diff --git a/src/ClaudeDo.Localization/locales/de.json b/src/ClaudeDo.Localization/locales/de.json
index d3d94c95..d40850a9 100644
--- a/src/ClaudeDo.Localization/locales/de.json
+++ b/src/ClaudeDo.Localization/locales/de.json
@@ -11,6 +11,7 @@
       "categoryGeneral": "Allgemein",
       "categoryExecution": "Ausführung",
       "categoryPrime": "Prime Claude",
+      "categoryPrimeHint": "Geplante Claude-Läufe: Ping, My Day füllen oder ein eigener Prompt, zu festen Zeiten.",
       "categoryWorktrees": "Worktrees",
       "categoryFiles": "Dateien & Prompts",
       "categorySkills": "Session Skills",
@@ -27,6 +28,7 @@
       "model": "Modell",
       "maxTurns": "Max. Durchläufe",
       "effort": "Effort",
+      "effortHint": "Denkaufwand, den das Modell anwendet. Höher kostet mehr und denkt tiefer.",
       "modelPresets": "Vorgaben pro Modell",
       "modelPresetsHint": "Effort und Durchlauf-Budget, mit denen eine Sitzung unter diesem Modell läuft. Overrides auf Listen- und Aufgabenebene gewinnen weiterhin.",
       "maxTurnsCeiling": "Max. Durchläufe – Obergrenze",
@@ -50,6 +52,7 @@
       "powerUseWindowPlaceholder": "HH:mm",
       "powerUseHandoffPct": "Übergabe bei (% von 5h)",
       "powerUseWindowHint": "Beide Zeiten leer = rund um die Uhr. Das Fenster wird bei jedem Start einer Handler-Phase geprüft — eine Kette, die über das Ende hinausläuft, macht ohne PowerUse weiter.",
+      "powerUseWindowInvalid": "PowerUse-Zeiten müssen im Format HH:mm sein oder leer bleiben.",
       "throttleStagesHeading": "Throttle-Stufen",
       "throttleStagesHint": "Die Parallelität wird vor dem harten Gate begrenzt: soft auf 2 Slots, hard auf 1. Eingestellt wird das durch Ziehen der Marker an den Gauges im Usage-Monitor. Der Monitor schließt diese Einstellungen — vorher speichern, wenn du etwas geändert hast.",
       "throttleStagesValues": "5h: soft {0}% / hard {1}%  ·  7d: soft {2}% / hard {3}%",
diff --git a/src/ClaudeDo.Localization/locales/en.json b/src/ClaudeDo.Localization/locales/en.json
index db94367c..3c27cca8 100644
--- a/src/ClaudeDo.Localization/locales/en.json
+++ b/src/ClaudeDo.Localization/locales/en.json
@@ -11,6 +11,7 @@
       "categoryGeneral": "General",
       "categoryExecution": "Execution",
       "categoryPrime": "Prime Claude",
+      "categoryPrimeHint": "Scheduled Claude runs: Ping, Fill My Day, or a custom prompt, at fixed times.",
       "categoryWorktrees": "Worktrees",
       "categoryFiles": "Files & Prompts",
       "categorySkills": "Session Skills",
@@ -27,6 +28,7 @@
       "model": "Model",
       "maxTurns": "Max turns",
       "effort": "Effort",
+      "effortHint": "Reasoning effort the model applies. Higher costs more and reasons deeper.",
       "modelPresets": "Per-model defaults",
       "modelPresetsHint": "Reasoning effort and turn budget used when a session runs under that model. List- and task-level max-turn overrides still win.",
       "maxTurnsCeiling": "Max turns ceiling",
@@ -50,6 +52,7 @@
       "powerUseWindowPlaceholder": "HH:mm",
       "powerUseHandoffPct": "Hand off at (% of 5h)",
       "powerUseWindowHint": "Leave both times empty for round the clock. A window is checked whenever a handler phase starts, so a chain that runs past the end time continues without PowerUse.",
+      "powerUseWindowInvalid": "PowerUse window times must be HH:mm, or left blank.",
       "throttleStagesHeading": "Throttle stages",
       "throttleStagesHint": "Parallelism is capped before the hard gate: soft caps at 2 slots, hard at 1. Edited by dragging the markers on the usage-monitor gauges. Opening the monitor closes these settings — save first if you changed anything.",
       "throttleStagesValues": "5h: soft {0}% / hard {1}%  ·  7d: soft {2}% / hard {3}%",
diff --git a/src/ClaudeDo.Ui/ViewModels/Modals/Settings/GeneralSettingsTabViewModel.cs b/src/ClaudeDo.Ui/ViewModels/Modals/Settings/GeneralSettingsTabViewModel.cs
index 7fb528cd..571ba865 100644
--- a/src/ClaudeDo.Ui/ViewModels/Modals/Settings/GeneralSettingsTabViewModel.cs
+++ b/src/ClaudeDo.Ui/ViewModels/Modals/Settings/GeneralSettingsTabViewModel.cs
@@ -119,8 +119,13 @@ public sealed partial class GeneralSettingsTabViewModel : ViewModelBase
         foreach (var row in ModelPresets)
             if (row.MaxTurns is < 1 or > 200)
                 return $"Max turns for {row.Model} must be between 1 and 200.";
+        if (!IsValidWindowBound(PowerUseWindowStart) || !IsValidWindowBound(PowerUseWindowEnd))
+            return Loc.T("settings.general.powerUseWindowInvalid");
         return null;
     }
+
+    private static bool IsValidWindowBound(string? value)
+        => string.IsNullOrWhiteSpace(value) || PowerUseWindow.Parse(value) is not null;
 }
 
 /// <summary>One row of the per-model preset table. Single consumer, so it lives here.</summary>
diff --git a/src/ClaudeDo.Ui/Views/Modals/SettingsModalView.axaml b/src/ClaudeDo.Ui/Views/Modals/SettingsModalView.axaml
index 4325820b..26a48f19 100644
--- a/src/ClaudeDo.Ui/Views/Modals/SettingsModalView.axaml
+++ b/src/ClaudeDo.Ui/Views/Modals/SettingsModalView.axaml
@@ -190,7 +190,8 @@
                            Opacity="0.6" FontSize="12" TextWrapping="Wrap"/>
                 <Grid ColumnDefinitions="90,12,*,12,110" Margin="0,4,0,0">
                   <TextBlock Grid.Column="0" Classes="eyebrow" Text="{loc:Tr settings.general.model}"/>
-                  <TextBlock Grid.Column="2" Classes="eyebrow" Text="{loc:Tr settings.general.effort}"/>
+                  <TextBlock Grid.Column="2" Classes="eyebrow" Text="{loc:Tr settings.general.effort}"
+                             ToolTip.Tip="{loc:Tr settings.general.effortHint}"/>
                   <TextBlock Grid.Column="4" Classes="eyebrow" Text="{loc:Tr settings.general.maxTurns}"/>
                 </Grid>
                 <ItemsControl ItemsSource="{Binding General.ModelPresets}">
@@ -307,7 +308,8 @@
           </ScrollViewer>
         </TabItem>
 
-        <TabItem Header="{loc:Tr settings.sidebar.categoryPrime}">
+        <TabItem Header="{loc:Tr settings.sidebar.categoryPrime}"
+                 ToolTip.Tip="{loc:Tr settings.sidebar.categoryPrimeHint}">
           <ScrollViewer>
             <StackPanel Spacing="12" Margin="0,8,0,0">
               <Border IsVisible="{Binding !HasLinkedRepo}"
diff --git a/tests/ClaudeDo.Ui.Tests/ViewModels/GeneralSettingsTabViewModelTests.cs b/tests/ClaudeDo.Ui.Tests/ViewModels/GeneralSettingsTabViewModelTests.cs
index 68db0ebe..5a866a66 100644
--- a/tests/ClaudeDo.Ui.Tests/ViewModels/GeneralSettingsTabViewModelTests.cs
+++ b/tests/ClaudeDo.Ui.Tests/ViewModels/GeneralSettingsTabViewModelTests.cs
@@ -30,4 +30,31 @@ public class GeneralSettingsTabViewModelTests
 
         Assert.Equal(expectedValid, vm.Validate() is null);
     }
+
+    [Theory]
+    [InlineData(null, true)]
+    [InlineData("", true)]
+    [InlineData("09:00", true)]
+    [InlineData("23:59", true)]
+    [InlineData("9:0", false)]
+    [InlineData("abc", false)]
+    [InlineData("25:00", false)]
+    public void Validate_PowerUseWindowStart_rejectsUnparseable(string? value, bool expectedValid)
+    {
+        var vm = new GeneralSettingsTabViewModel { PowerUseWindowStart = value };
+
+        Assert.Equal(expectedValid, vm.Validate() is null);
+    }
+
+    [Theory]
+    [InlineData(null, true)]
+    [InlineData("09:00", true)]
+    [InlineData("9:0", false)]
+    [InlineData("abc", false)]
+    public void Validate_PowerUseWindowEnd_rejectsUnparseable(string? value, bool expectedValid)
+    {
+        var vm = new GeneralSettingsTabViewModel { PowerUseWindowEnd = value };
+
+        Assert.Equal(expectedValid, vm.Validate() is null);
+    }
 }
diff --git a/tests/ClaudeDo.Ui.Tests/ViewModels/SettingsModalViewModelTests.cs b/tests/ClaudeDo.Ui.Tests/ViewModels/SettingsModalViewModelTests.cs
index 9cd69339..47bfc50b 100644
--- a/tests/ClaudeDo.Ui.Tests/ViewModels/SettingsModalViewModelTests.cs
+++ b/tests/ClaudeDo.Ui.Tests/ViewModels/SettingsModalViewModelTests.cs
@@ -153,10 +153,17 @@ public class SettingsModalViewModelTests : IDisposable
         Assert.Equal("18:00", vm.General.PowerUseWindowStart);
         Assert.Equal(88, vm.General.PowerUseHandoffPct);
 
-        // A typo in the window is normalized to null on save, not stored as a fake window.
+        // A typo in the window blocks Save instead of silently becoming "round the clock".
         vm.General.PowerUseWindowEnd = "not a time";
         await vm.SaveCommand.ExecuteAsync(null);
 
+        Assert.Null(worker.Saved);
+        Assert.NotEqual("", vm.ValidationError);
+
+        // Clearing it back out is still a valid "round the clock" window and saves.
+        vm.General.PowerUseWindowEnd = "";
+        await vm.SaveCommand.ExecuteAsync(null);
+
         Assert.True(worker.Saved!.PowerUseEnabled);
         Assert.Equal("18:00", worker.Saved.PowerUseWindowStart);
         Assert.Null(worker.Saved.PowerUseWindowEnd);