.NET: [Bug]: AddFanInBarrierEdge never invokes the target executor after all source executors complete

[MaguireKrist-ITK]

Description

AddFanInBarrierEdge(...) does not appear to advance the target executor, even when all source executors run successfully and each
produces a message/result.

I reproduced this in an isolated minimal console app using InProcessExecution.RunStreamingAsync(...).

What I observed:

• AddFanOutEdge(...) works as expected
• all fan-out source executors are invoked
• each source executor completes successfully
• the fan-in target executor is never invoked
• no workflow output is produced from the fan-in target

I reproduced this with:

1. generic Executor<TIn, TOut> executors that return values
2. explicit message-based executors that call context.SendMessageAsync(...)

Code Sample

using Microsoft.Agents.AI.Workflows;

  var start = new StartExec();
  var a = new AExec();
  var b = new BExec();
  var agg = new AggExec();

  var workflow = new WorkflowBuilder(start)
      .AddFanOutEdge(start, [a, b])
      .AddFanInBarrierEdge([a, b], agg)
      .WithOutputFrom(agg)
      .Build();

  await using var run = await InProcessExecution.RunStreamingAsync(workflow, "x");

  await foreach (var evt in run.WatchStreamAsync())
  {
      Console.WriteLine($"{evt.GetType().Name}: {evt}");
      if (evt is WorkflowOutputEvent output)
      {
          Console.WriteLine($"OUTPUT: {output.Data}");
      }
  }

  internal sealed class StartExec() : Executor<string, string>("start")
  {
      public override ValueTask<string> HandleAsync(
          string input,
          IWorkflowContext context,
          CancellationToken cancellationToken = default)
      {
          Console.WriteLine("start");
          return ValueTask.FromResult($"{input}-start");
      }
  }

  internal sealed class AExec() : Executor<string, string>("a")
  {
      public override ValueTask<string> HandleAsync(
          string input,
          IWorkflowContext context,
          CancellationToken cancellationToken = default)
      {
          Console.WriteLine("a");
          return ValueTask.FromResult($"{input}-a");
      }
  }

  internal sealed class BExec() : Executor<string, string>("b")
  {
      public override ValueTask<string> HandleAsync(
          string input,
          IWorkflowContext context,
          CancellationToken cancellationToken = default)
      {
          Console.WriteLine("b");
          return ValueTask.FromResult($"{input}-b");
      }
  }

  internal sealed class AggExec() : Executor<List<string>, string>("agg")
  {
      public override ValueTask<string> HandleAsync(
          List<string> input,
          IWorkflowContext context,
          CancellationToken cancellationToken = default)
      {
          Console.WriteLine($"agg:{input.Count}");
          return ValueTask.FromResult(string.Join(",", input));
      }
  }

Error Messages / Stack Traces

Package Versions

10.0.0-rc4

.NET Version

.NET 10.0

Additional Context

Expected Behavior

After both a and b complete, the barrier should release and invoke agg with both messages/results.

I expected output like:

• agg:2
• a WorkflowOutputEvent containing both values

Actual Behavior

Only start, a, and b run. agg is never invoked.

Additional Context

• I also reproduced the same behavior with non-generic Executor implementations that explicitly call context.SendMessageAsync(...)
  and declare routes in ConfigureProtocol(...).
• In that variation, fan-out still works, but the fan-in target still never runs.
• ExecutorOptions.Default reports AutoSendMessageHandlerResultObject = true and AutoYieldOutputHandlerResultObject = true, so this
  does not appear to be caused by those defaults.
• The package docs for AddFanInBarrierEdge(...) describe the barrier as waiting for one message from each source and then advancing
  the target on the following step.
• The public concurrent sample in the repo also suggests this pattern should work.

References:

• Docs:

https://learn.microsoft.com/en-us/dotnet/api/microsoft.agents.ai.workflows.workflowbuilder.addfaninbarrieredge?view=agent-framework-dotnet-latest

• Sample: https://github.com/microsoft/agent-framework/blob/main/dotnet/samples/03-workflows/Concurrent/Concurrent/Program.cs

[rogeriorfp]

@MaguireKrist-ITK Here is the aggregator executor with some adjustments.

[YieldsOutput(typeof(string))]
internal sealed class AggExec() : Executor<string>("agg")
{

    readonly List<string> inputs = [];
    public override async ValueTask HandleAsync(
        string input,
        IWorkflowContext context,
        CancellationToken cancellationToken = default)
    {
        inputs.Add(input);

        if (inputs is { Count: 2 })
        {
            Console.WriteLine($"agg:{inputs.Count}");
            await context.YieldOutputAsync(string.Join(",", inputs), cancellationToken);
        }
    }
}