diff --git a/CMakeLists.txt b/CMakeLists.txt
index b9885446..6d2f91c2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -41,8 +41,9 @@ endif()
 enable_testing()
 
 option(XCENGINE_ENABLE_MONO_SCRIPTING "Build the Mono-based C# scripting runtime" ON)
+option(XCENGINE_BUILD_XCUI_EDITOR_CORE "Build the XCUI editor product core library" ON)
 option(XCENGINE_BUILD_XCUI_EDITOR_APP "Build the XCUI editor shell app" ON)
-if(XCENGINE_BUILD_XCUI_EDITOR_APP)
+if(XCENGINE_BUILD_XCUI_EDITOR_CORE OR XCENGINE_BUILD_XCUI_EDITOR_APP)
     set(XCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT_DEFAULT ON)
 else()
     set(XCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT_DEFAULT OFF)
diff --git a/docs/plan/editor-core-refactor-plan.md b/docs/plan/editor-core-refactor-plan.md
index 952cdc72..3d151973 100644
--- a/docs/plan/editor-core-refactor-plan.md
+++ b/docs/plan/editor-core-refactor-plan.md
@@ -105,6 +105,10 @@ Completed boundary cuts:
   explicit app module include roots. It restores project panel, scene viewport,
   viewport render-plan, viewport object-id picker, and app input-routing tests
   without widening the include surface.
+- `XCEditorCore` is now controlled by `XCENGINE_BUILD_XCUI_EDITOR_CORE`
+  instead of the executable-host switch. `XCENGINE_BUILD_XCUI_EDITOR_APP=OFF`
+  can still build the product core and the app-facing test targets when
+  renderer editor support is enabled.
 - The old Win32 tab-drag-drop target test now covers the current reusable
   `XCEditor/Docking/UIEditorDockHostTransfer.h` API through
   `editor_windowing_phase1_tests`.
@@ -280,6 +284,8 @@ Completed follow-up:
 
 ## Phase 2: Introduce XCEditorCore
 
+Status: completed and build-graph enforced.
+
 Create:
 
 ```cmake
@@ -316,6 +322,19 @@ Important: do not hide Win32/D3D12 in `XCEditorCore` just to make the first
 CMake edit easier. If a source file needs `windows.h`, it belongs in the host
 side until a neutral interface exists.
 
+Completed target-graph cut:
+
+- `XCENGINE_BUILD_XCUI_EDITOR_CORE` controls `XCEditorCore` and defaults to
+  `ON`.
+- `XCENGINE_BUILD_XCUI_EDITOR_APP` controls only the concrete executable host
+  target `XCEditor`.
+- `XCEditor` requires `XCENGINE_BUILD_XCUI_EDITOR_CORE=ON`.
+- When either the core library or executable host is enabled, the root build
+  defaults `XCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT=ON` because editor core
+  owns viewport services that link renderer editor support.
+- The host-off validation shape is:
+  `-DXCENGINE_BUILD_XCUI_EDITOR_APP=OFF -DXCENGINE_BUILD_XCUI_EDITOR_CORE=ON -DXCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT=ON`.
+
 ## Phase 3: Restore App-Core And App Feature Tests
 
 Status: completed for the initial app-core and app feature/viewport suites.
@@ -512,6 +531,16 @@ cmake --build build --config Debug --target editor_app_feature_tests
 cmake --build build --config Debug --target editor_windowing_phase1_tests
 ```
 
+To prove the product core boundary without the executable host, configure a
+separate build directory with:
+
+```powershell
+cmake -S . -B build/editor_core_hostoff -DXCENGINE_BUILD_XCUI_EDITOR_APP=OFF -DXCENGINE_BUILD_XCUI_EDITOR_CORE=ON -DXCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT=ON
+cmake --build build/editor_core_hostoff --config Debug --target XCEditorCore
+cmake --build build/editor_core_hostoff --config Debug --target editor_app_core_tests
+cmake --build build/editor_core_hostoff --config Debug --target editor_app_feature_tests
+```
+
 When app smoke is available:
 
 ```powershell
diff --git a/editor/AGENTS.md b/editor/AGENTS.md
index 6ec55f83..b2d9260e 100644
--- a/editor/AGENTS.md
+++ b/editor/AGENTS.md
@@ -20,6 +20,11 @@ change.
   should be buildable without the concrete Win32/D3D12 executable host:
   composition, commands, state, project and scene services, feature panels,
   utility-window descriptors, viewport services, and editor window core.
+- `XCEditorCore` is controlled by `XCENGINE_BUILD_XCUI_EDITOR_CORE`, which
+  defaults to `ON` and is independent of `XCENGINE_BUILD_XCUI_EDITOR_APP`.
+  App-off core validation must configure with
+  `XCENGINE_BUILD_XCUI_EDITOR_APP=OFF` while keeping
+  `XCENGINE_BUILD_XCUI_EDITOR_CORE=ON`.
 - `editor_app_core_tests` links `XCEditorCore` directly and is the build
   gate that proves the product-core boundary outside the executable host.
 - `editor_app_feature_tests` also links `XCEditorCore` directly. It restores
@@ -33,9 +38,12 @@ change.
   Its implementation include paths enumerate concrete app module roots such as
   `app/Composition`, `app/Features`, `app/Windowing`, `app/Rendering`,
   `app/Scene`, `app/Services`, and `app/Support`.
+- `XCEditorCore` and `XCEditor` require
+  `XCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT=ON`; the root build defaults that
+  support on when either the core library or app executable is enabled.
 - `XCEditor` is built when `XCENGINE_BUILD_XCUI_EDITOR_APP=ON`; that mode
-  requires `XCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT=ON`. The executable target
-  is named `XCEditor`, but its output name is `XCEngine`.
+  also requires `XCENGINE_BUILD_XCUI_EDITOR_CORE=ON`. The executable target is
+  named `XCEditor`, but its output name is `XCEngine`.
 - The executable target owns process startup, Win32 hosting, D3D12 window
   rendering, native resources, and the final composition-root wiring.
 - Keep the CMake target named `XCEditor` so it does not collide with the engine
@@ -325,6 +333,10 @@ inside pure shell/widget code.
 
 - `XCEditorCore` is the app/product-core target. It exists to make the app
   boundary build-visible before finer-grained directories are cleaned up.
+- `XCEditorCore` is not gated by the executable-host option. Keep
+  `XCENGINE_BUILD_XCUI_EDITOR_CORE` as the switch for product-core builds and
+  `XCENGINE_BUILD_XCUI_EDITOR_APP` as the switch for the concrete executable
+  host.
 - `XCEditorCore` does not use `editor/app` as a private implementation include
   root. Its public usage requirements expose only `app/Core`, `app/Commands`,
   `app/State`, `app/Host/Interfaces`, `include`, and engine headers;
@@ -363,6 +375,11 @@ inside pure shell/widget code.
 - Build product editor core after app composition, service, feature, windowing,
   or host-contract changes:
   `cmake --build <build-dir> --config Debug --target XCEditorCore`.
+- Validate the product-core boundary without the executable host by configuring
+  a separate build with
+  `-DXCENGINE_BUILD_XCUI_EDITOR_APP=OFF -DXCENGINE_BUILD_XCUI_EDITOR_CORE=ON -DXCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT=ON`,
+  then building `XCEditorCore`, `editor_app_core_tests`, and
+  `editor_app_feature_tests`.
 - Build app-core tests after product editor core or app-facing test changes:
   `cmake --build <build-dir> --config Debug --target editor_app_core_tests`.
 - Build app feature/viewport tests after feature panel, scene viewport,
@@ -506,3 +523,7 @@ ownership rule.
   tests use `Viewport/ViewportObjectIdPicker.h`, and the old Win32 tab-drop
   test now covers `XCEditor/Docking/UIEditorDockHostTransfer.h` through
   `editor_windowing_phase1_tests`.
+- `XCEditorCore` is now controlled by `XCENGINE_BUILD_XCUI_EDITOR_CORE`
+  instead of `XCENGINE_BUILD_XCUI_EDITOR_APP`. The executable host option only
+  creates `XCEditor`, and app-off builds can still compile `XCEditorCore`,
+  `editor_app_core_tests`, and `editor_app_feature_tests`.
diff --git a/editor/CMakeLists.txt b/editor/CMakeLists.txt
index 5f4ee135..4ae60c44 100644
--- a/editor/CMakeLists.txt
+++ b/editor/CMakeLists.txt
@@ -20,9 +20,15 @@ function(xcui_editor_apply_common_target_settings target visibility)
 endfunction()
 
 if(XCENGINE_BUILD_XCUI_EDITOR_APP AND
+   NOT XCENGINE_BUILD_XCUI_EDITOR_CORE)
+    message(FATAL_ERROR
+        "XCEditor requires XCEditorCore; set XCENGINE_BUILD_XCUI_EDITOR_CORE=ON")
+endif()
+
+if((XCENGINE_BUILD_XCUI_EDITOR_CORE OR XCENGINE_BUILD_XCUI_EDITOR_APP) AND
    NOT XCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT)
     message(FATAL_ERROR
-        "XCEditor requires XCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT=ON")
+        "XCEditorCore requires XCENGINE_ENABLE_RENDERING_EDITOR_SUPPORT=ON")
 endif()
 
 set(XCUI_EDITOR_FOUNDATION_SOURCES
@@ -209,13 +215,7 @@ set(XCUI_EDITOR_HOST_RENDERING_SOURCES
     app/Host/D3D12/D3D12WindowRenderLoop.cpp
 )
 
-if(XCENGINE_BUILD_XCUI_EDITOR_APP)
-    set(XCUI_EDITOR_APP_BOOTSTRAP_SOURCES
-        app/Bootstrap/EditorApp.rc
-        app/main.cpp
-        app/Bootstrap/Application.cpp
-    )
-
+if(XCENGINE_BUILD_XCUI_EDITOR_CORE)
     set(XCUI_EDITOR_APP_CORE_CONTRACT_SOURCES
         app/Core/UtilityWindows/EditorUtilityWindowRegistry.cpp
     )
@@ -284,19 +284,6 @@ if(XCENGINE_BUILD_XCUI_EDITOR_APP)
         app/Scene/EditorSceneBridge.cpp
     )
 
-    set(XCUI_EDITOR_APP_PLATFORM_SOURCES
-        app/Host/Win32/Diagnostics/Win32CrashTrace.cpp
-        app/Host/Win32/Windowing/EditorWindow.cpp
-        app/Host/Win32/Windowing/EditorFloatingWindowPlacement.cpp
-        app/Host/Win32/Windowing/EditorWindowSession.cpp
-        app/Host/Win32/Chrome/EditorWindowChromeController.cpp
-        app/Host/Win32/Resources/Win32EditorResourceService.cpp
-        app/Host/Win32/Runtime/EditorWindowInputController.cpp
-        app/Host/Win32/System/Win32SystemInteractionHost.cpp
-        app/Host/Win32/Windowing/EditorWindowHostRuntime.cpp
-        app/Host/Win32/Windowing/EditorWindowMessageDispatcher.cpp
-    )
-
     set(XCUI_EDITOR_APP_CORE_SOURCES
         ${XCUI_EDITOR_APP_CORE_CONTRACT_SOURCES}
         ${XCUI_EDITOR_APP_STATE_SOURCES}
@@ -347,6 +334,28 @@ if(XCENGINE_BUILD_XCUI_EDITOR_APP)
         XCEngineRenderingEditorSupport
     )
 
+endif()
+
+if(XCENGINE_BUILD_XCUI_EDITOR_APP)
+    set(XCUI_EDITOR_APP_BOOTSTRAP_SOURCES
+        app/Bootstrap/EditorApp.rc
+        app/main.cpp
+        app/Bootstrap/Application.cpp
+    )
+
+    set(XCUI_EDITOR_APP_PLATFORM_SOURCES
+        app/Host/Win32/Diagnostics/Win32CrashTrace.cpp
+        app/Host/Win32/Windowing/EditorWindow.cpp
+        app/Host/Win32/Windowing/EditorFloatingWindowPlacement.cpp
+        app/Host/Win32/Windowing/EditorWindowSession.cpp
+        app/Host/Win32/Chrome/EditorWindowChromeController.cpp
+        app/Host/Win32/Resources/Win32EditorResourceService.cpp
+        app/Host/Win32/Runtime/EditorWindowInputController.cpp
+        app/Host/Win32/System/Win32SystemInteractionHost.cpp
+        app/Host/Win32/Windowing/EditorWindowHostRuntime.cpp
+        app/Host/Win32/Windowing/EditorWindowMessageDispatcher.cpp
+    )
+
     set(XCUI_EDITOR_APP_HOST_SOURCES
         ${XCUI_EDITOR_HOST_PLATFORM_SOURCES}
         ${XCUI_EDITOR_HOST_RENDERING_SOURCES}
@@ -412,8 +421,6 @@ if(XCENGINE_BUILD_XCUI_EDITOR_APP)
     if(WIN32 AND XCENGINE_ENABLE_PHYSX)
         xcengine_copy_physx_runtime_dlls(XCEditor)
     endif()
-
 endif()
 
 
-