.NET: [Issue]:MCP content + structuredContent duplicate model input without a selection policy #9884

// Program.cs
using Microsoft.Agents.AI;
using ModelContextProtocol.Client;
using OpenAI;
using OpenAI.Responses;

var builder = WebApplication.CreateBuilder(args);
builder.WebHost.UseUrls("http://127.0.0.1:5017");
builder.Services.AddMcpServer()
    .WithHttpTransport()
    .WithTools<MyTools>();

await using var app = builder.Build();
app.MapMcp("/mcp");
await app.StartAsync();

try
{
    await using var mcpClient = await McpClient.CreateAsync(new HttpClientTransport(new()
    {
        Endpoint = new Uri("http://127.0.0.1:5017/mcp"),
    }));
    var tools = await mcpClient.ListToolsAsync();

    var openAi = new OpenAIClient(Environment.GetEnvironmentVariable("OPENAI_API_KEY")!);
    AIAgent agent = openAi.GetResponsesClient().AsAIAgent(
        model: "gpt-5.4",
        instructions: "Call get_report once and report its marker.",
        tools: [.. tools]);

    Console.WriteLine(await agent.RunAsync("Get the report."));
}
finally
{
    await app.StopAsync();
}

// MyTools.cs
using System.ComponentModel;
using ModelContextProtocol.Server;

[McpServerToolType]
public sealed class MyTools
{
    [McpServerTool(Name = "get_report", UseStructuredContent = true)]
    [Description("Returns a sample report.")]
    public Report GetReport() => new("PROBE_MARKER", 3);
}

public sealed record Report(string Marker, int Count);

{
  "content": [
    { "type": "text", "text": "{\"marker\":\"PROBE_MARKER\",\"count\":3}" }
  ],
  "structuredContent": {
    "marker": "PROBE_MARKER",
    "count": 3
  }
}

ficc.FunctionInvoker = async (context, cancellationToken) =>
{
    var result = await context.Function.InvokeAsync(
        context.Arguments, cancellationToken);

    if (context.Function.GetService<McpClientTool>() is not null &&
        result is JsonElement { ValueKind: JsonValueKind.Object } json)
    {
        if (json.TryGetProperty("isError", out var error) &&
            error.ValueKind == JsonValueKind.True)
        {
            return result;
        }
  // Example policy: `structured-first.`
        if (json.TryGetProperty("structuredContent", out var structured) &&
            structured.ValueKind is not (JsonValueKind.Null or JsonValueKind.Undefined))
        {
            return structured.Clone();
        }
    }

    return result;
};