How to make specific parameter be optional?

[KichangKim]

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Question Category

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

Your Question

Hi. I tried to implement MCP server tools which has optional parameters. Here is my current implemention (simplified):

[McpServerToolType]
public static class MyTools
{
    [McpServerTool, Description("Tool description ...")]
    public static async Task<string> ToolMethod1(
        [Description("Parameter 1 description ...")]
        string parameter1,
        [Description("Parameter 2 description ...")]
        string parameter2,
        [Description("Parameter 3 description ... Optional.")]
        List<MyParamType>? parameter3Optional = null)
    {
        // Some processing ...
    }

    [McpServerTool, Description("Tool description ...")]
    public static async Task<string> ToolMethod2(
        [Description("Parameter description ...")]
        MyParamClass parameter)
    {
        // Some processing ...
    }

    public class MyParamClass
    {
        [Description("Parameter 1 description ...")]
        public string? parameter1 { get; set; }
        [Description("Parameter 2 description ...")]
        public string? parameter2 { get; set; }
        [Description("Parameter 3 description ... Optional.")]
        public List<MyParamType> parameter3Optional { get; set; } = new List<MyParamType>();
    }
}

With both ToolMethod1 and ToolMethod2, when LLM calls these method without setting parameter3Optional, an error like "required argument is invalid (parameter3Optional)" is returned. Only works when LLM set "[]" (empty array) for parameter3Optional. How to make specific parameter be optional?

Thanks.

[PederHP]

This looks like a bug. You should be able to have optional array parameters as in the first example.

However, in the second example ToolMethod2, parameter3Optional is not optional, nor is parameter. So it must actually be provided.

As a side note, I would have caution against having too complex tool surfaces. Some models, especially smaller ones, can get confused by complex signatures and especially array of objects. Sometimes you do need a complex signature, but it's worth noting that models aren't provided with any special support to ensure they get the parameters right.

[eiriktsarpalis]

This is inherited from the current AIFunction defaults used in MEAI, which applies the OpenAI requirement that all parameters should be marked as required. This has created problems with other vendors, so we're actively moving away from this default, and this change will eventually find its way into the MCP sdk as well.

[Keerthan-GK]

Is there any option to pass a class as a single param. I would like to avoid multiple variable declaration in a method?