diff --git a/src/MikrotikInstaller.Core.Tests/Connectivity/DemoRouterOsClientTests.cs b/src/MikrotikInstaller.Core.Tests/Connectivity/DemoRouterOsClientTests.cs
new file mode 100644
index 0000000..28e2bf4
--- /dev/null
+++ b/src/MikrotikInstaller.Core.Tests/Connectivity/DemoRouterOsClientTests.cs
@@ -0,0 +1,40 @@
+using MikrotikInstaller.Core.Connectivity;
+using MikrotikInstaller.Core.Connectivity.Demo;
+
+namespace MikrotikInstaller.Core.Tests.Connectivity;
+
+public class DemoRouterOsClientTests
+{
+    [Fact]
+    public void Matches_OnlyExactTestCredentials()
+    {
+        Assert.True(DemoRouterOsClient.Matches(new RouterOsCredentials("127.127.127.127", "admin", "admin")));
+        Assert.False(DemoRouterOsClient.Matches(new RouterOsCredentials("192.168.88.1", "admin", "admin")));
+        Assert.False(DemoRouterOsClient.Matches(new RouterOsCredentials("127.127.127.127", "admin", "falsch")));
+    }
+
+    [Fact]
+    public async Task GetAsync_ReturnsSimulatedInterfacesAndResource()
+    {
+        var client = new DemoRouterOsClient();
+
+        var interfaces = await client.GetAsync("/interface");
+        var resource = await client.GetAsync("/system/resource");
+
+        Assert.NotEmpty(interfaces);
+        Assert.Contains(interfaces, i => i["name"] == "ether1");
+        Assert.Single(resource);
+    }
+
+    [Fact]
+    public async Task AddAsync_StoresRowAndReturnsId()
+    {
+        var client = new DemoRouterOsClient();
+
+        var id = await client.AddAsync("/interface/bridge", new Dictionary<string, string> { ["name"] = "bridge-lan" });
+        var rows = await client.GetAsync("/interface/bridge");
+
+        Assert.False(string.IsNullOrEmpty(id));
+        Assert.Contains(rows, r => r["name"] == "bridge-lan" && r[".id"] == id);
+    }
+}
diff --git a/src/MikrotikInstaller.Core/Connectivity/Demo/DemoRouterOsClient.cs b/src/MikrotikInstaller.Core/Connectivity/Demo/DemoRouterOsClient.cs
new file mode 100644
index 0000000..b402acb
--- /dev/null
+++ b/src/MikrotikInstaller.Core/Connectivity/Demo/DemoRouterOsClient.cs
@@ -0,0 +1,95 @@
+namespace MikrotikInstaller.Core.Connectivity.Demo;
+
+/// <summary>
+/// Simulierter RouterOS-Client für interne Tests des Assistenten, ganz ohne echtes Gerät.
+/// Wird ausschließlich aktiviert, wenn exakt die Test-Zugangsdaten verwendet werden
+/// (<see cref="Matches"/>) — es findet dabei keinerlei Netzwerkverbindung statt, alle Daten
+/// liegen nur im Arbeitsspeicher dieses Objekts. Nur in Debug-Builds erreichbar
+/// (siehe <see cref="RouterOsClientFactory"/>), damit dieser Testzugang nicht in der
+/// veröffentlichten Version landet.
+/// </summary>
+public sealed class DemoRouterOsClient : IRouterOsClient
+{
+    public const string DemoHost = "127.127.127.127";
+    public const string DemoUsername = "admin";
+    public const string DemoPassword = "admin";
+
+    private readonly Dictionary<string, List<Dictionary<string, string>>> _data = BuildInitialData();
+    private int _nextId = 1;
+
+    public static bool Matches(RouterOsCredentials credentials) =>
+        credentials.Host == DemoHost && credentials.Username == DemoUsername && credentials.Password == DemoPassword;
+
+    public RouterOsProtocol Protocol => RouterOsProtocol.Rest;
+
+    public Task<IReadOnlyList<IReadOnlyDictionary<string, string>>> GetAsync(string path, CancellationToken cancellationToken = default)
+    {
+        var rows = _data.TryGetValue(Normalize(path), out var list) ? list : [];
+        IReadOnlyList<IReadOnlyDictionary<string, string>> result = rows.Select(r => (IReadOnlyDictionary<string, string>)r).ToList();
+        return Task.FromResult(result);
+    }
+
+    public Task<string> AddAsync(string path, IReadOnlyDictionary<string, string> parameters, CancellationToken cancellationToken = default)
+    {
+        var id = $"*{_nextId++}";
+        var row = new Dictionary<string, string>(parameters) { [".id"] = id };
+
+        var key = Normalize(path);
+        if (!_data.TryGetValue(key, out var list))
+        {
+            list = [];
+            _data[key] = list;
+        }
+
+        list.Add(row);
+        return Task.FromResult(id);
+    }
+
+    public Task SetAsync(string path, string id, IReadOnlyDictionary<string, string> parameters, CancellationToken cancellationToken = default)
+    {
+        if (_data.TryGetValue(Normalize(path), out var list))
+        {
+            var row = list.FirstOrDefault(r => r.GetValueOrDefault(".id") == id);
+            if (row is not null)
+            {
+                foreach (var (key, value) in parameters)
+                {
+                    row[key] = value;
+                }
+            }
+        }
+
+        return Task.CompletedTask;
+    }
+
+    public Task RemoveAsync(string path, string id, CancellationToken cancellationToken = default)
+    {
+        if (_data.TryGetValue(Normalize(path), out var list))
+        {
+            list.RemoveAll(r => r.GetValueOrDefault(".id") == id);
+        }
+
+        return Task.CompletedTask;
+    }
+
+    public Task ExecuteAsync(string path, IReadOnlyDictionary<string, string>? parameters = null, CancellationToken cancellationToken = default) =>
+        Task.CompletedTask;
+
+    public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+
+    private static string Normalize(string path) => path.TrimEnd('/');
+
+    private static Dictionary<string, List<Dictionary<string, string>>> BuildInitialData() => new()
+    {
+        ["/system/resource"] = [new() { ["board-name"] = "CHR (Simulator)", ["version"] = "7.15.2 (stable)", ["architecture-name"] = "x86_64" }],
+        ["/system/identity"] = [new() { ["name"] = "Test-Router" }],
+        ["/system/routerboard"] = [new() { ["routerboard"] = "no" }],
+        ["/interface"] =
+        [
+            new() { [".id"] = "*1", ["name"] = "ether1", ["type"] = "ether", ["running"] = "true", ["disabled"] = "false" },
+            new() { [".id"] = "*2", ["name"] = "ether2", ["type"] = "ether", ["running"] = "true", ["disabled"] = "false" },
+            new() { [".id"] = "*3", ["name"] = "ether3", ["type"] = "ether", ["running"] = "false", ["disabled"] = "false" },
+            new() { [".id"] = "*4", ["name"] = "wlan1", ["type"] = "wlan", ["running"] = "false", ["disabled"] = "true" },
+        ],
+    };
+}
diff --git a/src/MikrotikInstaller.Core/Connectivity/RouterOsClientFactory.cs b/src/MikrotikInstaller.Core/Connectivity/RouterOsClientFactory.cs
index 563f7b9..49a49a3 100644
--- a/src/MikrotikInstaller.Core/Connectivity/RouterOsClientFactory.cs
+++ b/src/MikrotikInstaller.Core/Connectivity/RouterOsClientFactory.cs
@@ -1,4 +1,5 @@
 using MikrotikInstaller.Core.Connectivity.Binary;
+using MikrotikInstaller.Core.Connectivity.Demo;
 using MikrotikInstaller.Core.Connectivity.Rest;
 
 namespace MikrotikInstaller.Core.Connectivity;
@@ -12,6 +13,16 @@ public static class RouterOsClientFactory
 {
     public static async Task<IRouterOsClient> ConnectAsync(RouterOsCredentials credentials, CancellationToken cancellationToken = default)
     {
+#if DEBUG
+        // Nur in Debug-Builds: interner Testzugang ohne echtes Gerät, siehe DemoRouterOsClient.
+        // Bewusst per Compile-Flag ausgeschlossen, damit dieser Zugang nicht in der für
+        // Endnutzer veröffentlichten Version landet.
+        if (DemoRouterOsClient.Matches(credentials))
+        {
+            return new DemoRouterOsClient();
+        }
+#endif
+
         try
         {
             return await RestRouterOsClient.ConnectAsync(credentials, cancellationToken);