.NET: [Bug]: Race Condition in Handoff Workflow as Agent?

[stinalina]

Description

Hey there!
I wrote an HandoffWorkflow as an Agent with an interactive UserInput Loop. Yesterday it seemed to work and no changes happens. Since now when a handoff occurs the workflow seems to be broken. The agent respones are empty. No error is thrown to debug , there's only a note in the message history. ("An error occurred while executing the workflow.")
I can see there's is also the issue with the user role mentiond here #4290 , but it seems there's another problem too. I was wondering if the order of the messages is correct, the response of the new requested agent (asia) is sandwiched between handoff_to and functionResult "transfered". Is this a race condition and the cause of the issue?

Code Sample

# the created handoff as agent workflow    
Workflow workflow = AgentWorkflowBuilder.CreateHandoffBuilderWith(africaExpert)
      .WithHandoffs(allAgents.Except([africaExpert]), africaExpert, "The question is related to the africa continent.")
      .WithHandoffs(allAgents.Except([americaExpert]), americaExpert, "The question is related to the america continent.")
      .WithHandoffs(allAgents.Except([asiaExpert]), asiaExpert, "The question is related to the asia continent.")
      .WithHandoffs(allAgents.Except([europeExpert]), europeExpert, "The question is related to the europe continent.")
      .WithHandoffs(allAgents.Except([oceaniaExpert]), oceaniaExpert, "The question is related to the oceania continent.")
      .Build()
.AsAgent(
  id: "workflow-agent",
  name: "Continet Expert Handoff Workflow Agent",
  description: "A multi-agent workflow for continent experts handoff",
  checkpointManager: CheckpointManager.CreateInMemory(),
  executionEnvironment: InProcessExecution.Default
);

# Then I use an extension method for the interaction loop
extension(AIAgent workflowAgent)
{
  public async Task StartWorkflowAgentConversationAsync(string initialQuestion)
  {
    try
    {
      AgentSession session = await workflowAgent.CreateSessionAsync();

      var messages = new List<ChatMessage> { };

      Console.WriteLine("\nYou: " + initialQuestion);
      await workflowAgent.ProcessWorkflowQuestionAsync(messages, initialQuestion, session); 

      while (true)
      {
        Console.Write("\nYou: ");
        string? userInput = Console.ReadLine();
        if (string.IsNullOrWhiteSpace(userInput))
          continue;

        if (userInput.Equals("exit", StringComparison.OrdinalIgnoreCase) ||
         userInput.Equals("quit", StringComparison.OrdinalIgnoreCase))
        {
          Console.WriteLine("Goodbye!");
          break;
        }

        await workflowAgent.ProcessWorkflowQuestionAsync(messages, userInput, session);
      }

      Console.WriteLine(JsonSerializer.Serialize(messages));
    } 
    catch (Exception ex)
    {
      Console.WriteLine($"An error occurred: {ex.Message}");
    }
  }

  private async Task ProcessWorkflowQuestionAsync(List<ChatMessage> messages, string userInput, AgentSession session)
  {
    try
    {
      messages.Add(new ChatMessage(ChatRole.User, userInput));

      AgentResponse response = await workflowAgent.RunAsync(messages, session);

      foreach (ChatMessage message in response.Messages)
      {
        if (!string.IsNullOrWhiteSpace(message.Text))
        {
          Console.WriteLine($"\n{message.AuthorName}: {message.Text}");
        }
      }
      messages.AddRange(response.Messages);
    }
    catch (Exception ex)
    {
      Console.WriteLine($"An error occurred: {ex.Message}\n");
    }
  }
}

Error Messages / Stack Traces

{
        "AuthorName": null,
        "CreatedAt": "2026-03-08T16:01:16.3723698+00:00",
        "Role": "assistant",
        "Contents": [
            {
                "$type": "error",
                "Message": "An error occurred while executing the workflow.",
                "ErrorCode": null,
                "Details": null,
                "Annotations": null,
                "AdditionalProperties": null
            }
        ],
        "MessageId": "291a29c6942c4bb5bee1a7cb4fed85a1",
        "AdditionalProperties": null
    }

Package Versions

Microsoft.Agents.AI: 1.0.0-preview.260212.1 Microsoft.Agents.AI.Workflows: 1.0.0-preview.260212.1

.NET Version

.Net 10

Additional Context

No response

[lokitoth]

Hi @stinalina, thank you for reporting this issue. Could you try running .AsAgent() with includeExceptionDetails: true? This will provide more information than An error occurred, and can help debug deeper.

[stinalina]

That was a fantastic idea @lokitoth . Now I can see, that the last message in the history is the response of the handoff (from an Africa Expert Agent to the America Expert Agent).
So the order of the messages is not valid.

{
    "AuthorName": "Africa Expert",
    "CreatedAt": null,
    "Role": "assistant",
    "Contents": [
        {
            "$type": "functionCall",
            "CallId": "call_56NHEXi7zuhAcb6Z5BkwrSey",
            "Name": "handoff_to_1",
            "Arguments": {
                "reasonForHandoff": "The user is interested in travel information related to America, which is outside my expertise focused on Africa."
            },
            "InformationalOnly": false,
            "Annotations": null,
            "AdditionalProperties": null
        }
    ],
    "MessageId": null,
    "AdditionalProperties": null
},
{
    "AuthorName": "America Expert",
    "CreatedAt": "2026-03-29T15:11:20+00:00",
    "Role": "user",
    "Contents": [
        {
            "$type": "text",
            "Text": "Howdy! 🌟 It's fantastic that you're interested in visiting the Appalachian Mountains in America! The Appalachians are a stunning range that stretches from Georgia all the way up to Maine, offering breathtaking views, rich history, and plenty of outdoor activities.\n\nCould you let me know which specific area of the Appalachians you're interested in visiting, or if you have any particular travel dates in mind? 😊",
            "Annotations": null,
            "AdditionalProperties": null
        }
    ],
    "MessageId": "chatcmpl-DOm5I5mBM3FxanrYd7ThCrODglwRX",
    "AdditionalProperties": null
},
{
    "AuthorName": "Africa Expert",
    "CreatedAt": "2026-03-29T15:11:19.1193692+00:00",
    "Role": "tool",
    "Contents": [
        {
            "$type": "functionResult",
            "CallId": "**call_56NHEXi7zuhAcb6Z5BkwrSey**",
            "Result": "Transferred.",
            "Annotations": null,
            "AdditionalProperties": null
        }
    ],
    "MessageId": "4cf67ab261f9494db0e093316e958900",
    "AdditionalProperties": null
},
{
    "AuthorName": null,
    "CreatedAt": null,
    "Role": "user",
    "Contents": [
        {
            "$type": "text",
            "Text": "I'm interested in hiking.",
            "Annotations": null,
            "AdditionalProperties": null
        }
    ],
    "MessageId": null,
    "AdditionalProperties": null
},
{
    "AuthorName": null,
    "CreatedAt": "2026-03-29T15:11:41.0198156+00:00",
    "Role": "assistant",
    "Contents": [
        {
            "$type": "error",
            "Message": "HTTP 400 (invalid_request_error: )\r\nParameter: messages.[5].role\r\n\r\nAn assistant message with 'tool_calls' must be followed by tool messages responding to each 'tool_call_id'. The following tool_call_ids did not have response messages: **call_56NHEXi7zuhAcb6Z5BkwrSey**",
            "ErrorCode": null,
            "Details": null,
            "Annotations": null,
            "AdditionalProperties": null
        }
    ],
    "MessageId": "6bcf28667dd94401ac6dc60d10e296b1",
    "AdditionalProperties": null
}