mcp.registerTool(
    'send_email',
    {
      description: "Sends an email.",
      inputSchema: SendEmailInputSchema.shape,
      outputSchema: SendEmailOutputSchema.shape,
      annotations: {
        title: "Send Email",
        readOnlyHint: false,
        destructiveHint: false,
        idempotentHint: false,
        openWorldHint: true,
      } as ToolAnnotations,
   },
  async (args): Promise<CallToolResult> => {
    // ... implementation
    return {
      content: [{ type: 'text', text: 'Email sent successfully' }],
      structuredContent: {
        success: true,
        messageId: res.data.id || '',
        threadId: res.data.threadId || undefined,
      },
    };
  }

  async (args, _extra): Promise<CallToolResult & { structuredContent?: z.infer<typeof SendEmailOutputSchema> }> => {
    // ... implementation  
    return {
      content: [{ type: 'text', text: 'Email sent successfully' }],
      structuredContent: {
        success: true,
        messageId: res.data.id || '',
        threadId: res.data.threadId || undefined,
      },
    };
  }