diff --git a/src/ClaudeDo.Localization/locales/de.json b/src/ClaudeDo.Localization/locales/de.json
index d5737d5..027e8ab 100644
--- a/src/ClaudeDo.Localization/locales/de.json
+++ b/src/ClaudeDo.Localization/locales/de.json
@@ -422,7 +422,7 @@
     "planningBadge": { "active": "PLANUNG", "finalized": "GEPLANT" },
     "taskRow": { "createdPrefix": "Erstellt {0}", "stepsText": "{0}/{1} Schritte" },
     "tasksIsland": { "completedHeader": "ABGESCHLOSSEN", "completedHeaderCount": "ABGESCHLOSSEN · {0}" },
-    "diff": { "loadFailed": "Diff konnte nicht geladen werden: {0}", "noChanges": "Keine Änderungen anzuzeigen." },
+    "diff": { "loadFailed": "Diff konnte nicht geladen werden: {0}", "noChanges": "Keine Änderungen anzuzeigen.", "unavailable": "Diff nicht mehr verfügbar — Commit-Bereich unvollständig." },
     "planningDiff": { "hubError": "Kombinierte Vorschau konnte nicht erstellt werden (Hub-Fehler).", "conflict": "Kombinierte Vorschau nicht möglich: Teilaufgabe {0} steht im Konflikt mit einer früheren Teilaufgabe ({1} Dateien)." },
     "merge": { "commitMessage": "Merge-Aufgabe: {0}", "workerOfflineBranches": "Worker offline — Branches können nicht aufgelistet werden.", "loadBranchesFailed": "Branches konnten nicht geladen werden: {0}", "merged": "Zusammengeführt.", "conflict": "Merge-Konflikt — Ziel-Branch wiederhergestellt. Manuell oder über Fortsetzen lösen, dann erneut versuchen.", "blocked": "Blockiert: {0}", "unknownStatus": "Unbekannter Status: {0}", "mergeFailed": "Merge fehlgeschlagen: {0}" },
     "conflictResolution": { "vsCodeError": "VS Code konnte nicht gestartet werden: {0}. Die Pfade sind oben aufgeführt — kopiere sie manuell.", "subtaskPrefix": "Konflikte in Teilaufgabe: {0}", "targetPrefix": "Zusammenführen in: {0}" },
diff --git a/src/ClaudeDo.Localization/locales/en.json b/src/ClaudeDo.Localization/locales/en.json
index 4457637..3aeacbf 100644
--- a/src/ClaudeDo.Localization/locales/en.json
+++ b/src/ClaudeDo.Localization/locales/en.json
@@ -422,7 +422,7 @@
     "planningBadge": { "active": "PLANNING", "finalized": "PLANNED" },
     "taskRow": { "createdPrefix": "Created {0}", "stepsText": "{0}/{1} steps" },
     "tasksIsland": { "completedHeader": "COMPLETED", "completedHeaderCount": "COMPLETED · {0}" },
-    "diff": { "loadFailed": "Failed to load diff: {0}", "noChanges": "No changes to show." },
+    "diff": { "loadFailed": "Failed to load diff: {0}", "noChanges": "No changes to show.", "unavailable": "Diff no longer available — commit range incomplete." },
     "planningDiff": { "hubError": "Could not build combined preview (hub error).", "conflict": "Cannot build combined preview: subtask {0} conflicts with an earlier subtask ({1} files)." },
     "merge": { "commitMessage": "Merge task: {0}", "workerOfflineBranches": "Worker offline — cannot list branches.", "loadBranchesFailed": "Failed to load branches: {0}", "merged": "Merged.", "conflict": "Merge conflict — target branch restored. Resolve manually or via Continue, then retry.", "blocked": "Blocked: {0}", "unknownStatus": "Unknown status: {0}", "mergeFailed": "Merge failed: {0}" },
     "conflictResolution": { "vsCodeError": "Could not launch VS Code: {0}. Paths are listed above — copy them manually.", "subtaskPrefix": "Conflicts in subtask: {0}", "targetPrefix": "Merging into: {0}" },
diff --git a/src/ClaudeDo.Ui/ViewModels/Modals/DiffModalViewModel.cs b/src/ClaudeDo.Ui/ViewModels/Modals/DiffModalViewModel.cs
index c92caaf..82050a3 100644
--- a/src/ClaudeDo.Ui/ViewModels/Modals/DiffModalViewModel.cs
+++ b/src/ClaudeDo.Ui/ViewModels/Modals/DiffModalViewModel.cs
@@ -110,6 +110,12 @@ public sealed partial class DiffModalViewModel : ViewModelBase
         Files.Clear();
         StatusMessage = null;
 
+        if (FromCommitRange && (BaseRef is null || HeadCommit is null))
+        {
+            StatusMessage = Loc.T("vm.diff.unavailable");
+            return;
+        }
+
         string raw;
         try
         {
diff --git a/tests/ClaudeDo.Ui.Tests/ViewModels/DiffModalViewModelTests.cs b/tests/ClaudeDo.Ui.Tests/ViewModels/DiffModalViewModelTests.cs
new file mode 100644
index 0000000..3fb1d5b
--- /dev/null
+++ b/tests/ClaudeDo.Ui.Tests/ViewModels/DiffModalViewModelTests.cs
@@ -0,0 +1,54 @@
+using System.IO;
+using ClaudeDo.Localization;
+using ClaudeDo.Ui.Localization;
+using ClaudeDo.Ui.ViewModels.Modals;
+
+namespace ClaudeDo.Ui.Tests.ViewModels;
+
+public class DiffModalViewModelTests
+{
+    public DiffModalViewModelTests()
+    {
+        var dir = AppContext.BaseDirectory;
+        while (dir is not null && !Directory.Exists(Path.Combine(dir, "src", "ClaudeDo.Localization", "locales")))
+            dir = Path.GetDirectoryName(dir);
+        Loc.Current = new Localizer(
+            LocaleStore.Load(Path.Combine(dir!, "src", "ClaudeDo.Localization", "locales")), "en");
+    }
+
+    [Fact]
+    public async Task LoadAsync_CommitRange_NullHeadCommit_ShowsUnavailableState()
+    {
+        var vm = new DiffModalViewModel(null!)
+        {
+            WorktreePath = "/some/repo",
+            BaseRef = "abc123",
+            HeadCommit = null,
+            FromCommitRange = true,
+        };
+
+        await vm.LoadAsync();
+
+        Assert.Empty(vm.Files);
+        Assert.NotNull(vm.StatusMessage);
+        Assert.Contains("no longer available", vm.StatusMessage, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public async Task LoadAsync_CommitRange_NullBaseRef_ShowsUnavailableState()
+    {
+        var vm = new DiffModalViewModel(null!)
+        {
+            WorktreePath = "/some/repo",
+            BaseRef = null,
+            HeadCommit = "def456",
+            FromCommitRange = true,
+        };
+
+        await vm.LoadAsync();
+
+        Assert.Empty(vm.Files);
+        Assert.NotNull(vm.StatusMessage);
+        Assert.Contains("no longer available", vm.StatusMessage, StringComparison.OrdinalIgnoreCase);
+    }
+}