6 MCP Server Patterns That Turn JavaScript APIs Into AI-Callable Tools

MCP made AI tools call your code directly. Not your UI. Not your REST client. Your functions. Here are 6 patterns that convert existing JavaScript services into MCP servers you can ship this week. 1. Wrap a REST API as an MCP tool Most teams already have REST APIs. MCP sits on top of them. Before (Express route) // routes/jobs.ts app . get ( " /jobs " , async ( req , res ) => { const { tech , location } = req . query ; const jobs = await db . jobs . findMany ({ where : { tech , location } }); res . json ( jobs ); }); After (MCP tool wrapper) import { McpServer } from " @modelcontextprotocol/sdk/server/mcp.js " ; import { z } from " zod " ; const server = new McpServer ({ name : " jobs " , version : " 1.0.0 " }); server . tool ( " search_jobs " , " Search jobs by tech and location " , { tech : z . string (). optional (), location : z . string (). optional () }, async ({ tech , location }) => { const res = await fetch ( `https://api.myapp.com/jobs?tech= ${ tech } &location= ${ location }