If I create a tool like this:

[McpServerToolType]
public class EchoTool
{
    [McpServerTool(Name = "echo", UseStructuredContent = true), Description("Echoes the message back to the client.")]
    public static string Echo(string message) => $"Echo: {message}";
}

Then the response violates the spec in terms of the plain TextContent matching the StructuredContent:

{
  "content": [
    {
      "type": "text",
      "text": "Echo: Test"
    }
  ],
  "structuredContent": {
    "result": "Echo: Test"
  }
}

The text property should be the stringified version of the structuredContent object, as per
https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content

Note that if UseStructuredContent is not set in the attribute, this does not happen (as only plain text is returned), nor is there a discrepancy for object types, json nodes, etc.