[BUG] ChatClientAgent.RunAsync throws JSON parsing error with gpt-5 but works with gpt-4o

[dantatepico]

Library name and version

Microsoft.Agents.AI 1.0.0-preview.260128.1

Describe the bug

We’re encountering a JSON parsing exception when using ChatClientAgent.RunAsync with the gpt-5 model. The same code works correctly when switching to gpt-4o.

This appears to be either a service-side incompatibility with Agents or an SDK parsing issue when handling the final response block.

Expected behavior

ChatClientAgent.RunAsync returns an AgentResponse

Actual behavior

ChatClientAgent.RunAsync throws an exception:

The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. LineNumber: 0 | BytePositionInLine: 0.

Reproduction Steps

    var chatMessage = new ChatMessage
    {
      Role = ChatRole.User,
      Contents = [new DataContent(image, "image/png")]
    };

    var response = await _chatClientAgent.RunAsync(chatMessage, session, cancellationToken: ct);

Environment

.NET 10 Windows
Visual Studio 18.1.0

[wmmihaa]

The issue relates to the fact that keepalive events are not handled. I've made a fix and submitted a PR #56258