diff --git a/src/IO.Ably.Tests.Shared/Realtime/ConnectionSandBoxSpecs.cs b/src/IO.Ably.Tests.Shared/Realtime/ConnectionSandBoxSpecs.cs
index 3cd5c62eb..66e450d35 100644
--- a/src/IO.Ably.Tests.Shared/Realtime/ConnectionSandBoxSpecs.cs
+++ b/src/IO.Ably.Tests.Shared/Realtime/ConnectionSandBoxSpecs.cs
@@ -620,7 +620,8 @@ await WaitFor(done =>
         [ProtocolData]
         [Trait("spec", "RTN15c6")]
         [Trait("spec", "RTN15c7")]
-        public async Task ResumeRequest_ConnectedProtocolMessageWithSameOrNewConnectionId_AttachesAllChannels(Protocol protocol)
+        public async Task ResumeRequest_ConnectedProtocolMessage_AttachAllChannelsAndProcessPendingMessages(
+            Protocol protocol)
         {
             var client = await GetRealtimeClient(protocol);
             var channelName = "RTN15c6.RTN15c7.".AddRandomSuffix();
@@ -636,6 +637,31 @@ public async Task ResumeRequest_ConnectedProtocolMessageWithSameOrNewConnectionI
                 channels.Add(channel as RealtimeChannel);
             }
 
+            TaskCompletionAwaiter connectedAwaiter = null;
+
+            // Should add messages to connection-wide pending queue
+            async Task PublishMessagesWhileNotConnected()
+            {
+                // Make sure new transport is created to apply BeforeProtocolMessageProcessed method
+                await client.WaitForState(ConnectionState.Connecting);
+                connectedAwaiter = new TaskCompletionAwaiter(15000);
+                client.BeforeProtocolMessageProcessed(message =>
+                {
+                    if (message.Action == ProtocolMessage.MessageAction.Connected)
+                    {
+                        connectedAwaiter.Task.Wait(); // Keep in connecting state
+                    }
+                });
+
+                foreach (var channel in channels)
+                {
+                    channel.Publish("eventName", "foo");
+                }
+
+                await client.ProcessCommands();
+                client.State.PendingMessages.Should().HaveCount(channelCount);
+            }
+
             async Task CheckForAttachedChannelsAfterResume()
             {
                 var attachedChannels = new List<RealtimeChannel>();
@@ -652,27 +678,44 @@ await WaitForMultiple(channelCount, partialDone =>
                             });
                         });
                     }
+
+                    connectedAwaiter.SetCompleted();
                 });
                 attachedChannels.Should().HaveCount(channelCount);
+            }
 
-                // TODO - check for processed pending messages
+            async Task CheckForProcessedPendingMessages()
+            {
+                client.State.PendingMessages.Should().HaveCount(0);
+                foreach (var channel in channels)
+                {
+                    var history = await channel.HistoryAsync();
+                    history.Items.Count.Should().BeGreaterOrEqualTo(1);
+                    history.Items[0].Data.Should().Be("foo");
+                }
             }
 
+            // resume successful - RTN15c6
             client.GetTestTransport().Close(false);
+            await PublishMessagesWhileNotConnected();
             await CheckForAttachedChannelsAfterResume();
+            await CheckForProcessedPendingMessages();
             client.GetTestTransport().ProtocolMessagesReceived.Count(x => x.Action == ProtocolMessage.MessageAction.Connected).Should().Be(1); // For every new transport, list is reset
             var connectedProtocolMessage = client.GetTestTransport().ProtocolMessagesReceived.First(x => x.Action == ProtocolMessage.MessageAction.Connected);
-            connectedProtocolMessage.ConnectionId.Should().Be(connectionId); // resume successful - RTN15c6
+            connectedProtocolMessage.ConnectionId.Should().Be(connectionId);
             connectedProtocolMessage.Error.Should().BeNull();
             client.Connection.ErrorReason.Should().BeNull();
 
+            // resume unsuccessful - RTN15c7
             client.State.Connection.Id = string.Empty;
             client.State.Connection.Key = "xxxxx!xxxxxxx-xxxxxxxx-xxxxxxxx"; // invalid connection key for next resume request
             client.GetTestTransport().Close(false);
+            await PublishMessagesWhileNotConnected(); // TODO - Fix process pending messages
             await CheckForAttachedChannelsAfterResume();
+            await CheckForProcessedPendingMessages();
             client.GetTestTransport().ProtocolMessagesReceived.Count(x => x.Action == ProtocolMessage.MessageAction.Connected).Should().Be(1); // For every new transport, list is reset
             connectedProtocolMessage = client.GetTestTransport().ProtocolMessagesReceived.First(x => x.Action == ProtocolMessage.MessageAction.Connected);
-            connectedProtocolMessage.ConnectionId.Should().NotBe(connectionId); // resume unsuccessful - RTN15c7
+            connectedProtocolMessage.ConnectionId.Should().NotBe(connectionId);
             connectedProtocolMessage.Error.Should().NotBeNull();
             client.Connection.ErrorReason.ToString().Should().Be(connectedProtocolMessage.Error.ToString());
             client.Connection.ErrorReason.Code.Should().Be(ErrorCodes.InvalidFormatForConnectionId);