diff --git a/Wino.Calendar.ViewModels/CalendarPageViewModel.cs b/Wino.Calendar.ViewModels/CalendarPageViewModel.cs
index a8ffb1a7..5d613ecb 100644
--- a/Wino.Calendar.ViewModels/CalendarPageViewModel.cs
+++ b/Wino.Calendar.ViewModels/CalendarPageViewModel.cs
@@ -963,6 +963,15 @@ public partial class CalendarPageViewModel : CalendarBaseViewModel,
         base.OnCalendarItemUpdated(calendarItem, source);
         Debug.WriteLine($"Calendar item updated: {calendarItem.Id}");
 
+        // Local-only calendar operations are persisted immediately without real network I/O.
+        // Ignore optimistic client updates to prevent applying the same mutation twice.
+        var isLocalCalendarUpdate = string.IsNullOrWhiteSpace(calendarItem.RemoteEventId) ||
+                                    calendarItem.RemoteEventId.StartsWith("local-", StringComparison.OrdinalIgnoreCase);
+        if (isLocalCalendarUpdate && source == CalendarItemUpdateSource.ClientUpdated)
+        {
+            return;
+        }
+
         // Series master events should not be visible on the UI.
         if (calendarItem.IsRecurringParent)
         {
diff --git a/Wino.Core/Synchronizers/ImapSynchronizer.cs b/Wino.Core/Synchronizers/ImapSynchronizer.cs
index 60978d84..61863b8c 100644
--- a/Wino.Core/Synchronizers/ImapSynchronizer.cs
+++ b/Wino.Core/Synchronizers/ImapSynchronizer.cs
@@ -705,7 +705,10 @@ public class ImapSynchronizer : WinoSynchronizer<ImapRequest, ImapMessageCreatio
 
         foreach (var item in batchedRequests)
         {
-            item.Request.ApplyUIChanges();
+            if (ShouldApplyOptimisticUIChanges(item.Request))
+            {
+                item.Request.ApplyUIChanges();
+            }
         }
 
         // All task bundles will execute on the same client.
@@ -734,7 +737,10 @@ public class ImapSynchronizer : WinoSynchronizer<ImapRequest, ImapMessageCreatio
                 // Client pool failed to get a client.
                 // Requests may not be executed at this point.
 
-                item.Request.RevertUIChanges();
+                if (ShouldApplyOptimisticUIChanges(item.Request))
+                {
+                    item.Request.RevertUIChanges();
+                }
 
                 isCrashed = true;
                 throw;
@@ -768,7 +774,10 @@ public class ImapSynchronizer : WinoSynchronizer<ImapRequest, ImapMessageCreatio
 
                 if (!handled)
                 {
-                    item.Request.RevertUIChanges();
+                    if (ShouldApplyOptimisticUIChanges(item.Request))
+                    {
+                        item.Request.RevertUIChanges();
+                    }
                     throw;
                 }
             }
@@ -782,6 +791,21 @@ public class ImapSynchronizer : WinoSynchronizer<ImapRequest, ImapMessageCreatio
         }
     }
 
+    private bool ShouldApplyOptimisticUIChanges(IRequestBase request)
+    {
+        // Mail changes are always applied.
+        // Calendar changes are applied only if calendar is not in local mode.
+        // Database updates are immidiate and will be reflected in the UI right after the request is processed, so no need for optimistic changes.
+
+        if (request is not ICalendarActionRequest)
+        {
+            return true;
+        }
+
+        var mode = Account.ServerInformation?.CalendarSupportMode ?? ImapCalendarSupportMode.Disabled;
+        return mode != ImapCalendarSupportMode.LocalOnly;
+    }
+
     /// <summary>
     /// Assigns special folder type for the given local folder.
     /// If server doesn't support special folders, we can't determine the type. MailKit will throw for GetFolder.