diff --git a/AppKit/CPSplitViewController.j b/AppKit/CPSplitViewController.j
new file mode 100644
index 000000000..fe23f53df
--- /dev/null
+++ b/AppKit/CPSplitViewController.j
@@ -0,0 +1,388 @@
+/*
+ * CPSplitViewController.j
+ *
+ * Created by Daniel Boehringer on September 2, 2025.
+ * Copyright (c) 2025 Daniel Boehringer. All rights reserved.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+@import "CPViewController.j"
+@import "CPSplitView.j"
+
+var CPSplitViewControllerAutomaticDimension = -1.0;
+
+/**
+ * A container view controller that manages two or more child view
+ * controllers in a split view interface.
+ *
+ * This class provides a controller-level abstraction for CPSplitView,
+ * managing the addition, removal, and arrangement of view controllers
+ * through CPSplitViewItem instances.
+ *
+ * This implementation was synthesized with the assistance of an LLM,
+ * directed by the author.
+ */
+@implementation CPSplitViewController : CPViewController <CPSplitViewDelegate>
+{
+    /** @private The underlying split view that arranges the child views. */
+    CPSplitView         _splitView;
+
+    /** @private An array of CPSplitViewItem objects managed by this controller. */
+    CPMutableArray      _splitViewItems;
+
+    /** @private The minimum thickness for sidebars to be displayed inline. */
+    CPNumber            _minimumThicknessForInlineSidebars;
+}
+
+// MARK: - Initialization
+
+- (id)init
+{
+    if (self = [super init])
+    {
+        _splitViewItems = [CPMutableArray array];
+        _minimumThicknessForInlineSidebars = 20.0;
+    }
+    return self;
+}
+
+// MARK: - View Lifecycle
+
+- (void)loadView
+{
+    if (!_splitView)
+    {
+        _splitView = [[CPSplitView alloc] initWithFrame:CGRectMake(0,0,400,400)];
+        [_splitView setDelegate:self];
+    }
+    [self setView:_splitView];
+}
+
+- (void)viewDidLoad
+{
+    [super viewDidLoad];
+    for (var i = 0; i < [_splitViewItems count]; i++)
+    {
+        var viewController = [[_splitViewItems objectAtIndex:i] viewController];
+        [[self splitView] addArrangedSubview:[viewController view]];
+    }
+}
+
+// MARK: - Accessors
+
+/**
+ * Returns the CPSplitView instance managed by the controller.
+ *
+ * @returns {CPSplitView} The split view.
+ */
+- (CPSplitView)splitView
+{
+    return _splitView;
+}
+
+/**
+ * Sets a custom split view for the controller.
+ *
+ * @param {CPSplitView} splitView The custom split view to use.
+ */
+- (void)setSplitView:(CPSplitView)splitView
+{
+    if (_splitView !== splitView)
+    {
+        _splitView = splitView;
+        [_splitView setDelegate:self];
+        if ([self isViewLoaded])
+            [self setView:_splitView];
+    }
+}
+
+/**
+ * Returns the array of split view items.
+ *
+ * @returns {CPArray} The array of CPSplitViewItem objects.
+ */
+- (CPArray)splitViewItems
+{
+    return _splitViewItems;
+}
+
+/**
+ * Sets the array of split view items, replacing any existing items.
+ *
+ * @param {CPArray} splitViewItems An array of CPSplitViewItem objects.
+ */
+- (void)setSplitViewItems:(CPArray)splitViewItems
+{
+    // Remove all existing items
+    while ([_splitViewItems count] > 0)
+    {
+        [self removeSplitViewItem:[_splitViewItems lastObject]];
+    }
+
+    // Add new items
+    for (var i = 0; i < [splitViewItems count]; i++)
+    {
+        [self addSplitViewItem:[splitViewItems objectAtIndex:i]];
+    }
+}
+
+// MARK: - Managing Split View Items
+
+/**
+ * Adds a split view item to the end of the split view.
+ *
+ * @param {CPSplitViewItem} splitViewItem The split view item to add.
+ */
+- (void)addSplitViewItem:(CPSplitViewItem)splitViewItem
+{
+    [self insertSplitViewItem:splitViewItem atIndex:[_splitViewItems count]];
+}
+
+/**
+ * Inserts a split view item at a specific index.
+ *
+ * @param {CPSplitViewItem} splitViewItem The split view item to insert.
+ * @param {CPInteger} index The zero-based index at which to insert the item.
+ */
+- (void)insertSplitViewItem:(CPSplitViewItem)splitViewItem atIndex:(CPInteger)index
+{
+    [splitViewItem _setSplitViewController:self];
+    [_splitViewItems insertObject:splitViewItem atIndex:index];
+    [self addChildViewController:[splitViewItem viewController]];
+
+    if ([self isViewLoaded])
+        [[self splitView] insertArrangedSubview:[[splitViewItem viewController] view] atIndex:index];
+}
+
+/**
+ * Removes the specified split view item.
+ *
+ * @param {CPSplitViewItem} splitViewItem The split view item to remove.
+ */
+- (void)removeSplitViewItem:(CPSplitViewItem)splitViewItem
+{
+    var viewController = [splitViewItem viewController];
+    if ([self isViewLoaded])
+        [[viewController view] removeFromSuperview];
+
+    [viewController removeFromParentViewController];
+    [splitViewItem _setSplitViewController:nil];
+    [_splitViewItems removeObject:splitViewItem];
+}
+
+/**
+ * Retrieves the split view item associated with a given view controller.
+ *
+ * @param {CPViewController} viewController The view controller to find.
+ * @returns {CPSplitViewItem | null} The corresponding split view item, or nil if not found.
+ */
+- (CPSplitViewItem)splitViewItemForViewController:(CPViewController)viewController
+{
+    for (var i = 0; i < [_splitViewItems count]; i++)
+    {
+        var item = [_splitViewItems objectAtIndex:i];
+        if ([item viewController] === viewController)
+            return item;
+    }
+    return nil;
+}
+
+// MARK: - Managing Sidebars and Inspectors
+
+/**
+ * Toggles the collapsed state of the first split view item, typically a sidebar.
+ *
+ * @param {id} sender The object that initiated the action.
+ */
+- (void)toggleSidebar:(id)sender
+{
+    if ([_splitViewItems count] > 0)
+    {
+        var sidebarItem = [_splitViewItems objectAtIndex:0];
+        [sidebarItem setCollapsed:![sidebarItem isCollapsed]];
+    }
+}
+
+/**
+ * Toggles the collapsed state of the last split view item, typically an inspector.
+ *
+ * @param {id} sender The object that initiated the action.
+ */
+- (void)toggleInspector:(id)sender
+{
+    if ([_splitViewItems count] > 1)
+    {
+        var inspectorItem = [_splitViewItems lastObject];
+        [inspectorItem setCollapsed:![inspectorItem isCollapsed]];
+    }
+}
+
+/**
+ * Returns the minimum thickness for sidebars to be displayed inline.
+ *
+ * @returns {CPNumber} The minimum thickness.
+ */
+- (CPNumber)minimumThicknessForInlineSidebars
+{
+    return _minimumThicknessForInlineSidebars;
+}
+
+// MARK: - CPSplitViewDelegate Methods
+
+// Note: A more complete implementation would forward these delegate methods
+// to a separate delegate property on the CPSplitViewController itself.
+// For now, these are stubbed to demonstrate where they would be handled.
+
+- (BOOL)splitView:(CPSplitView)splitView canCollapseSubview:(CPView)subview
+{
+    // Default behavior: allow all subviews to be collapsed.
+    return YES;
+}
+
+- (CGFloat)splitView:(CPSplitView)splitView constrainMinCoordinate:(CGFloat)proposedMinimumPosition ofSubviewAt:(CPInteger)dividerIndex
+{
+    return proposedMinimumPosition;
+}
+
+- (CGFloat)splitView:(CPSplitView)splitView constrainMaxCoordinate:(CGFloat)proposedMaximumPosition ofSubviewAt:(CPInteger)dividerIndex
+{
+    return proposedMaximumPosition;
+}
+
+- (CGRect)splitView:(CPSplitView)splitView effectiveRect:(CGRect)proposedEffectiveRect forDrawnRect:(CGRect)drawnRect ofDividerAtIndex:(CPInteger)dividerIndex
+{
+    // Default behavior: return the proposed rectangle.
+    // This can be overridden to provide a larger or custom hit area for the divider.
+    return proposedEffectiveRect;
+}
+
+- (void)splitViewDidResizeSubviews:(CPNotification)notification
+{
+    // Can be used to respond to user-initiated resizing.
+}
+
+@end
+
+/**
+ * An object that manages a view controller within a CPSplitViewController.
+ *
+ * A CPSplitViewItem acts as a wrapper around a CPViewController,
+ * maintaining properties like its collapsed state within the parent
+ * split view controller.
+ */
+@implementation CPSplitViewItem : CPObject
+{
+    /** @private The view controller managed by this item. */
+    CPViewController        _viewController;
+
+    /** @private A boolean indicating whether the item is collapsed. */
+    BOOL                    _isCollapsed;
+
+    /** @private A weak reference to the owning split view controller. */
+    CPSplitViewController   _splitViewController;
+}
+
+// MARK: - Class Methods
+
+/**
+ * Creates and returns a new split view item with the specified view controller.
+ *
+ * @param {CPViewController} viewController The view controller for the new item.
+ * @returns {instancetype} A new CPSplitViewItem instance.
+ */
++ (instancetype)splitViewItemWithViewController:(CPViewController)viewController
+{
+    return [[self alloc] initWithViewController:viewController];
+}
+
+// MARK: - Initialization
+
+/**
+ * Initializes a new split view item with the specified view controller.
+ *
+ * @param {CPViewController} viewController The view controller for the new item.
+ * @returns {id} The initialized CPSplitViewItem instance.
+ */
+- (id)initWithViewController:(CPViewController)viewController
+{
+    if (self = [super init])
+    {
+        _viewController = viewController;
+        _isCollapsed = NO;
+    }
+    return self;
+}
+
+// MARK: - Accessors
+
+/**
+ * Returns the view controller associated with the item.
+ *
+ * @returns {CPViewController} The associated view controller.
+ */
+- (CPViewController)viewController
+{
+    return _viewController;
+}
+
+/**
+ * Returns a boolean value indicating whether the item is collapsed.
+ *
+ * @returns {BOOL} YES if the item is collapsed, otherwise NO.
+ */
+- (BOOL)isCollapsed
+{
+    return _isCollapsed;
+}
+
+/**
+ * Sets the collapsed state of the item.
+ * When collapsed, the view controller's view is hidden.
+ *
+ * @param {BOOL} shouldCollapse YES to collapse the item, NO to expand it.
+ */
+- (void)setCollapsed:(BOOL)shouldCollapse
+{
+    if (_isCollapsed === shouldCollapse)
+        return;
+
+    _isCollapsed = shouldCollapse;
+    [[_viewController view] setHidden:shouldCollapse];
+}
+
+/**
+ * Returns the split view controller that owns this item.
+ *
+ * @returns {CPSplitViewController | null} The parent split view controller.
+ */
+- (CPSplitViewController)splitViewController
+{
+    return _splitViewController;
+}
+
+/**
+ * @private
+ * Sets the owning split view controller. This method is for internal use by
+ * CPSplitViewController.
+ *
+ * @param {CPSplitViewController} splitViewController The parent controller.
+ */
+- (void)_setSplitViewController:(CPSplitViewController)splitViewController
+{
+    _splitViewController = splitViewController;
+}
+
+@end
diff --git a/Tests/AppKit/CPSplitViewControllerTests.j b/Tests/AppKit/CPSplitViewControllerTests.j
new file mode 100644
index 000000000..464dfae36
--- /dev/null
+++ b/Tests/AppKit/CPSplitViewControllerTests.j
@@ -0,0 +1,186 @@
+@import <AppKit/CPSplitViewController.j>
+@import <AppKit/CPViewController.j>
+
+// MARK: - Test Implementation
+
+@implementation CPSplitViewControllerTest : OJTestCase
+{
+    // Instance variables can be declared here if needed for tests.
+}
+
+// MARK: - Initialization Tests
+
+- (void)testInit
+{
+    var splitViewController = [[CPSplitViewController alloc] init];
+
+    [self assertNotNil:splitViewController message:@"CPSplitViewController should initialize."];
+    [self assertNotNil:[splitViewController splitViewItems] message:@"splitViewItems should be an empty array on init."];
+    [self assert:[[splitViewController splitViewItems] count] equals:0 message:@"splitViewItems should have 0 items on init."];
+}
+
+- (void)testViewLoading
+{
+    var splitViewController = [[CPSplitViewController alloc] init];
+
+    // Accessing the view property triggers loadView
+    var view = [splitViewController view];
+
+    [self assertNotNil:view message:@"The controller's view should be created."];
+    [self assertTrue:[view isKindOfClass:[CPSplitView class]] message:@"The controller's view should be a CPSplitView."];
+    [self assert:[splitViewController splitView] equals:view message:@"The splitView accessor should return the controller's view."];
+}
+
+- (void)testSplitViewItemInit
+{
+    var childVC = [[CPViewController alloc] init];
+    var item = [CPSplitViewItem splitViewItemWithViewController:childVC];
+
+    [self assertNotNil:item message:@"CPSplitViewItem should be created."];
+    [self assert:[item viewController] equals:childVC message:@"The item should hold the correct view controller."];
+    [self assertFalse:[item isCollapsed] message:@"A new item should not be collapsed."];
+}
+
+// MARK: - Item Management Tests
+
+- (void)testAddSplitViewItem
+{
+    var splitViewController = [[CPSplitViewController alloc] init];
+    [splitViewController view]; // Ensure view is loaded
+
+    var childVC1 = [[CPViewController alloc] init];
+    var item1 = [CPSplitViewItem splitViewItemWithViewController:childVC1];
+
+    [splitViewController addSplitViewItem:item1];
+
+    [self assert:[[splitViewController splitViewItems] count] equals:1 message:@"Should have 1 split view item."];
+    [self assert:[splitViewController splitViewItemForViewController:childVC1] equals:item1 message:@"Should be able to retrieve item by its view controller."];
+    [self assert:[item1 splitViewController] equals:splitViewController message:@"Item should have a reference to its parent controller."];
+    [self assert:[[[splitViewController splitView] arrangedSubviews] count] equals:1 message:@"The splitView should have one arranged subview."];
+    [self assert:[[[childVC1 view] superview] equals:[splitViewController splitView]] message:@"Child's view should be a subview of the splitView."];
+}
+
+- (void)testInsertSplitViewItem
+{
+    var splitViewController = [[CPSplitViewController alloc] init];
+    [splitViewController view];
+
+    var childVC1 = [[CPViewController alloc] init];
+    var childVC2 = [[CPViewController alloc] init];
+    var item1 = [CPSplitViewItem splitViewItemWithViewController:childVC1];
+    var item2 = [CPSplitViewItem splitViewItemWithViewController:childVC2];
+
+    [splitViewController addSplitViewItem:item1];
+    [splitViewController insertSplitViewItem:item2 atIndex:0];
+
+    [self assert:[[splitViewController splitViewItems] count] equals:2 message:@"Should have 2 split view items."];
+    [self assert:[[splitViewController splitViewItems] objectAtIndex:0] equals:item2 message:@"Item2 should be at index 0."];
+
+    var arrangedSubviews = [[splitViewController splitView] arrangedSubviews];
+    [self assert:[arrangedSubviews objectAtIndex:0] equals:[childVC2 view] message:@"The view of item2 should be the first arranged subview."];
+}
+
+- (void)testRemoveSplitViewItem
+{
+    var splitViewController = [[CPSplitViewController alloc] init];
+    [splitViewController view];
+
+    var childVC1 = [[CPViewController alloc] init];
+    var item1 = [CPSplitViewItem splitViewItemWithViewController:childVC1];
+
+    [splitViewController addSplitViewItem:item1];
+    [self assert:[[splitViewController splitViewItems] count] equals:1 message:@"Pre-condition: Should have 1 item."];
+
+    [splitViewController removeSplitViewItem:item1];
+
+    [self assert:[[splitViewController splitViewItems] count] equals:0 message:@"Should have 0 split view items after removal."];
+    [self assertNil:[item1 splitViewController] message:@"Item's reference to parent should be nil after removal."];
+    [self assert:[[[splitViewController splitView] arrangedSubviews] count] equals:0 message:@"The splitView should have no arranged subviews after removal."];
+    [self assertNil:[[childVC1 view] superview] message:@"Child's view should be removed from the splitView."];
+}
+
+- (void)testAddingItemsBeforeViewLoads
+{
+    var splitViewController = [[CPSplitViewController alloc] init];
+
+    var childVC1 = [[CPViewController alloc] init];
+    var childVC2 = [[CPViewController alloc] init];
+    var item1 = [CPSplitViewItem splitViewItemWithViewController:childVC1];
+    var item2 = [CPSplitViewItem splitViewItemWithViewController:childVC2];
+
+    [splitViewController addSplitViewItem:item1];
+    [splitViewController addSplitViewItem:item2];
+
+    [self assert:[[[splitViewController childViewControllers] objectAtIndex:0] equals:childVC1] message:@"Child view controllers should be added immediately."];
+
+    // View is not loaded yet, so it should have no arranged subviews
+    [self assertNil:[splitViewController _splitView] message:@"SplitView should not exist yet."];
+
+    // Trigger viewDidLoad
+    [splitViewController view];
+
+    // Now check if the views were added
+    var arrangedSubviews = [[splitViewController splitView] arrangedSubviews];
+    [self assert:[arrangedSubviews count] equals:2 message:@"SplitView should have 2 subviews after loading."];
+    [self assert:[arrangedSubviews objectAtIndex:0] equals:[childVC1 view]];
+    [self assert:[arrangedSubviews objectAtIndex:1] equals:[childVC2 view]];
+}
+
+
+// MARK: - Toggling and State Tests
+
+- (void)testToggleSidebar
+{
+    var splitViewController = [[CPSplitViewController alloc] init];
+    var childVC1 = [[CPViewController alloc] init];
+    var item1 = [CPSplitViewItem splitViewItemWithViewController:childVC1];
+    [splitViewController addSplitViewItem:item1];
+
+    [splitViewController toggleSidebar:self];
+    [self assertTrue:[item1 isCollapsed] message:@"Sidebar item should be collapsed."];
+    [self assertTrue:[[childVC1 view] isHidden] message:@"Sidebar view should be hidden."];
+
+    [splitViewController toggleSidebar:self];
+    [self assertFalse:[item1 isCollapsed] message:@"Sidebar item should be expanded again."];
+    [self assertFalse:[[childVC1 view] isHidden] message:@"Sidebar view should be visible again."];
+}
+
+- (void)testToggleInspector
+{
+    var splitViewController = [[CPSplitViewController alloc] init];
+    var childVC1 = [[CPViewController alloc] init];
+    var childVC2 = [[CPViewController alloc] init];
+    var item1 = [CPSplitViewItem splitViewItemWithViewController:childVC1];
+    var item2 = [CPSplitViewItem splitViewItemWithViewController:childVC2];
+    [splitViewController addSplitViewItem:item1];
+    [splitViewController addSplitViewItem:item2];
+
+    [splitViewController toggleInspector:self];
+    [self assertTrue:[item2 isCollapsed] message:@"Inspector item should be collapsed."];
+    [self assertTrue:[[childVC2 view] isHidden] message:@"Inspector view should be hidden."];
+
+    [splitViewController toggleInspector:self];
+    [self assertFalse:[item2 isCollapsed] message:@"Inspector item should be expanded again."];
+    [self assertFalse:[[childVC2 view] isHidden] message:@"Inspector view should be visible again."];
+}
+
+- (void)testSetCollapsed
+{
+    var childVC = [[CPViewController alloc] init];
+    var item = [CPSplitViewItem splitViewItemWithViewController:childVC];
+    var childView = [childVC view];
+
+    [item setCollapsed:YES];
+    [self assertTrue:[item isCollapsed] message:@"Item's collapsed property should be YES."];
+    [self assertTrue:[childView isHidden] message:@"The item's view should be hidden."];
+
+    [item setCollapsed:NO];
+    [self assertFalse:[item isCollapsed] message:@"Item's collapsed property should be NO."];
+    [self assertFalse:[childView isHidden] message:@"The item's view should be visible."];
+
+    // Test idempotency
+    [item setCollapsed:NO];
+    [self assertFalse:[item isCollapsed] message:@"Setting the same value should have no effect."];
+}
+
+@end
\ No newline at end of file