Currently, mcp.AddTool takes a ToolHandlerFor[In, Out], which is defined as

func(context.Context, *CallToolRequest, In) (*CallToolResult, Out, error)

As argued on #386, the request and result are seldom used and confusing. In particular, it is unclear whether to use the StructuredOutput and IsError fields of CallToolResult, or to return results via the second and third return values.

The proposal is to change AddTool to take a handler with signature

func(context.Context, In) (Out, error)

That will satisfy the majority of tool authors and remove the confusion.