diff --git a/Source/MQTTnet/Server/MqttClientConnection.cs b/Source/MQTTnet/Server/MqttClientConnection.cs
index e71d1a8..ed36b6a 100644
--- a/Source/MQTTnet/Server/MqttClientConnection.cs
+++ b/Source/MQTTnet/Server/MqttClientConnection.cs
@@ -118,13 +118,13 @@ namespace MQTTnet.Server
             _cancellationToken.Dispose();
         }
 
-        public Task<MqttClientDisconnectType> RunAsync()
+        public Task<MqttClientDisconnectType> RunAsync(MqttConnectionValidatorContext connectionValidatorContext)
         {
-            _packageReceiverTask = RunInternalAsync();
+            _packageReceiverTask = RunInternalAsync(connectionValidatorContext);
             return _packageReceiverTask;
         }
 
-        private async Task<MqttClientDisconnectType> RunInternalAsync()
+        private async Task<MqttClientDisconnectType> RunInternalAsync(MqttConnectionValidatorContext connectionValidatorContext)
         {
             var disconnectType = MqttClientDisconnectType.NotClean;
             try
@@ -142,12 +142,8 @@ namespace MQTTnet.Server
                 _keepAliveMonitor.Start(ConnectPacket.KeepAlivePeriod, _cancellationToken.Token);
 
                 await SendAsync(
-                    new MqttConnAckPacket
-                    {
-                        ReturnCode = MqttConnectReturnCode.ConnectionAccepted,
-                        ReasonCode = MqttConnectReasonCode.Success,
-                        IsSessionPresent = !Session.IsCleanSession
-                    }).ConfigureAwait(false);
+                    _channelAdapter.PacketFormatterAdapter.DataConverter.CreateConnAckPacket(connectionValidatorContext)
+                    ).ConfigureAwait(false);
 
                 Session.IsCleanSession = false;
 
diff --git a/Source/MQTTnet/Server/MqttClientSessionsManager.cs b/Source/MQTTnet/Server/MqttClientSessionsManager.cs
index db70e95..82d3c76 100644
--- a/Source/MQTTnet/Server/MqttClientSessionsManager.cs
+++ b/Source/MQTTnet/Server/MqttClientSessionsManager.cs
@@ -240,10 +240,10 @@ namespace MQTTnet.Server
                     return;
                 }
 
-                clientId = connectPacket.ClientId;
-
                 var connectionValidatorContext = await ValidateConnectionAsync(connectPacket, channelAdapter).ConfigureAwait(false);
 
+                clientId = connectPacket.ClientId;
+
                 if (connectionValidatorContext.ReasonCode != MqttConnectReasonCode.Success)
                 {
                     // Send failure response here without preparing a session. The result for a successful connect
@@ -258,7 +258,7 @@ namespace MQTTnet.Server
 
                 await _eventDispatcher.HandleClientConnectedAsync(clientId).ConfigureAwait(false);
                 
-                disconnectType = await connection.RunAsync().ConfigureAwait(false);
+                disconnectType = await connection.RunAsync(connectionValidatorContext).ConfigureAwait(false);
             }
             catch (OperationCanceledException)
             {
diff --git a/Tests/MQTTnet.Core.Tests/MQTTv5/Client_Tests.cs b/Tests/MQTTnet.Core.Tests/MQTTv5/Client_Tests.cs
index 3b813c3..c4c7634 100644
--- a/Tests/MQTTnet.Core.Tests/MQTTv5/Client_Tests.cs
+++ b/Tests/MQTTnet.Core.Tests/MQTTv5/Client_Tests.cs
@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
@@ -57,6 +58,65 @@ namespace MQTTnet.Tests.MQTTv5
                 Assert.AreEqual(2, receivedMessage.UserProperties.Count);
             }
         }
+        [TestMethod]
+        public async Task Connect_With_AssignedClientId()
+        {
+            using (var testEnvironment = new TestEnvironment())
+            {
+                string serverConnectedClientId = null;
+                string serverDisconnectedClientId = null;
+                string clientAssignedClientId = null;
+
+                // Arrange server
+                var disconnectedMre = new ManualResetEventSlim();
+                var serverOptions = new MqttServerOptionsBuilder()
+                    .WithConnectionValidator((context) =>
+                    {
+                        if (string.IsNullOrEmpty(context.ClientId))
+                        {
+                            context.AssignedClientIdentifier = "test123";
+                            context.ReasonCode = MqttConnectReasonCode.Success;
+                        }
+                    });
+                await testEnvironment.StartServerAsync(serverOptions);
+                testEnvironment.Server.UseClientConnectedHandler((args) =>
+                {
+                    serverConnectedClientId = args.ClientId;
+                });
+                testEnvironment.Server.UseClientDisconnectedHandler((args) =>
+                {
+                    serverDisconnectedClientId = args.ClientId;
+                    disconnectedMre.Set();
+                });
+
+                // Arrange client
+                var client = testEnvironment.CreateClient();
+                client.UseConnectedHandler((args) =>
+                {
+                    clientAssignedClientId = args.AuthenticateResult.AssignedClientIdentifier;
+                });
+
+                // Act
+                await client.ConnectAsync(new MqttClientOptionsBuilder()
+                    .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
+                    .WithProtocolVersion(MqttProtocolVersion.V500)
+                    .WithClientId(null)
+                    .Build());
+                await client.DisconnectAsync();
+
+                // Wait for ClientDisconnectedHandler to trigger
+                disconnectedMre.Wait(500);
+
+                // Assert
+                Assert.IsNotNull(serverConnectedClientId);
+                Assert.IsNotNull(serverDisconnectedClientId);
+                Assert.IsNotNull(clientAssignedClientId);
+                Assert.AreEqual("test123", serverConnectedClientId);
+                Assert.AreEqual("test123", serverDisconnectedClientId);
+                Assert.AreEqual("test123", clientAssignedClientId);
+
+            }
+        }
 
         [TestMethod]
         public async Task Connect()