Your first API call

Saying hi to Claude might warm your heart, but it's not really useful. In this lesson we'll send Claude something real and get structured insight back — in just under 20 lines of code.

Get set up

First, grab an API key from platform.claude.com. You'll need to purchase some credits beforehand.

Take the API key and store it in a .env.local file so it stays out of your version control. Hardcoding keys in source files is how they end up leaked on GitHub — keep them in environment files instead.

Next, install the SDK:

npm install @anthropic-ai/sdk

The anatomy of a request

Every API call goes through the messages.create function. You specify three things:

• A model — which Claude model handles the request
• A max tokens limit — a cap on how long the response can be
• A list of messages — objects with either user or assistant roles, structured similarly to how you'd have a conversation with Claude elsewhere

Here's what that looks like in its most basic form:

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic();

const msg = await client.messages.create({
  model: "claude-sonnet-5",
  max_tokens: 2048,
  messages: [{
    role: "user",
    content: "Hello, Claude",
  }],
});

A real example: reviewing buggy code

Let's give Claude something a little more interesting than "hello." We'll point it at some buggy code and ask for a review. Here's the whole thing — one file, about 20 lines of code:

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic();

const buggyCode = `
function add(a, b) {
  return a - b;
}
`;

const response = await client.messages.create({
  model: "claude-sonnet-5",
  max_tokens: 2048,
  system: "You are a terse senior code reviewer. Give feedback in one paragraph.",
  messages: [
    { role: "user", content: `Review this code:\n${buggyCode}` },
  ],
});

for (const block of response.content) {
  if (block.type === "text") {
    console.log(block.text);
  }
}