feat(attachments): drag-and-drop file attachments on the detail pane

Drop a file anywhere on the detail pane to attach it: pane-wide drop target
with a 'Drop to attach' hover overlay (Copy cursor, gated on an idle selected
task), an explicit lingering confirmation/error line, plus an Attachments list
with size, remove, and an Add file… picker in the DETAILS card. ComposedPreview
now shows the reference files too. en/de keys added.

src/ClaudeDo.Localization/locales/de.json

@@ -182,7 +182,17 @@
     "descriptionPlaceholder": "Aufgabendetails hinzufügen (Markdown unterstützt)...",
     "prepTitle": "Tagesvorbereitung",
     "planDay": "Tag planen",
-    "prepEmpty": "Heute noch keine Vorbereitung — klick Tag planen"
+    "prepEmpty": "Heute noch keine Vorbereitung — klick Tag planen",
+    "attachments": {
+      "sectionLabel": "ANHÄNGE",
+      "dropToAttach": "Zum Anhängen ablegen",
+      "addFile": "Datei hinzufügen…",
+      "removeTip": "Anhang entfernen",
+      "addedSummary": "✓ Hinzugefügt: {0} ({1} Datei(en))",
+      "overLimitError": "Konnte {0} nicht hinzufügen: {1}",
+      "invalidNameError": "Konnte {0} nicht hinzufügen: {1}",
+      "selectIdleTask": "Zuerst eine inaktive Aufgabe auswählen"
+    }
   },
   "agent": {
     "stopTip": "Agent stoppen",

src/ClaudeDo.Localization/locales/en.json

@@ -182,7 +182,17 @@
     "descriptionPlaceholder": "Add task details (markdown supported)...",
     "prepTitle": "Daily prep",
     "planDay": "Plan day",
-    "prepEmpty": "No prep run today yet — click Plan day"
+    "prepEmpty": "No prep run today yet — click Plan day",
+    "attachments": {
+      "sectionLabel": "ATTACHMENTS",
+      "dropToAttach": "Drop to attach",
+      "addFile": "Add file…",
+      "removeTip": "Remove attachment",
+      "addedSummary": "✓ Added {0} ({1} file(s))",
+      "overLimitError": "Could not add {0}: {1}",
+      "invalidNameError": "Could not add {0}: {1}",
+      "selectIdleTask": "Select an idle task first"
+    }
   },
   "agent": {
     "stopTip": "Stop agent",

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

@@ -12,6 +12,7 @@ using ClaudeDo.Ui.Services.Interfaces;
 using ClaudeDo.Ui.ViewModels.Modals;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.DependencyInjection;
+using System.IO;
 
 namespace ClaudeDo.Ui.ViewModels.Islands;
 
@@ -138,7 +139,10 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
 
     public string ComposedPreview =>
         ClaudeDo.Data.TaskPromptComposer.Compose(
-            EditableTitle, EditableDescription, Subtasks.Select(s => (s.Title, s.Done)));
+            EditableTitle, EditableDescription, Subtasks.Select(s => (s.Title, s.Done)),
+            Task is not null
+                ? Attachments.Select(a => Path.Combine(new AttachmentStore().TaskDir(Task.Id), a.FileName))
+                : null);
 
     [ObservableProperty]
     [NotifyPropertyChangedFor(nameof(IsOutputTab))]
@@ -257,6 +261,7 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
         OnPropertyChanged(nameof(ShowRoadblockCard));
         AgentSettings.IsRunning = IsRunning;
         NotifySessionSections();
+        OnPropertyChanged(nameof(CanAcceptDrop));
     }
 
     [ObservableProperty] private string? _model;
@@ -293,6 +298,12 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
     public ObservableCollection<LogLineViewModel> Log { get; } = new();
     public ObservableCollection<SubtaskRowViewModel> Subtasks { get; } = new();
     public ObservableCollection<ChildOutcomeRowViewModel> ChildOutcomes { get; } = new();
+    public ObservableCollection<AttachmentRowViewModel> Attachments { get; } = new();
+
+    [ObservableProperty] private bool _isDragOver;
+    [ObservableProperty] private string? _dropStatus;
+
+    public bool CanAcceptDrop => Task is not null && !Task.IsRunning;
     public bool HasChildOutcomes => ChildOutcomes.Count > 0;
 
     public int ChildrenNeedingAttention => ChildOutcomes.Count(c =>
@@ -601,6 +612,8 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
         Log.Clear();
         Subtasks.Clear();
         ChildOutcomes.Clear();
+        Attachments.Clear();
+        DropStatus = null;
         OnPropertyChanged(nameof(HasChildOutcomes));
         SessionOutcome = null;
         Roadblocks = null;
@@ -683,6 +696,12 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
             foreach (var s in subs)
                 Subtasks.Add(new SubtaskRowViewModel { Id = s.Id, Title = s.Title, Done = s.Completed });
 
+            var attachmentRepo = new TaskAttachmentRepository(ctx);
+            var attachments = await attachmentRepo.ListByTaskIdAsync(row.Id, ct);
+            ct.ThrowIfCancellationRequested();
+            foreach (var a in attachments)
+                Attachments.Add(new AttachmentRowViewModel { FileName = a.FileName, ByteSize = a.ByteSize });
+
             if (entity.PlanningPhase != ClaudeDo.Data.Models.PlanningPhase.None)
                 await LoadPlanningChildrenAsync(row.Id, ct);
             await LoadChildOutcomesAsync(row.Id, ct);
@@ -931,6 +950,7 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
     {
         Merge.SyncTaskContext(Task?.Id, Task?.Title, Task?.IsPlanningParent == true);
         NotifySessionSections();
+        OnPropertyChanged(nameof(CanAcceptDrop));
     }
 
     [RelayCommand]
@@ -1196,6 +1216,106 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
         catch { /* stale review action; broadcast reconciles */ }
     }
 
+    private async System.Threading.Tasks.Task ReloadAttachmentsAsync()
+    {
+        if (Task is null) return;
+        try
+        {
+            await using var ctx = await _dbFactory.CreateDbContextAsync();
+            var attachments = await new TaskAttachmentRepository(ctx).ListByTaskIdAsync(Task.Id);
+            Attachments.Clear();
+            foreach (var a in attachments)
+                Attachments.Add(new AttachmentRowViewModel { FileName = a.FileName, ByteSize = a.ByteSize });
+            OnPropertyChanged(nameof(ComposedPreview));
+        }
+        catch { /* best-effort */ }
+    }
+
+    public async System.Threading.Tasks.Task AddFilesAsync(IReadOnlyList<(string FileName, Stream Content)> files)
+    {
+        if (Task is null || Task.IsRunning)
+        {
+            DropStatus = Loc.T("details.attachments.selectIdleTask");
+            return;
+        }
+
+        var store = new AttachmentStore();
+        var successes = new List<string>();
+        var failures = new List<string>();
+
+        foreach (var (fileName, content) in files)
+        {
+            try
+            {
+                var byteSize = await store.SaveAsync(Task.Id, fileName, content);
+                await using var ctx = await _dbFactory.CreateDbContextAsync();
+                var repo = new TaskAttachmentRepository(ctx);
+                var existing = await repo.GetAsync(Task.Id, fileName);
+                if (existing is not null)
+                {
+                    existing.ByteSize = byteSize;
+                    await repo.UpdateAsync(existing);
+                }
+                else
+                {
+                    await repo.AddAsync(new ClaudeDo.Data.Models.TaskAttachmentEntity
+                    {
+                        Id = Guid.NewGuid().ToString(),
+                        TaskId = Task.Id,
+                        FileName = fileName,
+                        ByteSize = byteSize,
+                        CreatedAt = DateTime.UtcNow,
+                    });
+                }
+                successes.Add(fileName);
+            }
+            catch (InvalidOperationException ex)
+            {
+                failures.Add(string.Format(Loc.T("details.attachments.overLimitError"), fileName, ex.Message));
+            }
+            catch (ArgumentException ex)
+            {
+                failures.Add(string.Format(Loc.T("details.attachments.invalidNameError"), fileName, ex.Message));
+            }
+            catch (Exception ex)
+            {
+                failures.Add($"{fileName}: {ex.Message}");
+            }
+        }
+
+        await ReloadAttachmentsAsync();
+
+        if (failures.Count == 0)
+        {
+            var names = string.Join(", ", successes);
+            DropStatus = string.Format(Loc.T("details.attachments.addedSummary"), names, successes.Count);
+        }
+        else if (successes.Count == 0)
+        {
+            DropStatus = string.Join(" · ", failures);
+        }
+        else
+        {
+            var names = string.Join(", ", successes);
+            var addedPart = string.Format(Loc.T("details.attachments.addedSummary"), names, successes.Count);
+            DropStatus = addedPart + " · " + string.Join(" · ", failures);
+        }
+    }
+
+    [RelayCommand]
+    private async System.Threading.Tasks.Task RemoveAttachment(AttachmentRowViewModel? row)
+    {
+        if (row is null || Task is null) return;
+        try
+        {
+            new AttachmentStore().DeleteFile(Task.Id, row.FileName);
+            await using var ctx = await _dbFactory.CreateDbContextAsync();
+            await new TaskAttachmentRepository(ctx).DeleteAsync(Task.Id, row.FileName);
+            await ReloadAttachmentsAsync();
+        }
+        catch { /* best-effort */ }
+    }
+
     internal static (int Additions, int Deletions) ParseDiffStat(string? stat)
     {
         if (string.IsNullOrEmpty(stat)) return (0, 0);
@@ -1208,6 +1328,18 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
     }
 }
 
+public sealed class AttachmentRowViewModel
+{
+    public required string FileName { get; init; }
+    public required long ByteSize { get; init; }
+    public string SizeText => ByteSize switch
+    {
+        >= 1024 * 1024 => $"{ByteSize / (1024.0 * 1024.0):F1} MB",
+        >= 1024 => $"{ByteSize / 1024.0:F1} KB",
+        _ => $"{ByteSize} B",
+    };
+}
+
 public sealed partial class SubtaskRowViewModel : ViewModelBase
 {
     public required string Id { get; init; }

src/ClaudeDo.Ui/Views/Islands/Detail/DescriptionStepsCard.axaml

@@ -161,6 +161,58 @@
           </StackPanel>
         </Border>
 
+        <!-- Attachments section -->
+        <Border BorderBrush="{DynamicResource LineBrush}"
+                BorderThickness="0,1,0,0"
+                Padding="0,8,0,0">
+          <StackPanel Spacing="6">
+            <TextBlock Classes="section-label" Text="{loc:Tr details.attachments.sectionLabel}"/>
+
+            <!-- Attachment rows -->
+            <ItemsControl ItemsSource="{Binding Attachments}">
+              <ItemsControl.ItemTemplate>
+                <DataTemplate DataType="vm:AttachmentRowViewModel">
+                  <Grid ColumnDefinitions="*,Auto,Auto" Margin="0,2,0,2">
+                    <TextBlock Grid.Column="0"
+                               Text="{Binding FileName}"
+                               VerticalAlignment="Center"
+                               FontSize="{StaticResource FontSizeBody}"
+                               Foreground="{DynamicResource TextDimBrush}"
+                               TextTrimming="CharacterEllipsis"/>
+                    <TextBlock Grid.Column="1"
+                               Text="{Binding SizeText}"
+                               VerticalAlignment="Center"
+                               Margin="8,0"
+                               FontSize="{StaticResource FontSizeBody}"
+                               Foreground="{DynamicResource TextMuteBrush}"/>
+                    <Button Grid.Column="2"
+                            Classes="icon-btn"
+                            ToolTip.Tip="{loc:Tr details.attachments.removeTip}"
+                            Command="{Binding $parent[ItemsControl].((vm:DetailsIslandViewModel)DataContext).RemoveAttachmentCommand}"
+                            CommandParameter="{Binding}">
+                      <PathIcon Data="{StaticResource Icon.X}" Width="11" Height="11"/>
+                    </Button>
+                  </Grid>
+                </DataTemplate>
+              </ItemsControl.ItemTemplate>
+            </ItemsControl>
+
+            <!-- Add file button -->
+            <Button Classes="btn"
+                    Padding="8,3"
+                    HorizontalAlignment="Left"
+                    Click="OnAddFileClick"
+                    Content="{loc:Tr details.attachments.addFile}"/>
+
+            <!-- Drop status / confirmation -->
+            <TextBlock Text="{Binding DropStatus}"
+                       IsVisible="{Binding DropStatus, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
+                       FontSize="{StaticResource FontSizeBody}"
+                       Foreground="{DynamicResource TextMuteBrush}"
+                       TextWrapping="Wrap"/>
+          </StackPanel>
+        </Border>
+
       </StackPanel>
       </ScrollViewer>
     </DockPanel>

src/ClaudeDo.Ui/Views/Islands/Detail/DescriptionStepsCard.axaml.cs

@@ -2,6 +2,7 @@ using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
+using Avalonia.Platform.Storage;
 using ClaudeDo.Ui.ViewModels.Islands;
 
 namespace ClaudeDo.Ui.Views.Islands.Detail;
@@ -34,4 +35,32 @@ public partial class DescriptionStepsCard : UserControl
             && vm.CommitSubtaskEditCommand.CanExecute(row))
             vm.CommitSubtaskEditCommand.Execute(row);
     }
+
+    private async void OnAddFileClick(object? sender, RoutedEventArgs e)
+    {
+        if (DataContext is not DetailsIslandViewModel vm) return;
+        var topLevel = TopLevel.GetTopLevel(this);
+        if (topLevel is null) return;
+
+        var picked = await topLevel.StorageProvider.OpenFilePickerAsync(
+            new FilePickerOpenOptions { AllowMultiple = true });
+
+        if (picked.Count == 0) return;
+
+        var files = new List<(string FileName, System.IO.Stream Content)>();
+        foreach (var item in picked)
+        {
+            var stream = await item.OpenReadAsync();
+            files.Add((item.Name, stream));
+        }
+
+        try
+        {
+            await vm.AddFilesAsync(files);
+        }
+        finally
+        {
+            foreach (var (_, s) in files) await s.DisposeAsync();
+        }
+    }
 }

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

@@ -5,7 +5,9 @@
              xmlns:detail="using:ClaudeDo.Ui.Views.Islands.Detail"
              xmlns:loc="using:ClaudeDo.Ui.Localization"
              x:Class="ClaudeDo.Ui.Views.Islands.DetailsIslandView"
-             x:DataType="vm:DetailsIslandViewModel">
+             x:DataType="vm:DetailsIslandViewModel"
+             DragDrop.AllowDrop="True">
+  <Panel>
   <DockPanel>
 
     <!-- ── Metadata footer (sticky bottom) — created-at + close — task detail only ── -->
@@ -124,4 +126,21 @@
     </Grid>
 
   </DockPanel>
+
+  <!-- Drop overlay — shown while dragging files over the pane -->
+  <Border IsVisible="{Binding IsDragOver}"
+          Background="{DynamicResource AccentSoftBrush}"
+          BorderBrush="{DynamicResource AccentBrush}"
+          BorderThickness="2"
+          CornerRadius="14"
+          IsHitTestVisible="False">
+    <TextBlock Text="{loc:Tr details.attachments.dropToAttach}"
+               HorizontalAlignment="Center"
+               VerticalAlignment="Center"
+               Foreground="{DynamicResource AccentBrush}"
+               FontSize="16"
+               FontWeight="Medium"/>
+  </Border>
+
+  </Panel>
 </UserControl>

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

@@ -1,8 +1,10 @@
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Input;
+using Avalonia.Interactivity;
 using Avalonia.Layout;
 using Avalonia.Media;
+using Avalonia.Platform.Storage;
 using Avalonia.Reactive;
 using ClaudeDo.Ui.ViewModels.Islands;
 using ClaudeDo.Ui.Views.Modals;
@@ -27,6 +29,78 @@ public partial class DetailsIslandView : UserControl
         // row Min/Max during a drag, so the console stops shrinking at 1/3.
         DetailBodyGrid.GetObservable(BoundsProperty)
             .Subscribe(new AnonymousObserver<Rect>(_ => UpdateRowLimits()));
+
+        AddHandler(DragDrop.DragEnterEvent, OnDragEnter);
+        AddHandler(DragDrop.DragOverEvent, OnDragOver);
+        AddHandler(DragDrop.DragLeaveEvent, OnDragLeave);
+        AddHandler(DragDrop.DropEvent, OnDrop);
+    }
+
+    private static bool IsFilesDrop(DragEventArgs e)
+        => e.DataTransfer?.Contains(DataFormat.File) == true;
+
+    private void OnDragEnter(object? sender, DragEventArgs e)
+    {
+        if (_vm is { CanAcceptDrop: true } && IsFilesDrop(e))
+        {
+            e.DragEffects = DragDropEffects.Copy;
+            _vm.IsDragOver = true;
+        }
+        else
+        {
+            e.DragEffects = DragDropEffects.None;
+        }
+        e.Handled = true;
+    }
+
+    private void OnDragOver(object? sender, DragEventArgs e)
+    {
+        if (_vm is { CanAcceptDrop: true } && IsFilesDrop(e))
+        {
+            e.DragEffects = DragDropEffects.Copy;
+            _vm.IsDragOver = true;
+        }
+        else
+        {
+            e.DragEffects = DragDropEffects.None;
+        }
+        e.Handled = true;
+    }
+
+    private void OnDragLeave(object? sender, RoutedEventArgs e)
+    {
+        if (_vm != null) _vm.IsDragOver = false;
+    }
+
+    private async void OnDrop(object? sender, DragEventArgs e)
+    {
+        if (_vm != null) _vm.IsDragOver = false;
+        if (_vm is not { CanAcceptDrop: true } || !IsFilesDrop(e)) return;
+        e.Handled = true;
+
+        var items = e.DataTransfer.TryGetFiles();
+        if (items is null) return;
+
+        var files = new List<(string FileName, System.IO.Stream Content)>();
+        foreach (var item in items)
+        {
+            if (item is IStorageFile sf)
+            {
+                var stream = await sf.OpenReadAsync();
+                files.Add((sf.Name, stream));
+            }
+        }
+
+        if (files.Count == 0) return;
+
+        try
+        {
+            await _vm.AddFilesAsync(files);
+        }
+        finally
+        {
+            foreach (var (_, s) in files) await s.DisposeAsync();
+        }
     }
 
     private void UpdateRowLimits()