Claude API JavaScript Examples (2026)

Complete guide to using the Claude API with JavaScript in 2026. Node.js quickstart, streaming, tool use, and prompt caching code examples using the official Anthropic SDK.

The official Anthropic JavaScript/TypeScript SDK (@anthropic-ai/sdk) works in Node.js 18+, Deno, and modern bundlers. This guide covers the most common patterns from a single message to streaming tool use.

Install

npm install @anthropic-ai/sdk

Or with Bun: bun add @anthropic-ai/sdk

First message

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

const client = new Anthropic(); // reads ANTHROPIC_API_KEY from env

const message = await client.messages.create({
  model: "claude-sonnet-4-6",
  max_tokens: 1024,
  messages: [{ role: "user", content: "Explain async/await in JavaScript." }]
});

console.log(message.content[0].text);

With a system prompt

const message = await client.messages.create({
  model: "claude-sonnet-4-6",
  max_tokens: 1024,
  system: "You are a concise JavaScript expert. Reply in under 100 words.",
  messages: [{ role: "user", content: "When should I use WeakRef?" }]
});

Streaming responses

const stream = client.messages.stream({
  model: "claude-sonnet-4-6",
  max_tokens: 1024,
  messages: [{ role: "user", content: "Write a quicksort in JS." }]
});

for await (const event of stream) {
  if (event.type === "content_block_delta" && event.delta.type === "text_delta") {
    process.stdout.write(event.delta.text);
  }
}
await stream.finalMessage();

Tool use (function calling)

const tools = [
  {
    name: "get_weather",
    description: "Returns current temperature for a city.",
    input_schema: {
      type: "object",
      properties: { city: { type: "string", description: "City name" } },
      required: ["city"]
    }
  }
];

const response = await client.messages.create({
  model: "claude-sonnet-4-6",
  max_tokens: 512,
  tools,
  messages: [{ role: "user", content: "What is the weather in Tokyo?" }]
});

if (response.stop_reason === "tool_use") {
  const toolUse = response.content.find(b => b.type === "tool_use");
  console.log("Tool called:", toolUse.name, toolUse.input);
}

Prompt caching (cut costs 90%)

const response = await client.messages.create({
  model: "claude-sonnet-4-6",
  max_tokens: 512,
  system: [
    {
      type: "text",
      text: "You are an expert on the Node.js docs. " + nodeDocs, // large static context
      cache_control: { type: "ephemeral" }
    }
  ],
  messages: [{ role: "user", content: "How do I use the crypto module?" }]
});

The first call writes the cache; subsequent calls within 5 minutes read at 90% off. See the Claude Cost Calculator to model your savings.

Counting tokens before a request

const count = await client.messages.countTokens({
  model: "claude-sonnet-4-6",
  messages: [{ role: "user", content: myPrompt }]
});
console.log("Input tokens:", count.input_tokens);

Error handling

import { APIError } from "@anthropic-ai/sdk";

try {
  const response = await client.messages.create({ /* ... */ });
} catch (err) {
  if (err instanceof APIError) {
    console.error(err.status, err.message); // 429, 500, etc.
    if (err.status === 429) { /* back off and retry */ }
  }
}

Useful links

• Claude Cost Calculator — estimate your monthly JS app bill
• Prompt-Pricing Recommender — Haiku vs Sonnet vs Opus for your use case

Frequently asked questions

Free tools

Related