Message and StepRun Deltas missing from eventMessage callback object

[StephenHodgson]

Discussed in #327

^{Originally posted by @rkuiper1 June 16, 2024}
Today I was lucky enough to have a decent internet connection and I took a look at your streaming implementation.
I focused on ThreadsEndpoint.cs and in particular the StreamRunAsync method.
There I found that all the delta's are neatly received and processed, but not passed to the streamEventHandler, which explains the empty content we see in our logfiles.
This tiny tweak made the difference:

                        case "thread.message.incomplete":
                            var partialMessage = sseResponse.Deserialize<MessageResponse>(ssEvent, client);
                            if (message == null)
                            {
                                message = partialMessage;
                            }
                            else
                            {
                                message.AppendFrom(partialMessage);
                            }

481                         streamEventHandler?.Invoke(partialMessage);
                            return;

Line 481 originally contains: streamEventHandler?.Invoke(message); where message does not contain the actual delta.
Changing it to streamEventHandler?.Invoke(partialMessage); will pass the message event that does contain the delta.

Next, I can print the delta's in my test project like this:

                    case MessageResponse messageResponse:
                        if (messageResponse.Delta != null)
                        {
                            Console.Write($"{messageResponse.Delta.Content.FirstOrDefault()?.Text}");
                        }
                        break;

Running the test project will show the familiar chat-gpt delta streaming I was trying to accomplish.
I know this is a quick hack and I can't oversee all consequences of this change, but the way it works would be good enough for me.