Your First LLM API Call in Node.js: Request and Response Anatomy

import Anthropic from "@anthropic-ai/sdk";
 
export const anthropic = new Anthropic({
  timeout: 60_000, // milliseconds
  maxRetries: 2, // retries 408, 409, 429, 5xx, and connection errors with backoff
});
 
export const MODEL = "claude-opus-5-5";

import { anthropic, MODEL } from "./lib/anthropic.ts";
 
const message = await anthropic.messages.create({
  model: MODEL,
  max_tokens: 2048,
  system: "You are a concise assistant for web developers. Answer in at most three sentences.",
  output_config: { effort: "low" },
  messages: [{ role: "user", content: "What is the difference between a process and a thread?" }],
});
 
console.log("id:", message.id);
console.log("model:", message.model);
console.log("stop_reason:", message.stop_reason);
console.log("usage:", message.usage);
console.log("request id (quote it in support tickets):", message._request_id);
 
for (const block of message.content) {
  if (block.type === "text") console.log("\n" + block.text);
}

node --env-file=.env.local first-call.ts

import { anthropic, MODEL } from "./lib/anthropic.ts";
 
const message = await anthropic.messages.create({
  model: MODEL,
  max_tokens: 1024,
  messages: [
    {
      role: "user",
      content: [
        // Images must be JPEG, PNG, GIF, or WebP. This is a PNG rendering of the React logo.
        {
          type: "image",
          source: { type: "url", url: "https://upload.wikimedia.org/wikipedia/commons/thumb/a/a7/React-icon.svg/500px-React-icon.svg.png" },
        },
        { type: "text", text: "What library does this logo represent? One sentence." },
      ],
    },
  ],
  metadata: { user_id: "user_7f3a" }, // an opaque ID, never personal data
});
 
console.log(message.content.flatMap((b) => (b.type === "text" ? [b.text] : [])).join(""));

{
  "id": "msg_01XFDUDYJgAACzvnptvVoYEL",
  "type": "message",
  "role": "assistant",
  "model": "claude-opus-5-5",
  "content": [
    { "type": "thinking", "thinking": "", "signature": "…" },
    { "type": "text", "text": "A process is an isolated program with its own memory…" }
  ],
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 42,
    "output_tokens": 118,
    "cache_creation_input_tokens": 0,
    "cache_read_input_tokens": 0
  }
}

import type Anthropic from "@anthropic-ai/sdk";
 
export class IncompleteAnswerError extends Error {}
export class RefusedError extends Error {}
 
/** The text of a finished answer, or a typed error explaining why there is none. */
export function answerText(message: Anthropic.Message): string {
  switch (message.stop_reason) {
    case "end_turn":
    case "stop_sequence":
      return message.content.flatMap((b) => (b.type === "text" ? [b.text] : [])).join("");
    case "max_tokens":
      throw new IncompleteAnswerError("The answer was cut off at max_tokens");
    case "refusal":
      throw new RefusedError("The model declined this request");
    case "tool_use":
    case "pause_turn":
      throw new Error(`stop_reason "${message.stop_reason}" needs the tool loop, not answerText()`);
    default:
      throw new Error(`Unexpected stop_reason: ${String(message.stop_reason)}`);
  }
}