IActionResult is not compatible with MCP #1042
Description
If I return the following IActionResult, the MCP output when the agent executes it is {"result":{}}
[HttpGet]
[McpServerTool(Name = "GetTodos"), Description("Returns the list of todos.")]
public ActionResult<Todo[]> GetTodos()
{
return Ok(todos);
}
But if I return instead a plain object, the agent gets the full list of todos.
[HttpGet]
[McpServerTool(Name = "GetTodos"), Description("Returns the list of todos.")]
public Todo[] GetTodos()
{
return todos;
}
IActionResult support is important to manage HTTP result in our existing APIs.
The existing code is:
using Microsoft.AspNetCore;
/*
Using directive is unnecessary.IDE0005
namespace Microsoft.AspNetCore
*/
// var builder = WebApplication.CreateSlimBuilder(args);
var builder = WebApplication.CreateBuilder(args);
// builder.Services.ConfigureHttpJsonOptions(options =>
// {
// });
builder.Services.AddControllers();
builder.Services.AddMcpServer()
.WithHttpTransport()
.WithToolsFromAssembly();
// Learn more about configuring OpenAPI at https://aka.ms/aspnet/openapi
builder.Services.AddOpenApi();
var app = builder.Build();
if (app.Environment.IsDevelopment())
{
app.MapOpenApi();
}
app.UseHttpsRedirection();
app.UseAuthorization();
app.MapControllers();
app.MapMcp("/mcp");
app.Run();
using System.ComponentModel;
using Microsoft.AspNetCore.Mvc;
using ModelContextProtocol.Server;
namespace API.Controllers;
[ApiController]
[Route("api/[controller]")]
[McpServerToolType]
public class TodosController : ControllerBase
{
private readonly Todo[] todos = [];
public record Todo(int Id, string? Title, DateOnly? DueBy = null, bool IsComplete = false);
public TodosController()
{
todos = [
new(1, "Walk the dog"),
new(2, "Do the dishes", DateOnly.FromDateTime(DateTime.Now)),
new(3, "Do the laundry", DateOnly.FromDateTime(DateTime.Now.AddDays(1))),
new(4, "Clean the bathroom"),
new(5, "Clean the car", DateOnly.FromDateTime(DateTime.Now.AddDays(2)))
];
}
[HttpGet]
// [Route(nameof(GetTodos))]
[McpServerTool(Name = "GetTodos"), Description("Returns the list of todos.")]
public ActionResult<Todo[]> GetTodos()
{
return Ok(todos);
}
[HttpGet("{id}")]
[McpServerTool(Name = "GetTodoById"), Description("Returns a todo by its ID.")]
public ActionResult<Todo> GetTodoById(int id)
{
var todo = todos.FirstOrDefault(a => a.Id == id);
return todo is not null ? Ok(todo) : NotFound();
}
}