Pie Testing

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Play-In-Editor (PIE) Testing Skill

PIE is the only way to validate runtime behavior — Blueprint logic, AI ticking, animation, input, widget interaction, gameplay events. Without starting PIE, your "fix" is unverified.

PIE control — engine EditorAppToolset

Generic PIE start/stop/status is owned by Unreal 5.8's native EditorToolset.EditorAppToolset, invoked through call_tool (run describe_toolset on it for exact action names/params):

call_tool(toolset="EditorToolset.EditorAppToolset", tool="IsPIERunning")
call_tool(toolset="EditorToolset.EditorAppToolset", tool="StartPIE")
call_tool(toolset="EditorToolset.EditorAppToolset", tool="StopPIE")

PIE start/stop/query is not on WidgetService anymore for general testing — use the engine toolset above. WidgetService still owns the widget-in-PIE validation helpers below.

Standard Test Loop

# 1. Make sure you're starting from a clean state
call_tool(toolset="EditorToolset.EditorAppToolset", tool="StopPIE")   # no-op if not running

# 2. Start the session (uses the editor's current PIE settings — default map, viewport)
call_tool(toolset="EditorToolset.EditorAppToolset", tool="StartPIE")

# 3. Let the test run / inspect log output (LogsToolset) / interact via other services
# 4. Stop when done
call_tool(toolset="EditorToolset.EditorAppToolset", tool="StopPIE")

Validating widgets in PIE — WidgetService

VibeUE keeps a small set of widget-in-PIE helpers on unreal.WidgetService (run them via execute_python_code). These spawn and inspect live widget instances once PIE is running:

import unreal

# After StartPIE, spawn a widget instance into the running viewport
handle = unreal.WidgetService.spawn_widget_in_pie("/Game/UI/WBP_HUD", 0)

# Read a live property off the running instance
val = unreal.WidgetService.get_live_property(handle, "HealthText", "Text")

# Tear it down before stopping PIE
unreal.WidgetService.remove_widget_from_pie(handle)

unreal.WidgetService.is_pie_running() also still exists and is handy from inside Python; for tool-level control prefer the engine EditorAppToolset actions above.

Gotchas

• PIE start is asynchronous. StartPIE returns immediately after RequestPlaySession is queued. The world isn't actually playing until the editor processes the request on its next tick. If you need to act inside the running world, give it a tick or poll IsPIERunning.
• Already-running is treated as success. StartPIE succeeds if a PIE session already exists — it does NOT restart. Stop first if you need a fresh session.
• StopPIE tears down the world via RequestEndPlayMap. Spawned PIE widget instances should be removed with WidgetService.remove_widget_from_pie(handle) before stopping.
• Save before starting. Dirty asset changes are NOT picked up by PIE unless saved/compiled. Always compile_blueprint(...) before launching PIE to test Blueprint changes.
• Don't leave PIE running between tasks. Subsequent edits (recompiles, asset moves, hot reload) can fail or behave oddly while a PIE world is alive. Call StopPIE before returning control to the user.
• Map-load delegates never fire on PIE start. FCoreUObjectDelegates::PreLoadMap / PostLoadMapWithWorld (loading screens, post-load hooks, subsystem map handlers) are skipped because the PIE world is duplicated from the editor world, not loaded. To exercise them, trigger a real in-game transition once PIE is up: unreal.GameplayStatics.open_level(pie_world, "MapName").

When to use PIE

• Verifying a Blueprint event fires (combine with log inspection — see LogsToolset)
• Validating gameplay logic (damage, scoring, state transitions)
• Testing widgets in their real runtime context (combine with umg-widgets skill's spawn_widget_in_pie)
• Reproducing user-reported runtime bugs

When NOT to use PIE

• Pure asset/editor validation (use compile_blueprint, find_assets, etc.)
• Static introspection (use get_nodes_in_graph, get_node_pins)
• Anything you can verify without a live world — PIE is slow, save it for genuine runtime checks.