Building Production MCP Servers: Anthropic's Standard for Safe Database & API Tool Calling

import { Server } from "@modelcontextprotocol/sdk/server/index.js";
import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";
import { CallToolRequestSchema, ListToolsRequestSchema } from "@modelcontextprotocol/sdk/types.js";
import { z } from "zod";
import pg from "pg";

const pool = new pg.Pool({ connectionString: process.env.DATABASE_URL });

const server = new Server(
  { name: "enterprise-postgres-mcp", version: "1.0.0" },
  { capabilities: { tools: {} } }
);

const QuerySchema = z.object({
  sql: z.string().refine((q) => !/^(INSERT|UPDATE|DELETE|DROP|ALTER)/i.test(q), {
    message: "Only SELECT queries are permitted on this MCP endpoint",
  }),
});

server.setRequestHandler(ListToolsRequestSchema, async () => ({
  tools: [
    {
      name: "execute_safe_sql",
      description: "Executes a read-only SQL query against the enterprise warehouse",
      inputSchema: {
        type: "object",
        properties: { sql: { type: "string" } },
        required: ["sql"],
      },
    },
  ],
}));

server.setRequestHandler(CallToolRequestSchema, async (request) => {
  if (request.params.name === "execute_safe_sql") {
    const { sql } = QuerySchema.parse(request.params.arguments);
    const client = await pool.connect();
    try {
      await client.query("BEGIN TRANSACTION READ ONLY;");
      const result = await client.query(`${sql} LIMIT 100;`);
      await client.query("COMMIT;");
      return { content: [{ type: "text", text: JSON.stringify(result.rows, null, 2) }] };
    } finally {
      client.release();
    }
  }
  throw new Error("Tool not found");
});