Describe the tool

The server registers a tool with a name, description, and input shape.

server.registerTool(
  "get_current_time",
  {
    description: "Return the current time in UTC",
    inputSchema: z.object({}),
  },
  async () => {
    // The work goes here.
  },
);

The empty object means the tool needs no input. Its description tells the host and model what the tool is for.