.NET: Workflow.AsAIAgent drops a terminal executor's ChatMessage output unless includeWorkflowOutputsInResponse is set #9704

Description

When a workflow is hosted with Workflow.AsAIAgent(...) and includeWorkflowOutputsInResponse is left at its default (false), an executor designated with WithOutputFrom(...) reaches the agent response only if its output is a string or AIContent. The same executor yielding a ChatMessage or IEnumerable<ChatMessage> produces no update at all: the stream carries only an observability update for the WorkflowOutputEvent.

In WorkflowSession.cs (1.25.0, unchanged on main as of 2026-10-26), the WorkflowOutputEvent case computes includeTerminalOutput = this._workflow.IsTerminalOutput(output.ExecutorId) and then:

• forwards updateContents when _includeWorkflowOutputsInResponse || includeTerminalOutput;
• forwards updateMessages only when _includeWorkflowOutputsInResponse.

The comment above that block says response-compatible payloads are forwarded "when the host requests all workflow outputs, or when this executor is an explicit output source for the workflow", so the two branches disagree with it. The same gap was pointed out in an automated review comment on #6212, which introduced the terminal-output path.

Since #8020 the hosted AddAsAIAgent() sets includeWorkflowOutputsInResponse: true, so this shows up only when Workflow.AsAIAgent is called directly, for example to pass an execution environment (#9370).

Steps to reproduce

File-based app (dotnet run repro.cs), Microsoft.Agents.AI.Workflows 1.25.0. The same terminal executor is bound twice, once returning List<ChatMessage> and once returning string, each marked with WithOutputFrom:

#:package Microsoft.Agents.AI.Workflows@1.25.0
#pragma warning disable MAAIW001, MAAI001
using Microsoft.Agents.AI;
using Microsoft.Agents.AI.Workflows;
using Microsoft.Extensions.AI;

static string Last(List<ChatMessage> m) => m.LastOrDefault(x => x.Role == ChatRole.User)?.Text ?? "";

Func<List<ChatMessage>, List<ChatMessage>> asMessages = m => [new(ChatRole.Assistant, $"echo: {Last(m)}")];
Func<List<ChatMessage>, string> asString = m => $"echo: {Last(m)}";

foreach (var (label, answer) in new (string, ExecutorBinding)[]
{
    ("List<ChatMessage>", asMessages.BindAsExecutor("Answer", threadsafe: true)),
    ("string", asString.BindAsExecutor("Answer", threadsafe: true)),
})
{
    ExecutorBinding start = new Start();
    var workflow = new WorkflowBuilder(start).AddEdge(start, answer).WithOutputFrom(answer).Build();
    var agent = workflow.AsAIAgent(name: "repro");
    var text = "";
    await foreach (var u in agent.RunStreamingAsync("hi")) text += u.Text;
    Console.WriteLine($"{label,-18} streamed text: \"{text}\"");
}

sealed class Start() : ChatProtocolExecutor("Start", new ChatProtocolExecutorOptions { AutoSendTurnToken = false })
{
    protected override ValueTask TakeTurnAsync(List<ChatMessage> messages, IWorkflowContext context, bool? emitEvents, CancellationToken cancellationToken = default)
        => context.SendMessageAsync(messages, cancellationToken);
}

Actual behavior

List<ChatMessage>  streamed text: ""
string             streamed text: "echo: hi"

In our app (behind the AG-UI server) a branch that answered with a List<ChatMessage> reached the client as RUN_STARTED followed directly by RUN_FINISHED, with no text message, so the user saw an empty reply.

Expected behavior

An executor designated with WithOutputFrom(...) reaches the agent response whatever its payload type, a ChatMessage or a list of them included, as its string and AIContent outputs already do.

Versions

• Microsoft.Agents.AI.Workflows 1.25.0 (.NET 10)
• Same code path on main (commit at time of writing)