One-click editor tools for the user (Python)

Verified Oct 2026, UE 5.8.4, editor driven through ue_exec.py (see unreal-mcp). Built for a production where the user moves a camera rail and re-bakes an effect from a project menu.

A menu in the main menu bar

import unreal
menus = unreal.ToolMenus.get()
main = menus.find_menu("LevelEditor.MainMenu")
menu = menus.find_menu("LevelEditor.MainMenu.MyTools") or \
       main.add_sub_menu("MyTools", "", "MyTools", "My Tools", "tooltip")   # owner, section, name, label
menu.add_section("Rail", "Camera rail")                                      # section name, label
entry = unreal.ToolMenuEntry(name="Rebake", type=unreal.MultiBlockType.MENU_ENTRY)  # kwargs work here
entry.set_label("Re-bake")
entry.set_tool_tip("...")
entry.set_string_command(unreal.ToolMenuStringCommandType.PYTHON, "", "import my_menu; my_menu.rebake()")
menu.add_menu_entry("Rail", entry)        # same entry name replaces -> re-running register() is safe
menus.refresh_all_widgets()

The sub-menu's path is LevelEditor.MainMenu.<name>; it appears right after Help (verified with a window screenshot). Keep the callbacks in a module (import my_menu; my_menu.fn()), with the module's folder on sys.path.

Run it at every editor start

Config/DefaultEngine.ini:

[/Script/PythonScriptPlugin.PythonScriptPluginSettings]
+StartupScripts=D:/Project/Scripts/startup/my_menu.py

Entries run as commands (RunTimedStartupScript(..., bRunAsFile=false)), so a path to a .py file is executed as a file; use an absolute path without spaces. Runs before the first tick. For the session you're in, just exec the same file over remote execution.

Making actions safe for the user

• Undo: with unreal.ScopedEditorTransaction("Start rail from view"): and call obj.modify() on each actor / component before changing it. Verified: TRANSACTION UNDO (console) put a moved rail and its spline back exactly.
• Progress: with unreal.ScopedSlowTask(n, "Baking...") as t: t.make_dialog(False); t.enter_progress_frame(1).
• Message boxes block your remote calls: unreal.EditorDialog.show_message(title, text, unreal.AppMsgType.OK) is modal — called from ue_exec.py it waits until someone clicks OK. Route every message through one helper with a module flag (QUIET = True in tests → unreal.log instead).
• Long work (a ~45 s bake) freezes the editor; say so in the tooltip.
• Re-running a setup script from a menu: exec(compile(open(path).read(), path, "exec"), {"__name__": "x", "FIELD_ONLY": True}) — the dict is the script's globals, so globals().get(...) switches work, and anything it defines (e.g. a REPORT string) can be read back from the dict.

Viewport camera and piloting

ues = unreal.get_editor_subsystem(unreal.UnrealEditorSubsystem)
loc, rot = ues.get_level_viewport_camera_info()          # what the user looks at (also while piloting)
ues.set_level_viewport_camera_info(loc, rot)
les = unreal.get_editor_subsystem(unreal.LevelEditorSubsystem)
les.pilot_level_actor(actor); les.eject_pilot_level_actor()
les.editor_set_game_view(True)                           # = G: hides sprites, volume outlines, gizmos

• Piloting an actor moves it with every mouse / WASD move in that viewport. Never leave a pilot active after a test — the user may be using the viewport (verified: a rail moved 16 m while a test pilot was left on). Eject and check the actor's transform afterwards.
• Piloting a camera that is attached to a rig moves the CAMERA relative to its parent (an offset), not the rig. For "position the shot by flying there", read the viewport camera and move the rig to it ("Start rail from current view"), then zero the camera's offset.
• Sequencer's camera-cut lock keeps the viewport on the shot camera: LevelSequenceEditorBlueprintLibrary.set_lock_camera_cut_to_viewport(False) before piloting.
• Editor-only helpers (Blueprint sprites, PCG / post-process volume outlines, spline lines of selected actors) show up in viewport captures; Game View hides them and they never render in MRQ.

Screenshots of the editor window (to check a menu)

CaptureEditorImage (MCP) works; or PowerShell System.Drawing + GetWindowRect of the UnrealEditor process's main window, crop to the area you need. A maximised window can extend past the screen, so its bottom bar may not be on screen.