Unreal expert (technical director and agent protocol)

Expert Unreal work is a loop run against numbers: set the budget, build one stage, measure it, look at it, fix it, then advance. An agent without a mouse gets there by choosing the right channel for each action, working like the Details panel (one transaction, explicit saves), and never trusting its own eyes alone. This skill is the protocol, the shared toolkit in scripts/, and the map to the team. Target: UE 5.8 on macOS Apple Silicon. If a sibling skill named here is missing from your available skills, ask the user to install it (npx skills add scenario-labs/skills --skill <name>); unattended, proceed from tool schemas and flag the gap.

Status (2026-09-24): UE 5.8 is not installed. Every in-engine call in scripts/ and references/ is not yet run in Unreal. The pure-Python parts ran offline and pass (eight test files, 314 checks on Python 3.14, 311 on 3.9). First action once the engine is installed: tests/code/unreal-expert/run_all.sh --live with UE_TEST_PROJECT set to a scratch project, then references/ue-5.8-traps.md section 7.

1. Execution channels

Channel	Use for	Rules
Epic Unreal MCP (plugin ModelContextProtocol, Experimental), your MCP client via .mcp.json	structured edits covered by shipped toolsets (actors, scene, material instances, Sequencer, PCG, UMG, animation), in the open editor	Enable Unreal MCP and All Toolsets; if "connected but can do nothing", add Editor Toolset, which Epic's webinar says the docs omit (Sam Deiter, k0tgmrBuIJc [00:18:13]). Start with -ModelContextProtocolStartServer -ModelContextProtocolPort=8000 or ModelContextProtocol.StartServer; ModelContextProtocol.GenerateClientConfig ClaudeCode writes .mcp.json; editor first, agent second. Calls run serially on the game thread: never overlap them. describe_toolset only for the toolset you need now: Niagara's is "almost a quarter of a million" (k0tgmrBuIJc [00:46:04]). Loopback, no auth.
Editor Python in the running editor (ue_remote.PythonRemote, or a custom Python toolset on the MCP server)	loops, audits, anything toolsets lack, screenshots	Enable Python Remote Execution (Project Settings > Plugins > Python). Full unreal API; the editor ticks between calls. One call = one named transaction that reports what it did.
Headless commandlet (ue_run.run_python, UnrealEditor-Cmd <uproject> -run=pythonscript)	audits, imports, batch edits, validation, CI, when no editor has the project open	No level loaded (map= loads one), fewer modules, no rendering. The boot waits for the Asset Registry (get_asset_registry().wait_for_completion(), 5.8 batch example); a job passes only on its boot's job-id UE_RESULT line, never on the exit code. A commandlet that saves must not touch a project an open editor has loaded: the editor holds a write lock (Jamie Dale, 0guOMTiwmhk [01:17:18]).
Full editor, one shot (mode="editor": -ExecutePythonScript; mode="latent": ticking boot that quits after)	jobs needing full modules; latent: screenshots and renders without a running editor	-ExecutePythonScript shuts down right after the script, so nothing ticks; ue_run adds -ScriptErrorsAreFatal (5.5) so a failing script fails the process. Latent jobs are generators: yield = one tick.
Remote Control API (ue_remote.RemoteControl, 127.0.0.1:30010)	batched property writes on known objects, live presets, -game builds	Off until WebControl.StartServer. 5.8 disables remote UFUNCTION calls by default with an allow list (5.8 release notes). C++ names, WRITE_TRANSACTION_ACCESS for undo, public EditAnywhere properties only.
Console and cvars	stats, view modes, quality switches, HighResShot	From Python unreal.SystemLibrary.execute_console_command(world, cmd) [verify]; at launch -DPCVars= (before init, overrides read-only cvars; Profiling with Purpose, C-AjCqjKRSs [00:14:13]), not -ExecCmds. Restore what you change.
Commandlets and UAT (ue_run.run_commandlet, run_uat)	DataValidation, ResavePackages -fixupredirects, WorldPartitionBuilderCommandlet, BuildCookRun	Headless; parse the log (ue_run.scan_log).

Capability matrix (from the 5.8 docs and talks; tests/code/unreal-expert/jobs/job_00_probe.py confirms or corrects each row):

Action	MCP	Python, running editor	Headless commandlet	Remote Control
read and edit actors, assets, properties	toolsets	yes	yes (load the level first)	public EditAnywhere properties
material instances and parameters	MaterialInstanceTools	yes	yes	properties only
material graphs	toolset (large schema)	MaterialEditingLibrary, partial: some pins unexposed (Matt Oztalay, m6mJ9r7ytks [00:13:39])	same	no
Blueprint graphs, AnimGraph [verify]	Blueprint toolset [verify scope]	5.8 BlueprintGraphEditor [verify scope]	same	no
Niagara emitters and modules	toolset, huge schema	no emitter or module editing	no	user parameters
PCG graphs	PCG toolset, primitives, fire-and-forget calls	graph API [verify]	yes	no
screenshots, MRG stills, PIE	via Python toolset	yes	no (use mode="latent" or -game render CLI)	no
landscape sculpt, foliage paint, Fracture Mode	no	no	no	no
cook, package, validate	no	not the cook (in-editor cook removed in 5.9)	yes, UAT	no

Graph editors and viewport tools, substitutes: Blueprints: logic in C++ (code project, Xcode) with data-only Blueprints; template Blueprints duplicated and their defaults set; never 150 Delay nodes or unjustified Event Tick (k0tgmrBuIJc [00:56:45], [00:57:19]). Materials: instances of approved masters; node graphs, never a whole-HLSL Custom node unless required (k0tgmrBuIJc [00:39:27]). Niagara: duplicate template systems, set user parameters. PCG: compose Epic's primitives and example graphs (lDf_y-YPELo [00:05:20]). Control Rig: its "Copy Python Script". Sculpting, painting, fracturing: heightmap import, PCG, Dataflow graphs, or a human step, said plainly.

Write locks and source control: ue_run lists UnrealEditor processes holding the project (ps); pass lock_check="refuse" for jobs that save. 5.8's PythonScript commandlet enables source control before the script, so saves can check out [verify with Perforce]; ResavePackages needs -autocheckout (redirectors doc). Stop when a file you must edit is not checked out (Bardoux, MjjkWH0eT3U [00:55:27]). OFPA changelists are submitted from inside the editor: partial submits leave dangling references (World Partition doc). Content a builder regenerates, such as PCG partition actors, stays out of changelists (TbNZ4GKaTow [00:26:23]).

2. The expert loop

Name the engine call behind every toolkit call, in plans as in reports: a reader without scripts/ must be able to check each step. Write ue_review.screenshot(p, 1920, 1080) (AutomationLibrary.take_high_res_screenshot, fallback HighResShot 1920x1080), cvars and console commands literally, and close with Verified (what ran, with the number, log line or frame that proves it) and Assumed (not run in Unreal, [verify] names, defaults taken). Mapping and template: references/toolkit-map.md.

1. Brief in numbers. Platform and device class; frame budget in ms per thread and GPU (16.67 at 60 fps, 33.3 at 30; Ari Arnbjörnsson, GuIav71867E [00:03:53]); output resolution and upscaler; scale (world size, actors, NPCs); gameplay or cinematic; references. When silent, take the domain skill's default and write it down.
2. Preflight. ue_env.preflight() (chip, macOS, Xcode), ue_env.find_project() (plugins, path spaces), and the project settings that change the plan: r.Substrate, Lumen, VSM, Nanite, hardware ray tracing (ue_env.read_ini_value, or the probe). Upgraded projects keep old renderer settings [added].
3. Build one stage with small idempotent scripts: get-or-create by path, mark what you create, one transaction per logical operation, explicit saves.
4. Measure. ue_audit.verdict for content; stat unit to classify (Frame, Game, Draw, GPU, RHIT), then Insights (lean capture for "are we on budget", rich with -statnamedevents for "why", about 20% overhead, C-AjCqjKRSs [00:11:27]); ue_stat.budget_check on CSV profiler or TraceQuery JSONL. Profile a cooked Development or Test build on target hardware; this Mac is a proxy for a console. Measure before any rewrite (VMZftEVDuCE [00:16:33]); when Insights cannot explain a hitch, add a sampling profiler (C-AjCqjKRSs [00:17:30]).
5. Look AND check. Screenshots from fixed camera bookmarks or an MRG still, opened with the image reader, plus ue_review.image_checks: Epic's lighting agent approved an all-white frame (lDf_y-YPELo [00:17:29]). review_images gives flags and a contact sheet in one call. Judge with the domain skill's references/critique.md. After a render, look for 5.8's on-screen exposure-range warning; an MRG frame that differs from the viewport: suspect Game Overrides first (fVg5ihB8Wdc [00:03:33]).
6. Fix before advancing, one change at a time, with ue_review.compare for before and after parity from the same views.
7. Deliver with evidence in that format: saved assets and levels, audit and stat JSON, the frames you looked at with their checks, the Verified and Assumed lists.

Never report a level, material, shot or build as done without numbers and a frame you looked at.

3. Persona pipelines

Brief	Chain	Deliverable
Open world or level	scenario-unreal-world-building, scenario-unreal-materials (landscape, RVT), scenario-unreal-lighting-rendering, scenario-unreal-performance	streamed level, HLODs, budget report
Material kit	scenario-unreal-materials, then scenario-unreal-lighting-rendering (test bench), scenario-unreal-performance (shader cost)	masters, instances, permutation audit
Lit interior or hero still	scenario-unreal-lighting-rendering (scenario-unreal-materials for emissives), scenario-unreal-cinematics for MRG output	gameplay lighting at budget, EXR still
Gameplay prototype	scenario-unreal-gameplay, scenario-unreal-animation, scenario-unreal-vfx for abilities, scenario-unreal-performance	playable map, automation tests
Character from Maya	scenario-unreal-pipeline-automation (import), scenario-unreal-animation, scenario-unreal-gameplay	retargeted, locomotion-ready character
Trailer	scenario-unreal-cinematics (scenario-unreal-animation, scenario-unreal-lighting-rendering)	Sequencer shots, reviewed renders
VFX or destruction	scenario-unreal-vfx, scenario-unreal-performance	systems within budget
Batch import and build	scenario-unreal-pipeline-automation, scenario-unreal-performance (packaging)	report, validated package
Frame-rate rescue	scenario-unreal-performance, routing fixes to owners	proof capture, parity frames

Handoff contract: saved assets in a named folder, the ue_audit.verdict for the receiver's profile with no error, stat or trace numbers when performance matters, the frames the sender looked at with their image_checks, each toolkit call with its engine call, and the Verified and Assumed lists. Units are centimeters, Z-up [added]. Sister teams: scenario-maya-expert (1 unit = 1 cm, Y-up in Maya), scenario-zbrush-expert, scenario-blender-expert; scenario-3d for generated meshes to finish.

4. Shared toolkit (scripts/, in-engine parts not yet run in Unreal)

Import: import sys; sys.path.insert(0, "<this skill>/scripts"). PythonRemote.call adds that folder in the editor for one call.

• ue_env.py: find_engine(version=None) (editor, editor_cmd, uat, version; UE_ROOT override), find_project(path), preflight(), plan_plugins / enable_plugins (timestamped backup), read_ini_value, xcode_status.
• ue_run.py: run_python(uproject, script, args=None, map=None, timeout=1800, mode="commandlet"|"editor"|"latent", strict=False, lock_check="warn") returns the UE_RESULT envelope plus log scan and warnings; job side result(obj), args(), in_commandlet(), to_jsonable(), wait_for_registry(); run_commandlet(uproject, name, args), run_uat, build_buildcookrun, launch_editor(mcp_port=), stop_editor, editor_processes, scan_log, parse_ue_result.
• ue_remote.py: RemoteControl(host, port).info/describe/get_property/set_property/call/batch/search_assets/console, errors carry fix hints; PythonRemote().open/exec/call/close; McpHttp().health() (reachability, tool search); loopback_only(port), pie_path, cdo_path, python_cdo_path.
• ue_review.py: in editor set_camera, screenshot, wait_screenshot, screenshot_views, camera_views_from_actors, render_still (MRG or MRQ preset, own queue), wait_render; offline image_checks (histogram, clipped, crushed, EV-like, center vs surround, all_white, all_black, uniform, alpha), image_verdict, compare, contact_sheet, review_images, wait_for_file, build_render_command.
• ue_audit.py: in editor audit_assets(paths, rules, profile), collect_facts; anywhere evaluate, build_report, verdict(report, "game"|"cinematic"|"mobile"|"prototype"); Epic prefixes, texture size and compression, Nanite, collision, LODs, slots, instance parents, Custom HLSL, After-DOF translucency, redirectors.
• ue_stat.py: parse_stat_unit, parse_csv_profile + csv_frames, parse_jsonl + sniff_schema + trace_frames + timer_totals, budget_check(frames, target_ms), bound_by, summarize.

Domain skills add scripts/ue_<domain>.py; they never copy these. The engine call behind each function: references/toolkit-map.md.

5. Top 5.8 traps (full list: references/ue-5.8-traps.md)

• EditorLevelLibrary and EditorAssetLibrary are the old API: EditorActorSubsystem, LevelEditorSubsystem, UnrealEditorSubsystem, EditorAssetSubsystem via unreal.get_editor_subsystem. Epic's own 5.8 samples still use the old names. load_level discards unsaved changes; delete_asset force-deletes and may clear undo; destination_name is ignored under Interchange.
• Substrate defaults depend on project age: on for projects created in 5.7+, legacy for upgraded ones until opted in; Substrate materials render black with it off; no Metallic pin; Mac listed with known issues. Read r.Substrate before authoring.
• Mac feature support: Apple Silicon only; Nanite and VSM Beta on M2+; Lumen HWRT and MegaLights Experimental on M2+; path tracing via Metal, macOS 26.4+. Xcode: Epic lists 26.0 minimum, 26.1.1 recommended, 26.4 incompatible; this Mac has 26.6, unlisted: verify a C++ build.
• MRG renames: Movie Render Graph Production Ready, new features graph-only, MRQ presets legacy, queue default "Basic"; MoviePipelinePrimaryConfig (not Master); set_graph_preset() for graph jobs; end frames exclusive; H.264 node Windows only.
• Remote Control defaults: server off until started, remote UFUNCTION calls refused by default in 5.8, 127.0.0.1 only; opening DefaultBindAddress to the LAN likely exposes the unauthenticated MCP server too [added inference].

6. macOS specifics