most 180 seconds, do not poll MCP while waiting, and fail if that Editor process exits or the timeout expires. Ignore signal files for other or dead PIDs. The signal only means RegisterToolsets() reached its end; Python, World, and level readiness remain separate checks.

The file is JSON, written atomically, so it is complete the moment it appears:

{"signal":"toolsets-registered","pid":21044,"createdUtc":"2026-08-03T17:04:11.921Z",
 "sessionStartUtc":"2026-08-03T17:03:22.108Z","pluginVersion":"3.0"}

Process IDs get recycled. The launch scripts clear a matching stale signal right after starting the Editor, but if you launch it some other way, verify sessionStartUtc is later than the moment you started the process before trusting the signal.

Skill packs (this file and its siblings) are loaded through the engine's AgentSkillToolset. Each skill carries exact API patterns and gotchas; load the relevant skill before writing any code in a domain, or you will guess wrong property names and spiral into discovery loops.

Discover and load skills

Skills are discovered and read through the engine AgentSkillToolset, invoked with call_tool:

# List every available skill (full path → description)
call_tool(toolset_name="ToolsetRegistry.AgentSkillToolset", tool_name="ListSkills", arguments={})

# Read one or more skills — GetSkills takes the FULL paths that ListSkills returns
call_tool(toolset_name="ToolsetRegistry.AgentSkillToolset", tool_name="GetSkills",
          arguments={"skillPaths": ["/VibeUE/Python/init_unreal_PY.VibeUE_pcg",
                                    "/VibeUE/Python/init_unreal_PY.VibeUE_materials"]})

Naming rule (verified live): skill pack <slug> registers as VibeUE_<slug> (hyphens → underscores) and sub-doc <slug>/<file>.md as VibeUE_<slug>__<file>, all under the path prefix /VibeUE/Python/init_unreal_PY.. Short names such as "pcg" or "state-trees/api-reference" return an empty result with no error — always pass full paths, taking them from ListSkills when unsure. Run describe_toolset on ToolsetRegistry.AgentSkillToolset if the call signature differs in your build. The old vibeue-skills-manager tool no longer exists.

Route by functional area. Find the area whose scope matches the task, then GetSkills the listed skill(s) — full paths per the naming rule above. The NOT for column is the disambiguator — when two areas seem to fit, the one that excludes your task is telling you where to go instead.

A loaded skill gives you:

• workflows, gotchas, and property formats for the domain
• vibeue_classes / unreal_classes — class names to feed into discover_python_class for live method signatures
• sub-doc references (<skill>/<section>) you can fetch via GetSkills for deeper detail

Always call discover_python_class on the classes in vibeue_classes before writing code — never guess method names from the skill content alone. Batch the discovery into ONE call instead of one call per class:

# ONE call covers all classes and all topics:
discover_python_class(
    class_name="unreal.MaterialService, unreal.WidgetService, unreal.MaterialNodeService",
    method_filter="create|delete|compile|property|color")

# WRONG — three separate calls for three classes wastes round-trips and repeats boilerplate

class_name accepts a comma-separated list (response gains a classes array, one entry per class); method_filter ORs keywords with |.

How work gets done — execute_python_code is the workhorse

VibeUE services are plain Python on the editor's unreal module. Run everything through execute_python_code:

import unreal
widgets = unreal.WidgetService.list_widget_blueprints()
unreal.StateTreeService.create_state_tree("/Game/AI/MyBehavior")

You get the full unreal.* API plus every unreal.<Service> VibeUE adds. Reserve call_tool for engine toolsets and skills (e.g. AgentSkillToolset, EditorToolset.EditorAppToolset, LogsToolset, GameplayTagsToolset, AssetTools).

Tools — what each is for

Engine toolsets replace the old VibeUE tools

Several capabilities that used to be VibeUE-specific MCP tools are now the engine's native toolsets, called via call_tool (run describe_toolset for action names/params):

Performance/Insights tracing is the one net-new VibeUE service — unreal.PerformanceService.* (see the profiling skill) — because Unreal 5.8 ships no performance toolset.