feature: return response error code

[adlternative]

Problem Statement

I want to access JSONRPCResponse.Error.Code in requests like CallTool, which would be very helpful for determining the error type, but currently only JSONRPCResponse.Error.Message is exposed...

// sendRequest sends a JSON-RPC request to the server and waits for a response.
// Returns the raw JSON response message or an error if the request fails.
func (c *Client) sendRequest(
	ctx context.Context,
	method string,
	params any,
) (*json.RawMessage, error) {
	if !c.initialized && method != "initialize" {
		return nil, fmt.Errorf("client not initialized")
	}

	id := c.requestID.Add(1)

	request := transport.JSONRPCRequest{
		JSONRPC: mcp.JSONRPC_VERSION,
		ID:      mcp.NewRequestId(id),
		Method:  method,
		Params:  params,
	}

	response, err := c.transport.SendRequest(ctx, request)
	if err != nil {
		return nil, fmt.Errorf("transport error: %w", err)
	}

	if response.Error != nil {
		return nil, errors.New(response.Error.Message)
	}

	return &response.Result, nil
}

Proposed Solution

Wrap an error struct in the returned error, then the outside code can unwrap it.