diff --git a/LehrerApp.Api.Tests/SyncProtocolVersionMiddlewareTests.cs b/LehrerApp.Api.Tests/SyncProtocolVersionMiddlewareTests.cs
new file mode 100644
index 0000000..870795b
--- /dev/null
+++ b/LehrerApp.Api.Tests/SyncProtocolVersionMiddlewareTests.cs
@@ -0,0 +1,81 @@
+using LehrerApp.Sync;
+using Microsoft.AspNetCore.Http;
+using Xunit;
+
+namespace LehrerApp.Api.Tests;
+
+public sealed class SyncProtocolVersionMiddlewareTests
+{
+    [Theory]
+    [InlineData("/api/sync/push")]
+    [InlineData("/api/sync/plain/push")]
+    [InlineData("/api/sync/attachments/datei")]
+    [InlineData("/api/snapshot/upload")]
+    public async Task FehlendeVersion_BlockiertAlleSyncKanaeleVorDemEndpoint(string path)
+    {
+        var endpointReached = false;
+        var middleware = Middleware(() => endpointReached = true);
+        var context = Context(path);
+
+        await middleware.InvokeAsync(context);
+
+        Assert.False(endpointReached);
+        Assert.Equal(StatusCodes.Status426UpgradeRequired, context.Response.StatusCode);
+        Assert.Equal(SyncProtocol.CurrentVersion,
+            context.Response.Headers[SyncProtocol.VersionHeaderName].ToString());
+    }
+
+    [Fact]
+    public async Task AbweichendeVersion_BlockiertRequest()
+    {
+        var endpointReached = false;
+        var middleware = Middleware(() => endpointReached = true);
+        var context = Context("/api/sync/push");
+        context.Request.Headers[SyncProtocol.VersionHeaderName] = "0";
+
+        await middleware.InvokeAsync(context);
+
+        Assert.False(endpointReached);
+        Assert.Equal(StatusCodes.Status426UpgradeRequired, context.Response.StatusCode);
+    }
+
+    [Fact]
+    public async Task AktuelleVersion_LaesstRequestZumEndpointDurch()
+    {
+        var endpointReached = false;
+        var middleware = Middleware(() => endpointReached = true);
+        var context = Context("/api/sync/push");
+        context.Request.Headers[SyncProtocol.VersionHeaderName] = SyncProtocol.CurrentVersion;
+
+        await middleware.InvokeAsync(context);
+
+        Assert.True(endpointReached);
+        Assert.NotEqual(StatusCodes.Status426UpgradeRequired, context.Response.StatusCode);
+    }
+
+    [Fact]
+    public async Task NichtSyncEndpoint_BrauchtKeineVersion()
+    {
+        var endpointReached = false;
+        var middleware = Middleware(() => endpointReached = true);
+        var context = Context("/api/auth/login");
+
+        await middleware.InvokeAsync(context);
+
+        Assert.True(endpointReached);
+    }
+
+    private static SyncProtocolVersionMiddleware Middleware(Action reached) => new(_ =>
+    {
+        reached();
+        return Task.CompletedTask;
+    });
+
+    private static DefaultHttpContext Context(string path)
+    {
+        var context = new DefaultHttpContext();
+        context.Request.Path = path;
+        context.Response.Body = new MemoryStream();
+        return context;
+    }
+}
diff --git a/LehrerApp.Api/Program.cs b/LehrerApp.Api/Program.cs
index a718ddc..34dca9d 100644
--- a/LehrerApp.Api/Program.cs
+++ b/LehrerApp.Api/Program.cs
@@ -85,6 +85,8 @@ builder.Services.AddSingleton<PlainEventStore>(sp =>
 var app = builder.Build();
 app.UseForwardedHeaders();
 app.UseRateLimiter();
+// Muss vor Auth/Endpoints laufen: inkompatible Clients dürfen keinen Sync-Store erreichen.
+app.UseMiddleware<SyncProtocolVersionMiddleware>();
 app.UseAuthentication();
 app.UseAuthorization();
 app.MapAuthEndpoints(secret);
diff --git a/LehrerApp.Api/SyncProtocolVersionMiddleware.cs b/LehrerApp.Api/SyncProtocolVersionMiddleware.cs
new file mode 100644
index 0000000..2f02c82
--- /dev/null
+++ b/LehrerApp.Api/SyncProtocolVersionMiddleware.cs
@@ -0,0 +1,44 @@
+using System.Text.Json;
+using LehrerApp.Sync;
+
+namespace LehrerApp.Api;
+
+/// <summary>
+/// Stoppt veraltete Sync-Clients vor Authentifizierung und insbesondere vor jedem Store-Zugriff.
+/// Die Prüfung gilt auch für Anhänge, Plain-Sync und Device-Pairing-Snapshots.
+/// </summary>
+public sealed class SyncProtocolVersionMiddleware(RequestDelegate next)
+{
+    public async Task InvokeAsync(HttpContext context)
+    {
+        if (!IsProtectedSyncPath(context.Request.Path))
+        {
+            await next(context);
+            return;
+        }
+
+        context.Response.Headers[SyncProtocol.VersionHeaderName] = SyncProtocol.CurrentVersion;
+        var clientVersion = context.Request.Headers[SyncProtocol.VersionHeaderName].ToString();
+        if (string.Equals(clientVersion, SyncProtocol.CurrentVersion, StringComparison.Ordinal))
+        {
+            await next(context);
+            return;
+        }
+
+        context.Response.StatusCode = StatusCodes.Status426UpgradeRequired;
+        context.Response.ContentType = "application/problem+json";
+        await JsonSerializer.SerializeAsync(context.Response.Body, new
+        {
+            type = "https://lehrerapp.local/problems/sync-version-mismatch",
+            title = "Inkompatible Sync-Version",
+            status = StatusCodes.Status426UpgradeRequired,
+            detail = $"Der Server erwartet Sync-Version {SyncProtocol.CurrentVersion}.",
+            expectedVersion = SyncProtocol.CurrentVersion,
+            receivedVersion = string.IsNullOrEmpty(clientVersion) ? null : clientVersion,
+        });
+    }
+
+    private static bool IsProtectedSyncPath(PathString path) =>
+        path.StartsWithSegments("/api/sync", StringComparison.OrdinalIgnoreCase) ||
+        path.StartsWithSegments("/api/snapshot", StringComparison.OrdinalIgnoreCase);
+}
diff --git a/LehrerApp.Desktop.Tests/SyncAuthServiceProtocolTests.cs b/LehrerApp.Desktop.Tests/SyncAuthServiceProtocolTests.cs
new file mode 100644
index 0000000..ef5f5cb
--- /dev/null
+++ b/LehrerApp.Desktop.Tests/SyncAuthServiceProtocolTests.cs
@@ -0,0 +1,33 @@
+using System.Net;
+using LehrerApp.Desktop.Services;
+using LehrerApp.Sync;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class SyncAuthServiceProtocolTests
+{
+    [Fact]
+    public async Task TestConnectionAsync_InkompatibleVersion_WirdNichtAlsOfflineGemeldet()
+    {
+        HttpRequestMessage? sent = null;
+        var service = new SyncAuthService(new HttpClient(new Handler(request =>
+        {
+            sent = request;
+            return new HttpResponseMessage(HttpStatusCode.UpgradeRequired);
+        })));
+
+        var result = await service.TestConnectionAsync("https://sync.example.invalid", "token");
+
+        Assert.Equal(SyncConnectionTestResult.IncompatibleVersion, result);
+        Assert.Equal(SyncProtocol.CurrentVersion,
+            sent!.Headers.GetValues(SyncProtocol.VersionHeaderName).Single());
+    }
+
+    private sealed class Handler(Func<HttpRequestMessage, HttpResponseMessage> response) : HttpMessageHandler
+    {
+        protected override Task<HttpResponseMessage> SendAsync(
+            HttpRequestMessage request, CancellationToken cancellationToken) =>
+            Task.FromResult(response(request));
+    }
+}
diff --git a/LehrerApp.Desktop.Tests/SyncStatusViewModelTests.cs b/LehrerApp.Desktop.Tests/SyncStatusViewModelTests.cs
index caf940d..2e98515 100644
--- a/LehrerApp.Desktop.Tests/SyncStatusViewModelTests.cs
+++ b/LehrerApp.Desktop.Tests/SyncStatusViewModelTests.cs
@@ -67,6 +67,19 @@ public sealed class SyncStatusViewModelTests
         Assert.False(fired);
     }
 
+    [Fact]
+    public async Task InkompatibleSyncVersion_ZeigtKlareUpdateMeldung()
+    {
+        using var temp = new TempSyncEngine(new IncompatibleVersionStubHandler());
+        var vm = new SyncStatusViewModel(temp.Engine);
+
+        await temp.Engine.SyncNowAsync();
+
+        Assert.True(vm.IsIncompatibleVersion);
+        Assert.Equal("Update erforderlich", vm.StatusText);
+        Assert.Contains("Bitte aktualisiere die LehrerApp", SyncStatusViewModel.IncompatibleVersionMessage);
+    }
+
     private sealed class PullEventStubHandler : HttpMessageHandler
     {
         protected override Task<HttpResponseMessage> SendAsync(
@@ -100,6 +113,17 @@ public sealed class SyncStatusViewModelTests
             });
     }
 
+    private sealed class IncompatibleVersionStubHandler : HttpMessageHandler
+    {
+        protected override Task<HttpResponseMessage> SendAsync(
+            HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            var response = new HttpResponseMessage(HttpStatusCode.UpgradeRequired);
+            response.Headers.Add(SyncProtocol.VersionHeaderName, "2");
+            return Task.FromResult(response);
+        }
+    }
+
     private sealed class TempSyncEngine : IDisposable
     {
         private readonly string _queuePath = Path.Combine(
diff --git a/LehrerApp.Desktop/AppBootstrapper.cs b/LehrerApp.Desktop/AppBootstrapper.cs
index a6c9d89..0eb89af 100644
--- a/LehrerApp.Desktop/AppBootstrapper.cs
+++ b/LehrerApp.Desktop/AppBootstrapper.cs
@@ -253,7 +253,9 @@ public static class AppBootstrapper
         services.AddSingleton<MainWindowViewModel>();
         services.AddSingleton<DashboardViewModel>();
         services.AddSingleton(sp =>
-            new SyncStatusViewModel(sp.GetService<SyncEngine>()));
+            new SyncStatusViewModel(
+                sp.GetService<SyncEngine>(),
+                sp.GetRequiredService<NotificationService>()));
         services.AddSingleton<GroupListViewModel>();
         services.AddSingleton<StudentListViewModel>();
         services.AddSingleton<TimetableViewModel>();
diff --git a/LehrerApp.Desktop/Services/SyncAuthService.cs b/LehrerApp.Desktop/Services/SyncAuthService.cs
index af4582a..f973eeb 100644
--- a/LehrerApp.Desktop/Services/SyncAuthService.cs
+++ b/LehrerApp.Desktop/Services/SyncAuthService.cs
@@ -1,11 +1,12 @@
 using System.Net;
 using System.Net.Http.Json;
+using LehrerApp.Sync;
 
 namespace LehrerApp.Desktop.Services;
 
 public class SyncAuthException(string userMessage) : Exception(userMessage);
 
-public enum SyncConnectionTestResult { Ok, Unauthorized, Unreachable }
+public enum SyncConnectionTestResult { Ok, Unauthorized, IncompatibleVersion, Unreachable }
 
 /// <summary>
 /// Login/Verbindungstest gegen den eigenen Sync-Server (LehrerApp.Api) — getrennt vom bereits
@@ -27,7 +28,10 @@ public class SyncAuthService(HttpClient http)
         HttpResponseMessage resp;
         try
         {
-            resp = await http.PostAsJsonAsync(CombineUrl(serverUrl, "/api/auth/login"), new { username, password });
+            using var req = SyncProtocol.CreateRequest(HttpMethod.Post,
+                CombineUrl(serverUrl, "/api/auth/login"));
+            req.Content = JsonContent.Create(new { username, password });
+            resp = await http.SendAsync(req);
         }
         catch (HttpRequestException)
         {
@@ -49,6 +53,7 @@ public class SyncAuthService(HttpClient http)
     public async Task<SyncConnectionTestResult> TestConnectionAsync(string serverUrl, string? token)
     {
         using var req = new HttpRequestMessage(HttpMethod.Get, CombineUrl(serverUrl, "/api/sync/status"));
+        req.Headers.Add(SyncProtocol.VersionHeaderName, SyncProtocol.CurrentVersion);
         if (token is not null)
             req.Headers.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", token);
 
@@ -56,6 +61,8 @@ public class SyncAuthService(HttpClient http)
         {
             var resp = await http.SendAsync(req);
             if (resp.StatusCode == HttpStatusCode.Unauthorized) return SyncConnectionTestResult.Unauthorized;
+            if (resp.StatusCode == HttpStatusCode.UpgradeRequired)
+                return SyncConnectionTestResult.IncompatibleVersion;
             return resp.IsSuccessStatusCode ? SyncConnectionTestResult.Ok : SyncConnectionTestResult.Unreachable;
         }
         catch (HttpRequestException) { return SyncConnectionTestResult.Unreachable; }
diff --git a/LehrerApp.Desktop/ViewModels/Settings/SettingsViewModel.cs b/LehrerApp.Desktop/ViewModels/Settings/SettingsViewModel.cs
index 04c3e98..bfb8c2e 100644
--- a/LehrerApp.Desktop/ViewModels/Settings/SettingsViewModel.cs
+++ b/LehrerApp.Desktop/ViewModels/Settings/SettingsViewModel.cs
@@ -512,6 +512,9 @@ public partial class SettingsViewModel : ObservableObject
         {
             SyncConnectionTestResult.Ok => "Verbindung erfolgreich.",
             SyncConnectionTestResult.Unauthorized => "Server erreichbar, aber nicht angemeldet oder Anmeldung abgelaufen.",
+            SyncConnectionTestResult.IncompatibleVersion =>
+                "Diese App-Version ist veraltet oder nicht mehr mit dem Sync-Server kompatibel. " +
+                "Bitte aktualisiere die LehrerApp.",
             _ => "Server nicht erreichbar. Bitte Adresse und Internetverbindung prüfen.",
         };
     }
diff --git a/LehrerApp.Desktop/ViewModels/SyncStatusViewModel.cs b/LehrerApp.Desktop/ViewModels/SyncStatusViewModel.cs
index d6e4528..42f3e3f 100644
--- a/LehrerApp.Desktop/ViewModels/SyncStatusViewModel.cs
+++ b/LehrerApp.Desktop/ViewModels/SyncStatusViewModel.cs
@@ -1,5 +1,6 @@
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
+using LehrerApp.Desktop.Services;
 using LehrerApp.Sync;
 using LehrerApp.Sync.Models;
 
@@ -8,11 +9,19 @@ namespace LehrerApp.Desktop.ViewModels;
 public partial class SyncStatusViewModel : ObservableObject
 {
     private readonly SyncEngine? _engine;
+    private readonly NotificationService? _notifications;
+    private bool _incompatibleNotificationShown;
+
+    public const string IncompatibleVersionMessage =
+        "Diese App-Version ist veraltet oder nicht mit dem Sync-Server kompatibel. " +
+        "Bitte aktualisiere die LehrerApp. Die Synchronisierung wurde zum Schutz deiner Daten angehalten.";
 
     [ObservableProperty] private string _statusText = "Kein Server konfiguriert";
     [ObservableProperty] private string _lastSyncText = "";
     [ObservableProperty] private bool _isSyncing;
     [ObservableProperty] private bool _isServerConfigured;
+    [ObservableProperty] private bool _isIncompatibleVersion;
+    [ObservableProperty] private bool _canAttemptSync;
     [ObservableProperty] private int _pendingCount;
 
     /// <summary>Feuert, wenn ein Sync tatsächlich Daten angewendet hat (siehe SyncEngine.
@@ -21,10 +30,12 @@ public partial class SyncStatusViewModel : ObservableObject
     /// schreibt.</summary>
     public event Action? DataChanged;
 
-    public SyncStatusViewModel(SyncEngine? engine)
+    public SyncStatusViewModel(SyncEngine? engine, NotificationService? notifications = null)
     {
         _engine            = engine;
+        _notifications     = notifications;
         IsServerConfigured = engine is not null;
+        CanAttemptSync     = IsServerConfigured;
         if (_engine is not null)
         {
             _engine.StatusChanged += OnStatus;
@@ -41,6 +52,8 @@ public partial class SyncStatusViewModel : ObservableObject
     private void OnStatus(SyncStatus s)
     {
         IsSyncing    = s.State == SyncState.Syncing;
+        IsIncompatibleVersion = s.State == SyncState.IncompatibleVersion;
+        CanAttemptSync = IsServerConfigured && !IsIncompatibleVersion;
         PendingCount = s.PendingEvents;
         StatusText   = s.State switch
         {
@@ -48,9 +61,16 @@ public partial class SyncStatusViewModel : ObservableObject
             SyncState.Syncing => "Synchronisiere…",
             SyncState.Offline => "Offline",
             SyncState.Error   => $"Fehler: {s.ErrorMessage}",
+            SyncState.IncompatibleVersion => "Update erforderlich",
             _ => "",
         };
         LastSyncText = s.LastSyncAt.HasValue ? $"Zuletzt: {s.LastSyncAt:HH:mm}" : "Noch nie";
+
+        if (IsIncompatibleVersion && !_incompatibleNotificationShown)
+        {
+            _incompatibleNotificationShown = true;
+            _notifications?.ShowError(IncompatibleVersionMessage);
+        }
     }
 
     [RelayCommand(CanExecute = nameof(CanSync))]
diff --git a/LehrerApp.Desktop/Views/MainWindow.axaml b/LehrerApp.Desktop/Views/MainWindow.axaml
index 6369a89..3dfc8d5 100644
--- a/LehrerApp.Desktop/Views/MainWindow.axaml
+++ b/LehrerApp.Desktop/Views/MainWindow.axaml
@@ -30,8 +30,7 @@
   <DrawerPage x:Name="RootDrawer"
               DrawerLength="220"
               DrawerBehavior="Auto"
-              DrawerLayoutBehavior="CompactInline"
-              Content="{Binding CurrentPage}">
+              DrawerLayoutBehavior="CompactInline">
 
     <DrawerPage.DataTemplates>
       <DataTemplate DataType="vm:DashboardViewModel">
@@ -63,6 +62,26 @@
       </DataTemplate>
     </DrawerPage.DataTemplates>
 
+    <DrawerPage.Content>
+      <DockPanel>
+        <Border DockPanel.Dock="Top"
+                IsVisible="{Binding SyncStatus.IsIncompatibleVersion}"
+                Background="#B3261E" Padding="16,12">
+          <StackPanel Orientation="Horizontal" Spacing="10">
+            <TextBlock Text="⚠" FontSize="18" Foreground="White"
+                       VerticalAlignment="Center"/>
+            <StackPanel>
+              <TextBlock Text="Software-Update erforderlich"
+                         Foreground="White" FontWeight="SemiBold"/>
+              <TextBlock Text="{x:Static vm:SyncStatusViewModel.IncompatibleVersionMessage}"
+                         Foreground="White" Opacity="0.92" TextWrapping="Wrap"/>
+            </StackPanel>
+          </StackPanel>
+        </Border>
+        <ContentControl Content="{Binding CurrentPage}"/>
+      </DockPanel>
+    </DrawerPage.Content>
+
     <DrawerPage.Drawer>
       <DockPanel Classes.compact="{Binding !#RootDrawer.IsOpen}">
         <DockPanel.Styles>
diff --git a/LehrerApp.Desktop/Views/SyncStatusBar.axaml b/LehrerApp.Desktop/Views/SyncStatusBar.axaml
index 829f861..3a036ee 100644
--- a/LehrerApp.Desktop/Views/SyncStatusBar.axaml
+++ b/LehrerApp.Desktop/Views/SyncStatusBar.axaml
@@ -4,14 +4,19 @@
              x:Class="LehrerApp.Desktop.Views.SyncStatusBar"
              x:DataType="vm:SyncStatusViewModel">
   <Grid ColumnDefinitions="*,Auto">
-    <StackPanel Grid.Column="0">
+    <StackPanel Grid.Column="0" IsVisible="{Binding !IsIncompatibleVersion}">
       <TextBlock Text="{Binding StatusText}" FontSize="12" Opacity="0.7"
                  TextTrimming="CharacterEllipsis"/>
       <TextBlock Text="{Binding LastSyncText}" FontSize="10" Opacity="0.4"/>
     </StackPanel>
+    <StackPanel Grid.Column="0" IsVisible="{Binding IsIncompatibleVersion}">
+      <TextBlock Text="⚠ Update erforderlich" FontSize="12" FontWeight="SemiBold"
+                 Foreground="#D32F2F"/>
+      <TextBlock Text="Sync angehalten" FontSize="10" Foreground="#D32F2F" Opacity="0.8"/>
+    </StackPanel>
     <Button Grid.Column="1" Content="↻" FontSize="14"
             Command="{Binding SyncNowCommand}"
-            IsVisible="{Binding IsServerConfigured}"
+            IsVisible="{Binding CanAttemptSync}"
             Background="Transparent" Padding="6,4"
             ToolTip.Tip="Jetzt synchronisieren"/>
   </Grid>
diff --git a/LehrerApp.Sync.Tests/AttachmentSyncerTests.cs b/LehrerApp.Sync.Tests/AttachmentSyncerTests.cs
index 6b7d9ca..864f18c 100644
--- a/LehrerApp.Sync.Tests/AttachmentSyncerTests.cs
+++ b/LehrerApp.Sync.Tests/AttachmentSyncerTests.cs
@@ -34,6 +34,8 @@ public sealed class AttachmentSyncerTests
         var request = Assert.Single(handler.Requests);
         Assert.Equal(HttpMethod.Post, request.Method);
         Assert.Equal($"/api/sync/attachments/{storageId}", request.RequestUri!.AbsolutePath);
+        Assert.Equal(SyncProtocol.CurrentVersion,
+            request.Headers.GetValues(SyncProtocol.VersionHeaderName).Single());
         Assert.NotNull(uploadedEncrypted);
         Assert.Equal([1, 2, 3, 4], SyncCrypto.Decrypt(uploadedEncrypted!, Key));
         Assert.Empty(temp.Queue.GetPendingAttachmentUploads());
diff --git a/LehrerApp.Sync.Tests/SyncEngineTests.cs b/LehrerApp.Sync.Tests/SyncEngineTests.cs
index d1306ac..74c25f0 100644
--- a/LehrerApp.Sync.Tests/SyncEngineTests.cs
+++ b/LehrerApp.Sync.Tests/SyncEngineTests.cs
@@ -13,6 +13,33 @@ public sealed class SyncEngineTests
     private static LiteDbContext NewInMemoryContext() => new(new MemoryStream());
     private static readonly byte[] Key = SyncCrypto.GenerateKey();
 
+    [Fact]
+    public async Task SyncNowAsync_InkompatibleServerVersion_BrichtVorWeiterenRequestsAb()
+    {
+        using var temp = new TempEventQueue();
+        var pending = temp.Queue.Enqueue("this-device", DeviceType.Desktop,
+            "Lesson", Guid.NewGuid().ToString(), "Save", "lokale-daten");
+        var handler = new FakeHttpMessageHandler(req =>
+        {
+            var response = new HttpResponseMessage(HttpStatusCode.UpgradeRequired);
+            response.Headers.Add(SyncProtocol.VersionHeaderName, "2");
+            return response;
+        });
+        var engine = MakeEngine(temp, handler);
+
+        var result = await engine.SyncNowAsync();
+
+        Assert.False(result.Success);
+        Assert.Contains("Sync-Version 1", result.Reason);
+        Assert.Contains("Server Version 2", result.Reason);
+        Assert.Equal(SyncState.IncompatibleVersion, engine.Status.State);
+        Assert.Equal(pending.EventId, Assert.Single(temp.Queue.GetPending()).EventId);
+        var request = Assert.Single(handler.Requests);
+        Assert.Equal("/api/sync/push", request.RequestUri!.AbsolutePath);
+        Assert.Equal(SyncProtocol.CurrentVersion,
+            request.Headers.GetValues(SyncProtocol.VersionHeaderName).Single());
+    }
+
     /// Regression: EventApplier.ApplyAsync fing früher nur LiteException ab. Jede andere Ausnahme
     /// (z.B. eine ungültige/korrupte Payload eines einzelnen Ereignisses) fiel unbehandelt aus
     /// SyncEngine.PullAsync heraus, BEVOR _queue.SetLastServerSeq() erreicht wurde — der nächste
diff --git a/LehrerApp.Sync/AttachmentSyncer.cs b/LehrerApp.Sync/AttachmentSyncer.cs
index f71064a..25c0bad 100644
--- a/LehrerApp.Sync/AttachmentSyncer.cs
+++ b/LehrerApp.Sync/AttachmentSyncer.cs
@@ -27,9 +27,11 @@ public class AttachmentSyncer(LiteDbContext db, HttpClient http, byte[] syncKey)
             await raw.CopyToAsync(buffer);
             var encrypted = SyncCrypto.Encrypt(buffer.ToArray(), syncKey);
 
-            using var content = new ByteArrayContent(encrypted);
-            var resp = await http.PostAsync($"/api/sync/attachments/{storageId}", content);
-            resp.EnsureSuccessStatusCode();
+            using var request = SyncProtocol.CreateRequest(HttpMethod.Post,
+                $"/api/sync/attachments/{storageId}");
+            request.Content = new ByteArrayContent(encrypted);
+            using var resp = await http.SendAsync(request);
+            await SyncProtocol.EnsureCompatibleSuccessAsync(resp);
             queue.MarkAttachmentUploaded(storageId);
         }
     }
diff --git a/LehrerApp.Sync/EventApplier.cs b/LehrerApp.Sync/EventApplier.cs
index 43e4bcf..2d535cb 100644
--- a/LehrerApp.Sync/EventApplier.cs
+++ b/LehrerApp.Sync/EventApplier.cs
@@ -55,6 +55,12 @@ public class EventApplier(LiteDbContext db, byte[] syncKey, HttpClient? http = n
             versions?.SetKnownServerSeq(evt.EntityType, evt.EntityId, evt.SequenceNr);
             logger?.Info($"Sync: Ereignis angewendet - {evt.EntityType} {evt.Operation} EntityId={evt.EntityId}");
         }
+        catch (SyncProtocolMismatchException)
+        {
+            // Anders als ein einzelnes korruptes Ereignis betrifft dies den gesamten Batch. Der
+            // Pull-Cursor darf nicht vorrücken, solange Client und Server inkompatibel sind.
+            throw;
+        }
         catch (LiteException ex)
         {
             // Harte Constraint-Verletzung (z.B. Unique-Index) - dieses eine Ereignis
@@ -86,7 +92,11 @@ public class EventApplier(LiteDbContext db, byte[] syncKey, HttpClient? http = n
         foreach (var attachment in entity.Attachments)
         {
             if (db.Attachments.Exists(attachment.StorageId)) continue;
-            var resp = await http!.GetAsync($"/api/sync/attachments/{attachment.StorageId}");
+            using var request = SyncProtocol.CreateRequest(HttpMethod.Get,
+                $"/api/sync/attachments/{attachment.StorageId}");
+            using var resp = await http!.SendAsync(request);
+            if (resp.StatusCode == System.Net.HttpStatusCode.UpgradeRequired)
+                await SyncProtocol.EnsureCompatibleSuccessAsync(resp);
             if (!resp.IsSuccessStatusCode) continue;
             var encrypted = await resp.Content.ReadAsByteArrayAsync();
             var decrypted = SyncCrypto.Decrypt(encrypted, syncKey);
diff --git a/LehrerApp.Sync/Models/SyncModels.cs b/LehrerApp.Sync/Models/SyncModels.cs
index e909956..fbeebe9 100644
--- a/LehrerApp.Sync/Models/SyncModels.cs
+++ b/LehrerApp.Sync/Models/SyncModels.cs
@@ -114,4 +114,4 @@ public class SyncStatus
 // ── Enums ─────────────────────────────────────────────────────────────────────
 
 public enum DeviceType { Desktop, Companion }
-public enum SyncState  { Idle, Syncing, Error, Offline }
+public enum SyncState  { Idle, Syncing, Error, Offline, IncompatibleVersion }
diff --git a/LehrerApp.Sync/SnapshotService.cs b/LehrerApp.Sync/SnapshotService.cs
index 8856ba6..d7a9fb2 100644
--- a/LehrerApp.Sync/SnapshotService.cs
+++ b/LehrerApp.Sync/SnapshotService.cs
@@ -28,20 +28,24 @@ public class SnapshotService(
 
         // Schritt 1: Upload ohne Key → Code erhalten
         Report(SnapshotStep.Uploading, "Code wird angefordert…");
-        var r1 = await http.PostAsJsonAsync("/api/snapshot/upload",
-            new SnapshotUploadRequest { EncryptedPayload = encPayload, DeviceType = deviceType }, ct);
-        r1.EnsureSuccessStatusCode();
+        using var request1 = SyncProtocol.CreateRequest(HttpMethod.Post, "/api/snapshot/upload");
+        request1.Content = JsonContent.Create(
+            new SnapshotUploadRequest { EncryptedPayload = encPayload, DeviceType = deviceType });
+        using var r1 = await http.SendAsync(request1, ct);
+        await SyncProtocol.EnsureCompatibleSuccessAsync(r1);
         var init = await r1.Content.ReadFromJsonAsync<SnapshotUploadResponse>(ct)
             ?? throw new InvalidOperationException("Leere Server-Antwort.");
 
         // Schritt 2: Key mit Code verschlüsseln + erneut hochladen
         Report(SnapshotStep.Uploading, "Schlüssel wird verschlüsselt…");
         var encKey = SyncCrypto.EncryptKeyWithCode(syncKey, init.Code);
-        var r2 = await http.PostAsJsonAsync("/api/snapshot/upload",
+        using var request2 = SyncProtocol.CreateRequest(HttpMethod.Post, "/api/snapshot/upload");
+        request2.Content = JsonContent.Create(
             new SnapshotUploadRequest { EncryptedPayload = encPayload,
                                         EncryptedSyncKey = encKey, DeviceType = deviceType,
-                                        Code = init.Code }, ct);
-        r2.EnsureSuccessStatusCode();
+                                        Code = init.Code });
+        using var r2 = await http.SendAsync(request2, ct);
+        await SyncProtocol.EnsureCompatibleSuccessAsync(r2);
         var result = await r2.Content.ReadFromJsonAsync<SnapshotUploadResponse>(ct)
             ?? throw new InvalidOperationException("Leere Server-Antwort.");
         Report(SnapshotStep.Done, $"Bereit – Code: {result.Code}");
@@ -53,10 +57,11 @@ public class SnapshotService(
     {
         var sanitized = code.Trim().ToUpperInvariant();
         Report(SnapshotStep.Downloading, "Snapshot wird geladen…");
-        var resp = await http.GetAsync($"/api/snapshot/{sanitized}", ct);
+        using var request = SyncProtocol.CreateRequest(HttpMethod.Get, $"/api/snapshot/{sanitized}");
+        using var resp = await http.SendAsync(request, ct);
         if (resp.StatusCode == System.Net.HttpStatusCode.NotFound)
             throw new SnapshotNotFoundException($"Code '{sanitized}' nicht gefunden oder abgelaufen.");
-        resp.EnsureSuccessStatusCode();
+        await SyncProtocol.EnsureCompatibleSuccessAsync(resp);
         var dl = await resp.Content.ReadFromJsonAsync<SnapshotDownloadResponse>(ct)
             ?? throw new InvalidOperationException("Leere Server-Antwort.");
 
diff --git a/LehrerApp.Sync/SyncEngine.cs b/LehrerApp.Sync/SyncEngine.cs
index 42be336..ce56581 100644
--- a/LehrerApp.Sync/SyncEngine.cs
+++ b/LehrerApp.Sync/SyncEngine.cs
@@ -100,6 +100,12 @@ public class SyncEngine : IDisposable
                           $"{pulled} gepullt ({conflicts} Pull-Konflikte)");
             return new() { Success = true, EventsPushed = pushed, EventsPulled = pulled, Conflicts = conflicts };
         }
+        catch (SyncProtocolMismatchException ex)
+        {
+            _logger?.Error("Sync wegen inkompatibler Protokollversion abgebrochen", ex);
+            SetState(SyncState.IncompatibleVersion, ex.Message);
+            return new() { Reason = ex.Message };
+        }
         catch (HttpRequestException ex)
         {
             // Sammelt sowohl echte Netzwerkfehler als auch nicht-erfolgreiche HTTP-Antworten
@@ -128,8 +134,10 @@ public class SyncEngine : IDisposable
             evt.BasedOnServerSeq = _queue.GetKnownServerSeq(evt.EntityType, evt.EntityId);
         _logger?.Info($"Sync: Push - {pending.Count} Ereignis(se) ausstehend: " +
                       string.Join(", ", pending.Select(e => $"{e.EntityType}/{e.Operation}")));
-        var resp = await _http.PostAsJsonAsync("/api/sync/push", pending);
-        resp.EnsureSuccessStatusCode();
+        using var request = SyncProtocol.CreateRequest(HttpMethod.Post, "/api/sync/push");
+        request.Content = JsonContent.Create(pending);
+        using var resp = await _http.SendAsync(request);
+        await SyncProtocol.EnsureCompatibleSuccessAsync(resp);
         var result = await resp.Content.ReadFromJsonAsync<PushResponse>();
         if (result is null) { _logger?.Warn("Sync: Push - leere Server-Antwort."); return (0, 0); }
         _queue.Acknowledge(pending
@@ -196,7 +204,9 @@ public class SyncEngine : IDisposable
             HttpResponseMessage resp;
             try
             {
-                resp = await _http.GetAsync($"/api/sync/entity/{local.EntityType}/{local.EntityId}");
+                using var request = SyncProtocol.CreateRequest(HttpMethod.Get,
+                    $"/api/sync/entity/{local.EntityType}/{local.EntityId}");
+                resp = await _http.SendAsync(request);
             }
             catch (Exception ex)
             {
@@ -217,7 +227,7 @@ public class SyncEngine : IDisposable
                               "nächster Push behandelt sie als neu.");
                 continue;
             }
-            resp.EnsureSuccessStatusCode();
+            await SyncProtocol.EnsureCompatibleSuccessAsync(resp);
             var remote = await resp.Content.ReadFromJsonAsync<SyncEvent>();
             if (remote is null)
             {
@@ -253,8 +263,11 @@ public class SyncEngine : IDisposable
     {
         var since = _queue.GetLastServerSeq();
         _logger?.Info($"Sync: Pull - frage Server nach Ereignissen seit ServerSequenceNr={since}.");
-        var resp = await _http.GetFromJsonAsync<PullResponse>(
+        using var request = SyncProtocol.CreateRequest(HttpMethod.Get,
             $"/api/sync/pull?since={since}&deviceId={_config.DeviceId}");
+        using var response = await _http.SendAsync(request);
+        await SyncProtocol.EnsureCompatibleSuccessAsync(response);
+        var resp = await response.Content.ReadFromJsonAsync<PullResponse>();
         if (resp is null || resp.Events.Count == 0)
         {
             _logger?.Info("Sync: Pull - keine neuen Ereignisse vom Server.");
diff --git a/LehrerApp.Sync/SyncProtocol.cs b/LehrerApp.Sync/SyncProtocol.cs
new file mode 100644
index 0000000..6b20135
--- /dev/null
+++ b/LehrerApp.Sync/SyncProtocol.cs
@@ -0,0 +1,44 @@
+using System.Net;
+
+namespace LehrerApp.Sync;
+
+/// <summary>
+/// Version des Wire-Protokolls zwischen Desktop-App und Sync-Server.
+///
+/// Bei jeder inkompatiblen Änderung an Sync-Events, Snapshots oder deren Verarbeitung erhöhen.
+/// Der Server lehnt Clients mit einer anderen (oder keiner) Version ab, bevor Daten gelesen oder
+/// geschrieben werden. Dadurch können alte App-Versionen nach einem Server-Deployment keine
+/// nicht mehr kompatiblen Daten in den Server-Store schreiben.
+/// </summary>
+public static class SyncProtocol
+{
+    public const string CurrentVersion = "1";
+    public const string VersionHeaderName = "X-LehrerApp-Sync-Version";
+
+    public static HttpRequestMessage CreateRequest(HttpMethod method, string requestUri)
+    {
+        var request = new HttpRequestMessage(method, requestUri);
+        request.Headers.Add(VersionHeaderName, CurrentVersion);
+        return request;
+    }
+
+    public static Task EnsureCompatibleSuccessAsync(HttpResponseMessage response)
+    {
+        if (response.StatusCode == HttpStatusCode.UpgradeRequired)
+        {
+            var serverVersion = response.Headers.TryGetValues(VersionHeaderName, out var values)
+                ? values.FirstOrDefault()
+                : null;
+            var detail = serverVersion is null
+                ? $"Diese App verwendet Sync-Version {CurrentVersion}, der Server eine andere Version."
+                : $"Diese App verwendet Sync-Version {CurrentVersion}, der Server Version {serverVersion}.";
+            throw new SyncProtocolMismatchException(
+                $"{detail} Bitte App und Server auf denselben Stand aktualisieren.");
+        }
+
+        response.EnsureSuccessStatusCode();
+        return Task.CompletedTask;
+    }
+}
+
+public sealed class SyncProtocolMismatchException(string message) : Exception(message);