How to add field description for a third-party class which is a tool input

[fvdcx]

Tagging experts here! @halter73 @stephentoub

Hello! I have such a tool

public CallToolResponse testTool(Foo foo)
{
    return new CallToolResponse()
    {
        Content = [new Content { Text = "hello") }],
        IsError = false
    };
}

Note that Foo class is NOT my source code, it lies in a third-party package, which I can't control at all. It's a complex class having nested classes as childs and also inherits from other class, like

class ParentOfFoo
{
   string myStr;
}

class Foo : ParentOfFoo
{
    Bar bar;
}

class Bar
{
   Baz baz;
}

class Baz
{
   int myInt;
}

For sure, there isn't any [Description] attribute in the third-party code, by default, the InputSchema generated by sdk will be like.

{
        "name": "test_tool",
        "description": "A test tool that accepts a Foo object and returns a hello world string",
        "inputSchema": {
          "title": "test_tool",
          "description": "A test tool that accepts a Foo object and returns a hello world string",
          "type": "object",
          "properties": {
            "foo": {
              "description": "The Foo object containing nested Bar and Baz objects",
              "type": "object",
              "properties": {
                "bar": {
                  "type": "object",
                  "properties": {
                    "baz": {
                      "type": "object",
                      "properties": {
                        "myInt": {
                          "type": "integer"
                        }
                      }
                    }
                  }
                },
                "myStr": {
                  "type": "string"
                }
              }
            }
          },
          "required": [
            "foo"
          ]
        }
      }

My requirement: add [Description] on each field of the third-party classes, similar like(note: I can't modify their source code, just for illustration here). I need add descriptions for AI to understand schema for each single field.

class ParentOfFoo
{
  [Description("XXX")]
   string myStr;
}

class Foo : ParentOfFoo
{
    [Description("XXX")]
    Bar bar;
}

class Bar
{
   [Description("XXX")]
   Baz baz;
}

class Baz
{
   [Description("XXX")]
   int myInt;
}

I expect to see inputschema is like

{
        "name": "test_tool",
        "description": "A test tool that accepts a Foo object and returns a hello world string",
        "inputSchema": {
          "title": "test_tool",
          "description": "A test tool that accepts a Foo object and returns a hello world string",
          "type": "object",
          "properties": {
            "foo": {
              "description": "The Foo object containing nested Bar and Baz objects",
              "type": "object",
              "properties": {
                "bar": {
                  "type": "object",
                  "description": "XXX",
                  "properties": {
                    "baz": {
                      "type": "object",
                      "description": "XXX"
                      "properties": {
                        "myInt": {
                          "type": "integer",
                         "description": "XXX"
                        }
                      }
                    }
                  }
                },
                "myStr": {
                  "type": "string",
                  "description": "XXX"
                }
              }
            }
          },
          "required": [
            "foo"
          ]
        }
      }

Is there an elegant way to achieve? I have some hacky and bad ideas, like creating my own facade class (adding description) based on the third party class, and use it as tool input. I would have hunderds of tools in one mcp server, need an elegant and efficient way .

[eiriktsarpalis]

It should be possible to customize your generated schema using a schema transformer delegate. This can be found in the McpServerToolCreateOptions under the SchemaCreateOptions.TransformSchemaNode property. This lets you modify any subschema within your generated schema document using relevant context (the Type or MemberInfo from which the schema is being generated). You can find an example of how this is being done in this article.

[stephentoub]

And which I shared an example of in an answer to another question from you yesterday 😄
#543 (comment)