How to define tools in an MCP server

1// TypeScript
2import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
3import { z } from "zod";
4
5const server = new McpServer({
6  name: "my-tools-server",
7  version: "1.0.0",
8});
9
10// Python
11from mcp.server.fastmcp import FastMCP
12
13mcp = FastMCP("my-tools-server")

1// TypeScript
2server.registerTool("get-weather", {
3  description: "Get current weather for a city",
4  inputSchema: {
5    city: z.string().describe("City name, e.g. San Francisco"),
6    units: z.enum(["celsius", "fahrenheit"]).default("celsius")
7      .describe("Temperature units"),
8  },
9}, async ({ city, units }) => {
10  const temp = await fetchWeather(city, units);
11  return {
12    content: [{ type: "text", text: `${city}: ${temp}° ${units}` }],
13  };
14});
15
16# Python
17@mcp.tool()
18async def get_weather(city: str, units: str = "celsius") -> str:
19    """Get current weather for a city.
20
21    Args:
22        city: City name, e.g. San Francisco
23        units: Temperature units (celsius or fahrenheit)
24    """
25    temp = await fetch_weather(city, units)
26    return f"{city}: {temp}° {units}"

1// Successful result
2return {
3  content: [
4    { type: "text", text: "Operation completed successfully" },
5    { type: "text", text: JSON.stringify(data, null, 2) },
6  ],
7  isError: false,
8};
9
10// Error result
11return {
12  content: [
13    { type: "text", text: "Error: City not found in weather database" },
14  ],
15  isError: true,
16};

1// TypeScript — multiple related tools
2server.registerTool("db-query", {
3  description: "Run a read-only SQL query against the database",
4  inputSchema: {
5    sql: z.string().describe("SELECT query to execute"),
6  },
7}, async ({ sql }) => {
8  if (!sql.trim().toUpperCase().startsWith("SELECT")) {
9    return { content: [{ type: "text", text: "Error: Only SELECT queries allowed" }], isError: true };
10  }
11  const rows = await pool.query(sql);
12  return { content: [{ type: "text", text: JSON.stringify(rows, null, 2) }] };
13});
14
15server.registerTool("db-list-tables", {
16  description: "List all tables in the database",
17  inputSchema: {},
18}, async () => {
19  const tables = await pool.query("SELECT tablename FROM pg_tables WHERE schemaname = 'public'");
20  return { content: [{ type: "text", text: JSON.stringify(tables, null, 2) }] };
21});

1// TypeScript
2import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
3
4const transport = new StdioServerTransport();
5await server.connect(transport);
6console.error("Server running on stdio");
7
8# Python
9if __name__ == "__main__":
10    mcp.run(transport="stdio")