How to Build AI Agents in 2026: The Actually Simple Guide

Building an AI agent sounds complicated. It's not. By the end of this guide, you'll have a working agent that can search the web, remember conversations, and handle multi-step tasks. No frameworks, just TypeScript and an LLM API. What We're Building A research assistant agent that: Takes questions from users Uses tools (web search) when needed Remembers conversation history Handles errors without crashing Runs in about 150 lines of TypeScript This won't be production-ready, but it'll work and you'll understand every line. Prerequisites You need: Node.js 18 or higher Basic TypeScript knowledge An Anthropic API key ( get one free ) That's it. No prior AI experience needed. Setup (5 minutes) # Create project mkdir research-agent cd research-agent npm init -y # Install dependencies npm install @anthropic-ai/sdk dotenv # Install dev dependencies npm install -D typescript @types/node tsx # Initialize TypeScript npx tsc --init Create .env : ANTHROPIC_API_KEY = your-key-here Step 1: Define Your Types Create src/types.ts : export interface Message { role : ' user ' | ' assistant ' ; content : string ; } export interface Tool { name : string ; description : string ; input_schema : { type : ' object ' ; properties : Record < string , any > ; required ?: string []; }; execute : ( input : any ) => Promise < string > ; } Why these types matter: Strong typing prevents bugs. If you change how a tool works, TypeScript tells you everywhere that breaks. Step 2: Create a Simple Tool Create src/tools/search.ts : import { Tool } from ' ../types ' ; export const searchTool : Tool = { name : ' search_web ' , description : ' Search the internet for current information. Use this when you need facts, recent events, or data you do not know. ' , input_schema : { type : ' object ' , properties : { query : { type : ' string ' , description : ' The search query ' , }, }, required : [ ' query ' ], }, execute : async ( input : { query : string }) => { console . log ( `[Tool] Searching for: ${ input