How To: Accept a complex/class object in a McpServerTool function

[ksaye]

Is your feature request related to a problem? Please describe.
I am unable to pass a complex object, as I can with OpenAPI, to a method.

the following code

using ModelContextProtocol.Server;
using System.ComponentModel;

namespace VoiceAgentWeb
{
    [McpServerToolType]
    public class AzureCommunicationServices
    {
        [McpServerTool, Description("Makes a phone call to the user.")]
        public static Task CallUser(PersonToCall person) {
            
            Console.WriteLine($"Calling the user {person.name}");

            return Task.CompletedTask;
        }

    }

    //[McpServerResourceType, Description("The person to call.")]
    public class PersonToCall
    {
        public required string? phoneNumber { get; set; }
        public required string? name { get; set; }
        public required string? greeting { get; set; }
    }
}

Copilot studio get confused about the name and the person record

In MCP Inspector, I can manually pass the variables and it works:

Also, if I change the code to a non complex object, it works:

using ModelContextProtocol.Server;
using System.ComponentModel;

namespace VoiceAgentWeb
{
    [McpServerToolType]
    public class AzureCommunicationServices
    {
        [McpServerTool, Description("Makes a phone call to the user.")]
        public static Task CallUser(string name, string greeting, string phonenumber)
        { 
            Console.WriteLine($"Calling the user {name}");
            Console.WriteLine($"Greeting: {greeting}");
            Console.WriteLine($"Phone Number: {phonenumber}");

            return Task.CompletedTask;
        }
    }
}

I know OpenAPI defines the schema which helps the LLM. Is there something I am missing in MCP?

Describe the solution you'd like
I would like to provide the schema to the calling LLM for successful execution.

Describe alternatives you've considered
Either using OpenAPI or a non complex object.

Additional context
I hope I am just missing something in the code where this would work as defined.

[stephentoub]

I know OpenAPI defines the schema which helps the LLM. Is there something I am missing in MCP?

A schema is being exposed for your complex object. This is what the client is receiving:

{
    "title": "CallUser",
    "description": "Makes a phone call to the user.",
    "type": "object",
    "properties": {
        "person": {
            "type": "object",
            "properties": {
                "phoneNumber": {
                    "type": [
                        "string",
                        "null"
                    ]
                },
                "name": {
                    "type": [
                        "string",
                        "null"
                    ]
                },
                "greeting": {
                    "type": [
                        "string",
                        "null"
                    ]
                }
            },
            "required": [
                "phoneNumber",
                "name",
                "greeting"
            ]
        }
    },
    "required": [
        "person"
    ]
}

@madevoge, is there something about this schema that copilot studio doesn't like?

[ksaye]

@stephentoub, Thanks. In doing some botframework dev, I just saw a log file below, confirming what you say:

 "toolsList": [
          {
            "$kind": "DynamicServerToolDefinition",
            "displayName": "CallUser",
            "description": "Makes a phone call to the user.",
            "inputs": [
              {
                "$kind": "ToolInput",
                "name": "person",
                "description": "The person to call.",
                "type": {
                  "$kind": "Record",
                  "properties": {
                    "greeting": {
                      "$kind": "PropertyInfo",
                      "displayName": "greeting",
                      "isRequired": true,
                      "type": {
                        "$kind": "Record"
                      }
                    },
                    "name": {
                      "$kind": "PropertyInfo",
                      "displayName": "name",
                      "isRequired": true,
                      "type": {
                        "$kind": "Record"
                      }
                    },
                    "phoneNumber": {
                      "$kind": "PropertyInfo",
                      "displayName": "phoneNumber",
                      "isRequired": true,
                      "type": {
                        "$kind": "Record"
                      }
                    }
                  }
                },
                "propertyName": "person",
                "isRequired": true,
                "isAutomatic": true,
                "shouldPromptUser": true,
                "order": 0
              }
            ],
            "identifier": "CallUser",
            "schemaName": "cr7a3_voiceAgent1.action.MCP-Streamable-HTTP-MCPServerStreamableHTTP",
            "llmIdentifierPrefix": "MCP Server Streamable HTTP"
          },

[cdavernas]

It's not working for me either, and that's a big deal breaker for non-basic operations, where you end up with 20+ parameters.

Any clue on how to address?

[eiriktsarpalis]

@ksaye is there a way you could share the tool call parameters made by Copilot studio?

[ksaye]

@ksaye is there a way you could share the tool call parameters made by Copilot studio?

Yes, let me reach out to you directly.

[ksaye]

FYI, working with @eiriktsarpalis, this appears to be a 'Client' issue, where MCS [Microsoft Copilot Studio] is the client.

As shown below, MCS is getting confused and while it has the schema, does not seem to know how to create the correct person record:

Closing this issue and opening one with the MCS team. Until the issue is resolved, a workaround is to flatten (aka change from a complex object to a series of primitive objects which both MCP and MCS understand.

[ksaye]

Thanks to @krnese, the answer was in the MCP code. I had a required nullable string. Note below the string?

Example the following code failed:

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

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddMcpServer()
    .WithHttpTransport()
    .WithToolsFromAssembly();
var app = builder.Build();

app.MapMcp();

app.Run();

[McpServerToolType]
public static class EchoTool
{
    [McpServerTool, Description("Echoes the message back to the client.")]
    public static string Echo(string message)
    { 
        return $"hello {message}";
    }
}

[McpServerToolType]
public static class ComplexSing
{
    [McpServerTool, Description("Sings the message back to the client.")]
    public static string Sing(WhatToSing sing)
    {
        return $"Received sing request: {sing.Song} in {sing.Language}";
    }
}

public class WhatToSing
{
    public required string? Song { get; set; } = "default song";
    public required string? Language { get; set; } = "default language";
}

And this code works:

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

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddMcpServer()
    .WithHttpTransport()
    .WithToolsFromAssembly();
var app = builder.Build();

app.MapMcp();

app.Run();

[McpServerToolType]
public static class EchoTool
{
    [McpServerTool, Description("Echoes the message back to the client.")]
    public static string Echo(string message)
    { 
        return $"hello {message}";
    }
}

[McpServerToolType]
public static class ComplexSing
{
    [McpServerTool, Description("Sings the message back to the client.")]
    public static string Sing(WhatToSing sing)
    {
        return $"Received sing request: {sing.Song} in {sing.Language}";
    }
}

public class WhatToSing
{
    public required string Song { get; set; } = "default song";
    public required string Language { get; set; } = "default language";
}

Success is shown here: