Defining Tool Schemas — Tools & Functions

Tools Give AI Hands

Tools are functions the AI can call to fetch data or mutate state. They are the most powerful part of MCP.

Registering a Tool with registerTool & Zod

Modern MCP TypeScript SDKs use server.registerTool() (which replaced the deprecated v1 server.tool() shorthand) and support Standard Schema libraries like zod, Valibot, or ArkType, compiling schemas to JSON Schema 2020-12.

import { z } from "zod";

server.registerTool(
  "calculate_tax",
  {
    title: "Sales Tax Calculator",
    description: "Calculate sales tax for a given purchase amount",
    inputSchema: z.object({
      amount: z.number().describe("The total purchase amount"),
      param_state: z.string().describe("Two-letter state code")
    })
  },
  async ({ amount, param_state }) => {
    return { content: [{ type: "text", text: `Tax calculated for $${amount} in ${param_state}` }] };
  }
);