diff --git a/src/ClaudeDo.Localization/locales/de.json b/src/ClaudeDo.Localization/locales/de.json
index 6cc41b17..8532aab2 100644
--- a/src/ClaudeDo.Localization/locales/de.json
+++ b/src/ClaudeDo.Localization/locales/de.json
@@ -253,7 +253,9 @@
     }
   },
   "agent": {
-    "openWorktreeTip": "Worktree im Datei-Explorer öffnen"
+    "openWorktreeTip": "Worktree im Datei-Explorer öffnen — benötigt einen aktiven Worktree",
+    "openDiffTip": "Diff öffnen — benötigt einen Worktree oder einen gemergten Commit-Bereich",
+    "reviewCombinedDiffTip": "Nur für Planning-Tasks mit Subtasks oder Child-Outcomes verfügbar"
   },
   "notes": {
     "today": "Heute",
@@ -268,6 +270,9 @@
     "chipFailed": "FEHLGESCHLAGEN",
     "reviewContinueTip": "Dieses Feedback senden und die Aufgabe erneut ausführen",
     "reviewResetTip": "Alle Änderungen verwerfen und die Aufgabe auf Leerlauf zurücksetzen",
+    "mergeDrainHint": "Merge zieht Subtasks ein — Abbrechen entsperrt sich danach",
+    "continueNoSessionTip": "Setzt den letzten Lauf fort — benötigt eine vorherige Session",
+    "resetAndRetryInteractiveTip": "Interaktive Sitzung ist offen — erst schließen",
     "reviewFeedbackPlaceholder": "Feedback für nächsten Lauf"
   },
   "missionControl": {
diff --git a/src/ClaudeDo.Localization/locales/en.json b/src/ClaudeDo.Localization/locales/en.json
index 0b8e6a72..3fecce43 100644
--- a/src/ClaudeDo.Localization/locales/en.json
+++ b/src/ClaudeDo.Localization/locales/en.json
@@ -253,7 +253,9 @@
     }
   },
   "agent": {
-    "openWorktreeTip": "Open worktree in file explorer"
+    "openWorktreeTip": "Open worktree in file explorer — requires an active worktree",
+    "openDiffTip": "Open the diff — requires a worktree or a merged commit range",
+    "reviewCombinedDiffTip": "Only available for planning tasks with subtasks or child outcomes"
   },
   "notes": {
     "today": "Today",
@@ -268,7 +270,10 @@
     "chipFailed": "FAILED",
     "reviewContinueTip": "Send this feedback and re-run the task",
     "reviewResetTip": "Discard all changes and reset the task to Idle",
-    "reviewFeedbackPlaceholder": "Feedback for next run"
+    "reviewFeedbackPlaceholder": "Feedback for next run",
+    "mergeDrainHint": "Merge is draining subtasks — Cancel unlocks once it finishes",
+    "continueNoSessionTip": "Resumes the last run — needs a previous session to continue",
+    "resetAndRetryInteractiveTip": "Interactive session is open — close it first"
   },
   "missionControl": {
     "windowTitle": "Mission Control",
diff --git a/src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs b/src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs
index 1c07e0e4..35a9ad1b 100644
--- a/src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs
+++ b/src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs
@@ -1364,8 +1364,13 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
     // fix); this just keeps the button from inviting a click the worker will reject anyway.
     [ObservableProperty]
     [NotifyCanExecuteChangedFor(nameof(CancelReviewCommand))]
+    [NotifyPropertyChangedFor(nameof(ShowMergeDrainHint))]
     private bool _isMergeDraining;
 
+    // Drives the visible hint next to Cancel while a merge drain is in progress, so the
+    // silently-disabled button doesn't look broken.
+    public bool ShowMergeDrainHint => IsMergeDraining;
+
     [RelayCommand(CanExecute = nameof(CanCancelReview))]
     private async System.Threading.Tasks.Task CancelReviewAsync()
     {
diff --git a/src/ClaudeDo.Ui/Views/Islands/Detail/WorkConsole.axaml b/src/ClaudeDo.Ui/Views/Islands/Detail/WorkConsole.axaml
index 8f31a34e..0aeee9b1 100644
--- a/src/ClaudeDo.Ui/Views/Islands/Detail/WorkConsole.axaml
+++ b/src/ClaudeDo.Ui/Views/Islands/Detail/WorkConsole.axaml
@@ -174,10 +174,14 @@
           <StackPanel Orientation="Horizontal" Spacing="8">
             <Button Classes="btn accent" Content="Continue"
                     Command="{Binding ContinueCommand}"
-                    IsVisible="{Binding ShowContinue}" />
+                    IsVisible="{Binding ShowContinue}"
+                    ToolTip.Tip="{loc:Tr session.continueNoSessionTip}"
+                    ToolTip.ShowOnDisabled="True" />
             <Button Classes="btn" Content="Reset &amp; Retry"
                     Command="{Binding ResetAndRetryCommand}"
-                    IsVisible="{Binding ShowResetAndRetry}" />
+                    IsVisible="{Binding ShowResetAndRetry}"
+                    ToolTip.Tip="{loc:Tr session.resetAndRetryInteractiveTip}"
+                    ToolTip.ShowOnDisabled="True" />
           </StackPanel>
         </StackPanel>
       </Border>
@@ -341,9 +345,12 @@
               <!-- Inspect: diff / worktree / combined diff -->
               <WrapPanel Orientation="Horizontal">
                 <Button Classes="btn" Content="Open Diff" Margin="0,0,8,8"
+                        ToolTip.Tip="{loc:Tr agent.openDiffTip}"
+                        ToolTip.ShowOnDisabled="True"
                         Command="{Binding Merge.OpenDiffCommand}" />
                 <Button Classes="btn" Margin="0,0,8,8"
                         ToolTip.Tip="{loc:Tr agent.openWorktreeTip}"
+                        ToolTip.ShowOnDisabled="True"
                         Command="{Binding Merge.OpenWorktreeCommand}">
                   <StackPanel Orientation="Horizontal" Spacing="5">
                     <TextBlock Text="Worktree" />
@@ -351,6 +358,8 @@
                   </StackPanel>
                 </Button>
                 <Button Classes="btn" Content="Review Combined Diff" Margin="0,0,8,8"
+                        ToolTip.Tip="{loc:Tr agent.reviewCombinedDiffTip}"
+                        ToolTip.ShowOnDisabled="True"
                         Command="{Binding Merge.ReviewCombinedDiffCommand}" />
               </WrapPanel>
             </StackPanel>
@@ -371,6 +380,17 @@
                            Text="Open the diff to enable merge" />
               </StackPanel>
 
+              <!-- Gate: a unit merge is draining this task's subtasks, so Cancel is locked
+                   until it finishes (mirrors the worker-side guard). -->
+              <StackPanel Orientation="Horizontal" Spacing="6"
+                          IsVisible="{Binding ShowMergeDrainHint}">
+                <PathIcon Data="{StaticResource Icon.Warning}" Width="11" Height="11"
+                          Foreground="{DynamicResource AmberBrush}" VerticalAlignment="Center" />
+                <TextBlock Classes="meta" VerticalAlignment="Center"
+                           Foreground="{DynamicResource AmberBrush}"
+                           Text="{loc:Tr session.mergeDrainHint}" />
+              </StackPanel>
+
               <WrapPanel Orientation="Horizontal">
                 <StackPanel Orientation="Horizontal" Spacing="{StaticResource SpaceSm}" Margin="0,0,8,8">
                   <Button Classes="btn accent" Content="Approve &amp; Merge"
@@ -384,6 +404,8 @@
                   <controls:OperationIndicator Status="{Binding ParkOp}" />
                 </StackPanel>
                 <Button Classes="btn" Content="Cancel" Margin="0,0,8,8"
+                        ToolTip.Tip="{loc:Tr session.mergeDrainHint}"
+                        ToolTip.ShowOnDisabled="True"
                         Command="{Binding CancelReviewCommand}" />
               </WrapPanel>
 
diff --git a/tests/ClaudeDo.Ui.Tests/ViewModels/DetailsIslandReviewActionsTests.cs b/tests/ClaudeDo.Ui.Tests/ViewModels/DetailsIslandReviewActionsTests.cs
index efabc440..7ee5cabc 100644
--- a/tests/ClaudeDo.Ui.Tests/ViewModels/DetailsIslandReviewActionsTests.cs
+++ b/tests/ClaudeDo.Ui.Tests/ViewModels/DetailsIslandReviewActionsTests.cs
@@ -138,6 +138,24 @@ public class DetailsIslandReviewActionsTests : IDisposable
         Assert.False(vm.ApproveReviewCommand.CanExecute(null));
     }
 
+    [Fact]
+    public void ShowMergeDrainHint_IsTrueExactlyWhenMergeIsDraining()
+    {
+        var vm = BuildVm(new RecordingWorkerClient());
+        vm.Bind(new TaskRowViewModel { Id = "task-drain-1", Status = TaskStatus.WaitingForReview });
+
+        Assert.False(vm.ShowMergeDrainHint);
+        Assert.True(vm.CancelReviewCommand.CanExecute(null));
+
+        vm.IsMergeDraining = true;
+        Assert.True(vm.ShowMergeDrainHint);
+        Assert.False(vm.CancelReviewCommand.CanExecute(null));
+
+        vm.IsMergeDraining = false;
+        Assert.False(vm.ShowMergeDrainHint);
+        Assert.True(vm.CancelReviewCommand.CanExecute(null));
+    }
+
     private sealed class ThrowingWorkerClient : StubWorkerClient
     {
         public override bool IsConnected => true;