diff --git a/src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs b/src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs
index 404472b..5aa5e16 100644
--- a/src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs
+++ b/src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs
@@ -148,6 +148,9 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase
     // Set by the view so DeleteTaskCommand can prompt yes/no before deleting
     public Func<string, System.Threading.Tasks.Task<bool>>? ConfirmAsync { get; set; }
 
+    // Set by the view so ReviewCombinedDiffCommand can show the planning diff modal
+    public Func<ViewModels.Planning.PlanningDiffViewModel, System.Threading.Tasks.Task>? ShowPlanningDiffModal { get; set; }
+
     // Set by the view so DeleteTaskCommand can show an error message
     public Func<string, System.Threading.Tasks.Task>? ShowErrorAsync { get; set; }
 
@@ -570,8 +573,10 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase
     [RelayCommand(CanExecute = nameof(CanReviewDiff))]
     private async System.Threading.Tasks.Task ReviewCombinedDiffAsync()
     {
-        // TODO(Task 14): open PlanningDiffView once it exists
-        await System.Threading.Tasks.Task.CompletedTask;
+        if (Task is null || ShowPlanningDiffModal is null) return;
+        var vm = new ViewModels.Planning.PlanningDiffViewModel(_worker, Task.Id, SelectedMergeTarget ?? "main");
+        await vm.InitializeAsync();
+        await ShowPlanningDiffModal(vm);
     }
 
     private bool CanReviewDiff() => Task?.IsPlanningParent == true && Subtasks.Any();
diff --git a/src/ClaudeDo.Ui/ViewModels/Planning/PlanningDiffViewModel.cs b/src/ClaudeDo.Ui/ViewModels/Planning/PlanningDiffViewModel.cs
new file mode 100644
index 0000000..485ceed
--- /dev/null
+++ b/src/ClaudeDo.Ui/ViewModels/Planning/PlanningDiffViewModel.cs
@@ -0,0 +1,91 @@
+using System.Collections.ObjectModel;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using ClaudeDo.Ui.Services;
+
+namespace ClaudeDo.Ui.ViewModels.Planning;
+
+public sealed partial class PlanningDiffViewModel : ObservableObject
+{
+    private readonly IWorkerClient _worker;
+    private readonly string _planningTaskId;
+    private readonly string _targetBranch;
+
+    public ObservableCollection<SubtaskDiffRow> Subtasks { get; } = new();
+
+    [ObservableProperty] private SubtaskDiffRow? _selectedSubtask;
+    [ObservableProperty] private string _displayedDiff = "";
+    [ObservableProperty] private bool _isCombinedMode;
+    [ObservableProperty] private string? _combinedWarning;
+    [ObservableProperty] private bool _isLoadingCombined;
+
+    public Action? CloseAction { get; set; }
+
+    public PlanningDiffViewModel(IWorkerClient worker, string planningTaskId, string targetBranch)
+    {
+        _worker = worker;
+        _planningTaskId = planningTaskId;
+        _targetBranch = targetBranch;
+    }
+
+    [RelayCommand]
+    private void Close() => CloseAction?.Invoke();
+
+    public async Task InitializeAsync()
+    {
+        var items = await _worker.GetPlanningAggregateAsync(_planningTaskId);
+        Subtasks.Clear();
+        foreach (var i in items)
+            Subtasks.Add(new SubtaskDiffRow(i.SubtaskId, i.Title, i.DiffStat, i.UnifiedDiff));
+        SelectedSubtask = Subtasks.FirstOrDefault();
+    }
+
+    partial void OnSelectedSubtaskChanged(SubtaskDiffRow? value)
+    {
+        if (!IsCombinedMode)
+            DisplayedDiff = value?.UnifiedDiff ?? "";
+    }
+
+    [RelayCommand]
+    private async Task ToggleCombinedAsync()
+    {
+        if (IsCombinedMode)
+        {
+            IsLoadingCombined = true;
+            try
+            {
+                var result = await _worker.BuildPlanningIntegrationBranchAsync(_planningTaskId, _targetBranch);
+
+                if (result is null)
+                {
+                    DisplayedDiff = "";
+                    CombinedWarning = "Could not build combined preview (hub error).";
+                }
+                else if (result.Success)
+                {
+                    DisplayedDiff = result.UnifiedDiff ?? "";
+                    CombinedWarning = null;
+                }
+                else
+                {
+                    var files = result.ConflictedFiles?.Count ?? 0;
+                    CombinedWarning = $"Cannot build combined preview: subtask {result.FirstConflictSubtaskId} conflicts with an earlier subtask ({files} files).";
+                    DisplayedDiff = "";
+                }
+            }
+            finally
+            {
+                IsLoadingCombined = false;
+            }
+        }
+        else
+        {
+            DisplayedDiff = SelectedSubtask?.UnifiedDiff ?? "";
+            CombinedWarning = null;
+        }
+    }
+
+    partial void OnIsCombinedModeChanged(bool value) => ToggleCombinedCommand.Execute(null);
+}
+
+public sealed record SubtaskDiffRow(string Id, string Title, string? DiffStat, string UnifiedDiff);
diff --git a/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs b/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs
index 62016b7..9b2816f 100644
--- a/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs
+++ b/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs
@@ -5,6 +5,7 @@ using Avalonia.Layout;
 using Avalonia.Media;
 using ClaudeDo.Ui.ViewModels.Islands;
 using ClaudeDo.Ui.Views.Modals;
+using ClaudeDo.Ui.Views.Planning;
 
 namespace ClaudeDo.Ui.Views.Islands;
 
@@ -44,6 +45,14 @@ public partial class DetailsIslandView : UserControl
                 await modal.ShowDialog(owner);
             };
 
+            vm.ShowPlanningDiffModal = async (planningDiffVm) =>
+            {
+                var owner = TopLevel.GetTopLevel(this) as Window;
+                if (owner == null) return;
+                var modal = new PlanningDiffView { DataContext = planningDiffVm };
+                await modal.ShowDialog(owner);
+            };
+
             vm.ConfirmAsync = ShowConfirmAsync;
             vm.ShowErrorAsync = ShowErrorDialogAsync;
         }
diff --git a/src/ClaudeDo.Ui/Views/Planning/PlanningDiffView.axaml b/src/ClaudeDo.Ui/Views/Planning/PlanningDiffView.axaml
new file mode 100644
index 0000000..1db85a5
--- /dev/null
+++ b/src/ClaudeDo.Ui/Views/Planning/PlanningDiffView.axaml
@@ -0,0 +1,109 @@
+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:vm="using:ClaudeDo.Ui.ViewModels.Planning"
+        x:Class="ClaudeDo.Ui.Views.Planning.PlanningDiffView"
+        x:DataType="vm:PlanningDiffViewModel"
+        Title="Planning — Combined diff"
+        Width="1100" Height="700"
+        SystemDecorations="None"
+        ExtendClientAreaToDecorationsHint="True"
+        WindowStartupLocation="CenterOwner"
+        Background="{StaticResource SurfaceBrush}">
+
+  <Window.KeyBindings>
+    <KeyBinding Gesture="Escape" Command="{Binding CloseCommand}"/>
+  </Window.KeyBindings>
+
+  <Border Background="{StaticResource SurfaceBrush}"
+          BorderBrush="{StaticResource LineBrush}"
+          BorderThickness="1">
+    <Grid RowDefinitions="36,Auto,*">
+
+      <!-- Title bar / drag handle -->
+      <Border Grid.Row="0"
+              x:Name="TitleBar"
+              Background="{StaticResource Surface2Brush}"
+              BorderBrush="{StaticResource LineBrush}"
+              BorderThickness="0,0,0,1"
+              PointerPressed="TitleBar_PointerPressed">
+        <Grid ColumnDefinitions="*,Auto" Margin="14,0">
+          <TextBlock Text="Planning — Combined diff"
+                     VerticalAlignment="Center"
+                     FontFamily="{StaticResource MonoFamily}"
+                     FontSize="12"
+                     Foreground="{StaticResource TextDimBrush}"/>
+          <Button Grid.Column="1"
+                  Classes="icon-btn"
+                  Content="✕"
+                  FontSize="12"
+                  Command="{Binding CloseCommand}"
+                  VerticalAlignment="Center"/>
+        </Grid>
+      </Border>
+
+      <!-- Toolbar row -->
+      <StackPanel Grid.Row="1"
+                  Orientation="Horizontal"
+                  Spacing="8"
+                  Margin="8,6">
+        <ToggleButton Content="Preview combined" IsChecked="{Binding IsCombinedMode}"/>
+        <TextBlock Text="{Binding CombinedWarning}"
+                   Foreground="Orange"
+                   VerticalAlignment="Center"
+                   IsVisible="{Binding CombinedWarning, Converter={x:Static ObjectConverters.IsNotNull}}"/>
+        <TextBlock Text="Loading…"
+                   VerticalAlignment="Center"
+                   IsVisible="{Binding IsLoadingCombined}"/>
+      </StackPanel>
+
+      <!-- Two-pane body -->
+      <Grid Grid.Row="2" ColumnDefinitions="240,*">
+
+        <!-- Subtask list (left pane) -->
+        <Border Grid.Column="0"
+                BorderBrush="{StaticResource LineBrush}"
+                BorderThickness="0,0,1,0"
+                Background="{StaticResource DeepBrush}">
+          <ListBox ItemsSource="{Binding Subtasks}"
+                   SelectedItem="{Binding SelectedSubtask}"
+                   IsEnabled="{Binding !IsCombinedMode}"
+                   Background="Transparent"
+                   BorderThickness="0"
+                   ScrollViewer.HorizontalScrollBarVisibility="Disabled">
+            <ListBox.ItemTemplate>
+              <DataTemplate x:DataType="vm:SubtaskDiffRow">
+                <Border Padding="10,8" Background="Transparent">
+                  <StackPanel Spacing="2">
+                    <TextBlock Text="{Binding Title}"
+                               FontWeight="SemiBold"
+                               TextTrimming="CharacterEllipsis"/>
+                    <TextBlock Text="{Binding DiffStat}"
+                               Opacity="0.7"
+                               FontFamily="{StaticResource MonoFamily}"
+                               FontSize="11"/>
+                  </StackPanel>
+                </Border>
+              </DataTemplate>
+            </ListBox.ItemTemplate>
+          </ListBox>
+        </Border>
+
+        <!-- Diff content (right pane) -->
+        <Grid Grid.Column="1" Background="{StaticResource VoidBrush}">
+          <ScrollViewer HorizontalScrollBarVisibility="Auto"
+                        VerticalScrollBarVisibility="Auto">
+            <TextBox Text="{Binding DisplayedDiff, Mode=OneWay}"
+                     IsReadOnly="True"
+                     AcceptsReturn="True"
+                     FontFamily="Consolas,Menlo,monospace"
+                     FontSize="12"
+                     Background="Transparent"
+                     BorderThickness="0"
+                     Padding="8"/>
+          </ScrollViewer>
+        </Grid>
+
+      </Grid>
+    </Grid>
+  </Border>
+</Window>
diff --git a/src/ClaudeDo.Ui/Views/Planning/PlanningDiffView.axaml.cs b/src/ClaudeDo.Ui/Views/Planning/PlanningDiffView.axaml.cs
new file mode 100644
index 0000000..7886098
--- /dev/null
+++ b/src/ClaudeDo.Ui/Views/Planning/PlanningDiffView.axaml.cs
@@ -0,0 +1,27 @@
+using Avalonia.Controls;
+using Avalonia.Input;
+using CommunityToolkit.Mvvm.Input;
+using ClaudeDo.Ui.ViewModels.Planning;
+
+namespace ClaudeDo.Ui.Views.Planning;
+
+public partial class PlanningDiffView : Window
+{
+    public PlanningDiffView()
+    {
+        InitializeComponent();
+    }
+
+    protected override void OnDataContextChanged(EventArgs e)
+    {
+        base.OnDataContextChanged(e);
+        if (DataContext is PlanningDiffViewModel vm)
+            vm.CloseAction = Close;
+    }
+
+    private void TitleBar_PointerPressed(object? sender, PointerPressedEventArgs e)
+    {
+        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
+            BeginMoveDrag(e);
+    }
+}
diff --git a/tests/ClaudeDo.Ui.Tests/ViewModels/PlanningDiffViewModelTests.cs b/tests/ClaudeDo.Ui.Tests/ViewModels/PlanningDiffViewModelTests.cs
new file mode 100644
index 0000000..7d693df
--- /dev/null
+++ b/tests/ClaudeDo.Ui.Tests/ViewModels/PlanningDiffViewModelTests.cs
@@ -0,0 +1,163 @@
+using System.ComponentModel;
+using ClaudeDo.Data.Models;
+using ClaudeDo.Ui.Services;
+using ClaudeDo.Ui.ViewModels.Planning;
+
+namespace ClaudeDo.Ui.Tests.ViewModels;
+
+public class PlanningDiffViewModelTests
+{
+    private sealed class FakePlanningWorker : IWorkerClient
+    {
+        public event PropertyChangedEventHandler? PropertyChanged;
+        public event Action<string, string, DateTime>? TaskStartedEvent;
+        public event Action<string, string, string, DateTime>? TaskFinishedEvent;
+        public event Action<string>? TaskUpdatedEvent;
+        public event Action<string>? WorktreeUpdatedEvent;
+        public event Action<string, string>? TaskMessageEvent;
+        public event Action<string, string>? PlanningMergeStartedEvent;
+        public event Action<string, string>? PlanningSubtaskMergedEvent;
+        public event Action<string, string, IReadOnlyList<string>>? PlanningMergeConflictEvent;
+        public event Action<string>? PlanningMergeAbortedEvent;
+        public event Action<string>? PlanningCompletedEvent;
+
+        public bool IsConnected => false;
+
+        public IReadOnlyList<SubtaskDiffDto> AggregateResult { get; set; } = Array.Empty<SubtaskDiffDto>();
+        public CombinedDiffResultDto? CombinedResult { get; set; }
+
+        public Task WakeQueueAsync() => Task.CompletedTask;
+        public Task RunNowAsync(string taskId) => Task.CompletedTask;
+        public Task ContinueTaskAsync(string taskId, string followUpPrompt) => Task.CompletedTask;
+        public Task ResetTaskAsync(string taskId) => Task.CompletedTask;
+        public Task CancelTaskAsync(string taskId) => Task.CompletedTask;
+        public Task<List<AgentInfo>> GetAgentsAsync() => Task.FromResult(new List<AgentInfo>());
+        public Task<ListConfigDto?> GetListConfigAsync(string listId) => Task.FromResult<ListConfigDto?>(null);
+        public Task UpdateTaskAgentSettingsAsync(UpdateTaskAgentSettingsDto dto) => Task.CompletedTask;
+        public Task StartPlanningSessionAsync(string taskId, CancellationToken ct = default) => Task.CompletedTask;
+        public Task ResumePlanningSessionAsync(string taskId, CancellationToken ct = default) => Task.CompletedTask;
+        public Task DiscardPlanningSessionAsync(string taskId, CancellationToken ct = default) => Task.CompletedTask;
+        public Task FinalizePlanningSessionAsync(string taskId, bool queueAgentTasks = true, CancellationToken ct = default) => Task.CompletedTask;
+        public Task<int> GetPendingDraftCountAsync(string taskId, CancellationToken ct = default) => Task.FromResult(0);
+        public Task<MergeTargetsDto?> GetMergeTargetsAsync(string taskId) => Task.FromResult<MergeTargetsDto?>(null);
+        public Task<IReadOnlyList<SubtaskDiffDto>> GetPlanningAggregateAsync(string planningTaskId) =>
+            Task.FromResult(AggregateResult);
+        public Task<CombinedDiffResultDto?> BuildPlanningIntegrationBranchAsync(string planningTaskId, string targetBranch) =>
+            Task.FromResult(CombinedResult);
+        public Task MergeAllPlanningAsync(string planningTaskId, string targetBranch) => Task.CompletedTask;
+        public Task ContinuePlanningMergeAsync(string planningTaskId) => Task.CompletedTask;
+        public Task AbortPlanningMergeAsync(string planningTaskId) => Task.CompletedTask;
+    }
+
+    [Fact]
+    public async Task InitializeAsync_PopulatesSubtasks()
+    {
+        var fake = new FakePlanningWorker
+        {
+            AggregateResult = new[]
+            {
+                new SubtaskDiffDto("s1", "First",  "branch-1", "base1", "head1", "+1 -0", "diff1"),
+                new SubtaskDiffDto("s2", "Second", "branch-2", "base2", "head2", "+2 -1", "diff2"),
+            }
+        };
+        var vm = new PlanningDiffViewModel(fake, "plan-1", "main");
+
+        await vm.InitializeAsync();
+
+        Assert.Equal(2, vm.Subtasks.Count);
+        Assert.Equal(vm.Subtasks[0], vm.SelectedSubtask);
+    }
+
+    [Fact]
+    public async Task SelectingSubtask_InGroupedMode_SetsDisplayedDiff()
+    {
+        var fake = new FakePlanningWorker
+        {
+            AggregateResult = new[]
+            {
+                new SubtaskDiffDto("s1", "First",  "b1", "base1", "head1", null, "DIFF-A"),
+                new SubtaskDiffDto("s2", "Second", "b2", "base2", "head2", null, "DIFF-B"),
+            }
+        };
+        var vm = new PlanningDiffViewModel(fake, "plan-1", "main");
+        await vm.InitializeAsync();
+
+        vm.SelectedSubtask = vm.Subtasks[1];
+
+        Assert.Equal("DIFF-B", vm.DisplayedDiff);
+    }
+
+    [Fact]
+    public async Task ToggleCombined_Success_DisplaysUnifiedDiff()
+    {
+        var fake = new FakePlanningWorker
+        {
+            AggregateResult = new[]
+            {
+                new SubtaskDiffDto("s1", "First", "b1", "base1", "head1", null, "DIFF-A"),
+            },
+            CombinedResult = new CombinedDiffResultDto(true, "integration-branch", "COMBINED-DIFF", null, null),
+        };
+        var vm = new PlanningDiffViewModel(fake, "plan-1", "main");
+        await vm.InitializeAsync();
+
+        vm.IsCombinedMode = true;
+
+        // Wait for the async toggle command to complete
+        var deadline = DateTime.UtcNow.AddSeconds(5);
+        while (DateTime.UtcNow < deadline && vm.IsLoadingCombined)
+            await Task.Delay(10);
+
+        Assert.Equal("COMBINED-DIFF", vm.DisplayedDiff);
+        Assert.Null(vm.CombinedWarning);
+    }
+
+    [Fact]
+    public async Task ToggleCombined_Conflict_ShowsWarning()
+    {
+        var fake = new FakePlanningWorker
+        {
+            AggregateResult = new[]
+            {
+                new SubtaskDiffDto("s1", "First", "b1", "base1", "head1", null, "DIFF-A"),
+            },
+            CombinedResult = new CombinedDiffResultDto(false, null, null, "subtask-42", new[] { "a.cs", "b.cs" }),
+        };
+        var vm = new PlanningDiffViewModel(fake, "plan-1", "main");
+        await vm.InitializeAsync();
+
+        vm.IsCombinedMode = true;
+
+        var deadline = DateTime.UtcNow.AddSeconds(5);
+        while (DateTime.UtcNow < deadline && vm.IsLoadingCombined)
+            await Task.Delay(10);
+
+        Assert.NotNull(vm.CombinedWarning);
+        Assert.Contains("subtask-42", vm.CombinedWarning);
+        Assert.Contains("2 files", vm.CombinedWarning);
+    }
+
+    [Fact]
+    public async Task ToggleCombined_HubReturnsNull_ShowsError()
+    {
+        var fake = new FakePlanningWorker
+        {
+            AggregateResult = new[]
+            {
+                new SubtaskDiffDto("s1", "First", "b1", "base1", "head1", null, "DIFF-A"),
+            },
+            CombinedResult = null,
+        };
+        var vm = new PlanningDiffViewModel(fake, "plan-1", "main");
+        await vm.InitializeAsync();
+
+        vm.IsCombinedMode = true;
+
+        var deadline = DateTime.UtcNow.AddSeconds(5);
+        while (DateTime.UtcNow < deadline && vm.IsLoadingCombined)
+            await Task.Delay(10);
+
+        Assert.NotNull(vm.CombinedWarning);
+        Assert.NotEmpty(vm.CombinedWarning!);
+    }
+}