How do you control which parameters are included in a tool's request schema?

[wazzamatazz]

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Question Category

• Protocol Specification
• SDK Usage
• Server Implementation
• General Implementation
• Documentation
• Other

Your Question

I have the following types:

public abstract record ToolRequest {

    [JsonPropertyName("reason")]
    [Description("A **SHORT** (1-2 sentences max.) plain text explanation for why the agent is calling the tool.")]
    public required string Reason { get; init; }

}

public sealed record CurrentTimeGetRequest : ToolRequest { }

[McpServerToolType]
public static class MyTools {

    [McpServerTool(Name = "get_current_time")]
    [Description("Gets the current UTC date and time.")]
    public static DateTimeOffset GetCurrentTime(CurrentTimeGetRequest request, TimeProvider timeProvider) => timeProvider.GetUtcNow();

}

The TimeProvider parameter in my get_current_time tool is provided by the DI container, but I am finding that it is being included in the generated input schema for the tool:

{
  "name": "get_current_time",
  "title": null,
  "description": "Gets the current UTC date and time.",
  "inputSchema": {
    "type": "object",
    "properties": {
      "request": {
        "type": "object",
        "properties": {
          "reason": {
            "description": "A **SHORT** (1-2 sentences max.) plain text explanation for why the agent is calling the tool.",
            "type": "string"
          }
        },
        "required": [
          "reason"
        ]
      },
      "timeProvider": {
        "type": "object",
        "properties": {
          "localTimeZone": {
            "type": [
              "object",
              "null"
            ],
            "properties": {
              "id": {
                "type": [
                  "string",
                  "null"
                ]
              },
              "hasIanaId": {
                "type": "boolean"
              },
              "displayName": {
                "type": [
                  "string",
                  "null"
                ]
              },
              "standardName": {
                "type": [
                  "string",
                  "null"
                ]
              },
              "daylightName": {
                "type": [
                  "string",
                  "null"
                ]
              },
              "baseUtcOffset": {
                "$comment": "Represents a System.TimeSpan value.",
                "type": "string"
              },
              "supportsDaylightSavingTime": {
                "type": "boolean"
              }
            }
          },
          "timestampFrequency": {
            "type": "integer"
          }
        }
      }
    },
    "required": [
      "request",
      "timeProvider"
    ]
  },
  "outputSchema": null,
  "annotations": null,
  "_meta": null
}

How does the SDK decide which parameters to include in the schema? Is there any way that I can make it ignore parameters that come from DI without switching from attribute-based tool registration to manual tool registration?

[eiriktsarpalis]

I believe the best way to do this is by accepting TimeProvider as a constructor parameter in your tool class.

[wazzamatazz]

Thanks - I'll try that out. All of the examples use static classes and methods so I thought that was maybe a requirement. I've also found that this behaviour does not occur with other methods that I'm injecting different dependencies into (e.g. a typed API client) so it does seem to be related to how the schema generator decides which parameters to include.

[wazzamatazz]

Okay, I've figured out the answer. I had assumed that TimeProvider was added to the service collection by default when creating the web application builder, but is actually only registered when you add authentication services to the builder:

var builder = WebApplication.CreateBuilder();

builder.Services.AddAuthentication();

await using var app = builder.Build();

var isRegistered = app.Services.GetRequiredService<IServiceProviderIsService>().IsService(typeof(TimeProvider));

Console.WriteLine(isRegistered); // true

var builder = WebApplication.CreateBuilder();

//builder.Services.AddAuthentication();

await using var app = builder.Build();

var isRegistered = app.Services.GetRequiredService<IServiceProviderIsService>().IsService(typeof(TimeProvider));

Console.WriteLine(isRegistered); // false

In my server app I have not implemented authentication yet, so TimeProvider was not being registered hence why the MCP tool registration thought that it was a user-provided parameter.