src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs

@@ -139,6 +139,9 @@ 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 DeleteTaskCommand can show an error message
+    public Func<string, System.Threading.Tasks.Task>? ShowErrorAsync { get; set; }
+
     public DetailsIslandViewModel(IDbContextFactory<ClaudeDoDbContext> dbFactory, WorkerClient worker, IServiceProvider services)
     {
         _dbFactory = dbFactory;
@@ -537,9 +540,20 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase
             var ok = await ConfirmAsync($"Delete \"{row.Title}\"? This cannot be undone.");
             if (!ok) return;
         }
+        try
+        {
             await using var ctx = _dbFactory.CreateDbContext();
             var repo = new TaskRepository(ctx);
             await repo.DeleteAsync(row.Id);
+        }
+        catch (DbUpdateException ex) when (
+               ex.Message.Contains("FOREIGN KEY", StringComparison.OrdinalIgnoreCase)
+            || ex.InnerException?.Message.Contains("FOREIGN KEY", StringComparison.OrdinalIgnoreCase) == true)
+        {
+            if (ShowErrorAsync != null)
+                await ShowErrorAsync("This task has child tasks. Discard the planning session or delete child tasks first.");
+            return;
+        }
         if (DeleteFromList != null)
             await DeleteFromList(row);
         CloseDetail?.Invoke();

src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs

@@ -45,9 +45,49 @@ public partial class DetailsIslandView : UserControl
             };
 
             vm.ConfirmAsync = ShowConfirmAsync;
+            vm.ShowErrorAsync = ShowErrorDialogAsync;
         }
     }
 
+    private async System.Threading.Tasks.Task ShowErrorDialogAsync(string message)
+    {
+        var owner = TopLevel.GetTopLevel(this) as Window;
+        if (owner == null) return;
+
+        var ok = new Button { Content = "OK", MinWidth = 90 };
+
+        var dialog = new Window
+        {
+            Title = "Error",
+            Width = 360,
+            SizeToContent = SizeToContent.Height,
+            CanResize = false,
+            WindowStartupLocation = WindowStartupLocation.CenterOwner,
+            ShowInTaskbar = false,
+            Background = this.FindResource("SurfaceBrush") as IBrush,
+            Content = new StackPanel
+            {
+                Spacing = 16,
+                Margin = new Thickness(20),
+                Children =
+                {
+                    new TextBlock { Text = message, TextWrapping = TextWrapping.Wrap },
+                    new StackPanel
+                    {
+                        Orientation = Orientation.Horizontal,
+                        Spacing = 8,
+                        HorizontalAlignment = HorizontalAlignment.Right,
+                        Children = { ok }
+                    }
+                }
+            }
+        };
+
+        ok.Click += (_, _) => dialog.Close();
+
+        await dialog.ShowDialog(owner);
+    }
+
     private async System.Threading.Tasks.Task<bool> ShowConfirmAsync(string message)
     {
         var owner = TopLevel.GetTopLevel(this) as Window;