`mcp.CallToolResult.IsError` usage is unclear

[katiehockman]

I find the usage of CallToolResult.IsError (bool) confusing alongside the error return parameter.

For example, in this code snippet, which is correct?

(1)

func SessionByID(ctx context.Context, _ *mcp.CallToolRequest, params SessionIDParams) (*mcp.CallToolResult, SessionsResult, error) {
	session, exists := sessionByID(params.SessionID)
	if !exists {
		return nil, SessionsResult{}, errors.New("session not found")
	}
	return nil, SessionsResult{Sessions: []Session{session}}, nil
}

(2)

func SessionByID(ctx context.Context, _ *mcp.CallToolRequest, params SessionIDParams) (*mcp.CallToolResult, SessionsResult, error) {
	session, exists := sessionByID(params.SessionID)
	if !exists {
		return &mcp.CallToolResult{
			IsError: true,
			Content: []mcp.Content{
				&mcp.TextContent{Text: "session not found"},
			}
		}, SessionsResult{}, nil
	}
	return nil, SessionsResult{Sessions: []Session{session}}, nil
}

If the former (1), then I find the godoc for IsError confusing, as it's not clear when to use this. If the latter (2), then I would prefer Error with an error type instead of IsError with a bool type and then needing to wrap the error in a mcp.TextContent which makes the code messier. In my testing I did (1) and the LLM worked just fine and the code was much simpler.

Either way, I think the godoc could be made clearer with examples of usage.

[findleyr]

Yes, it is (1). We should improve the godoc.