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MCP Tools: Actions AI Can Perform

Tools are the most commonly used MCP capability. They represent actions the AI can perform — like creating a file, querying a database, or sending a message. Each tool has a name, description, input schema, and a handler function.

typescript

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";

const server = new McpServer({ name: "demo", version: "1.0.0" });

// Basic tool with typed parameters
server.tool(
  "search_users",
  "Search for users by name or email",
  {
    query: z.string().describe("Search query"),
    limit: z.number().optional().default(10).describe("Max results"),
  },
  async ({ query, limit }) => {
    const users = await db.users.search(query, limit);
    return {
      content: [{
        type: "text",
        text: JSON.stringify(users, null, 2),
      }],
    };
  }
);

// Tool that returns an error gracefully
server.tool(
  "delete_user",
  "Delete a user by ID",
  { userId: z.string() },
  async ({ userId }) => {
    try {
      await db.users.delete(userId);
      return { content: [{ type: "text", text: `User ${userId} deleted` }] };
    } catch (error) {
      return {
        isError: true,
        content: [{ type: "text", text: `Failed: ${error.message}` }],
      };
    }
  }
);

Tip:Write clear descriptions for your tools — the AI uses them to decide WHEN to call each tool. Good descriptions lead to better AI behavior.