Your first API call

Install

npm install @anthropic-ai/sdk

Get a key at console.anthropic.com and put it in the ANTHROPIC_API_KEY environment variable. Never hardcode the key in the repository.

Minimal call in TypeScript

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

const client = new Anthropic({
  apiKey: process.env.ANTHROPIC_API_KEY, // picked up automatically, but be explicit
});

const message = await client.messages.create({
  model: "claude-sonnet-4-5",
  max_tokens: 1024,
  messages: [
    { role: "user", content: "Explain closures in JS in 3 sentences." },
  ],
});

console.log(message.content[0].type === "text" ? message.content[0].text : "");

What's in the response

The Message object contains:

id — the response identifier.
content — an array of content blocks, not a string. A text block has type: "text" and a text field.
stop_reason — "end_turn", "max_tokens", "tool_use", "stop_sequence".
usage — input_tokens and output_tokens for cost accounting.

Same in Python

import anthropic

client = anthropic.Anthropic()  # reads ANTHROPIC_API_KEY from the environment

msg = client.messages.create(
    model="claude-sonnet-4-5",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Hi"}],
)
print(msg.content[0].text)

max_tokens is required — it's the response length limit, not the context limit. Set a realistic value: too small truncates the answer (stop_reason: "max_tokens").

Report a bug

Task grader

Prompt sandbox

Quiz — 2 questions

Discussion

Install

Minimal call in TypeScript

What's in the response

Same in Python