Errors using tools with list or arrays as parameters

[albertaixendri]

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Question Category

• Protocol Specification
• SDK Usage
• Server Implementation
• General Implementation
• Documentation
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Your Question

Hi!
I'm getting errors trying to call tools that have arrays or lists as parameters. Here are two simple examples:

        [McpServerTool, Description("Sum a list of integers")]
        public static int SumIntegerList(
            [Description("An list of integers")] List<int> integers
            )
        {
            return integers.Sum();
        }

        [McpServerTool, Description("Concatenate a list of strings")]
        public static string ConcatenateListOfString(
            [Description("A list of strings")] string[] strings
            )
        {
            StringBuilder sb = new StringBuilder();
            foreach (var s in strings)
            {
                sb.Append(s);
                sb.Append(" ");
            }
            return sb.ToString();
        }

Using either MCP CLI or oterm I get this error:

I didn't see many examples with these parameters, so I don't know what can be wrong.
Thanks in advance!

[albertaixendri]

Two other tests on my side:

• Using MCP Inspector it works well as I show here:
  
• Using a MCP client build on dotnet fails

So, I'm wondering if it could be related to the serialization on the client side?