Anthropic API Integration (Claude)

Anthropic's Claude 3.5 Sonnet is widely considered the best model for coding and complex reasoning.

Setup

Install the official SDK:

npm install @anthropic-ai/sdk

Get your API key from the Anthropic Console.

Basic Usage

Unlike OpenAI, Anthropic's API requires max_tokens to be set, and the system prompt is a top-level parameter (not part of the messages array).

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

const anthropic = new Anthropic({
  apiKey: process.env.ANTHROPIC_API_KEY,
});

async function main() {
  const message = await anthropic.messages.create({
    model: 'claude-3-5-sonnet-20241022',
    max_tokens: 1024,
    system: "You are a concise assistant.",
    messages: [
      { role: "user", content: "Explain quantum computing in one sentence." }
    ],
  });

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

main();

Streaming Responses

Anthropic provides a helper method .stream() that simplifies consuming streams.

const stream = await anthropic.messages.stream({
  model: 'claude-3-5-sonnet-20241022',
  max_tokens: 1024,
  messages: [{ role: 'user', content: 'Write a short story.' }],
})
.on('text', (text) => {
  console.log(text);
});

const finalMessage = await stream.finalMessage();
console.log("Stream complete");

Tool Use

Claude's tool use (function calling) is very robust.

const msg = await anthropic.messages.create({
  model: "claude-3-5-sonnet-20241022",
  max_tokens: 1024,
  tools: [
    {
      name: "get_weather",
      description: "Get the current weather in a given location",
      input_schema: {
        type: "object",
        properties: {
          location: { type: "string" }
        },
        required: ["location"]
      }
    }
  ],
  messages: [{ role: "user", content: "What is the weather in London?" }],
});

if (msg.stop_reason === "tool_use") {
  const toolBlock = msg.content.find(block => block.type === "tool_use");
  console.log("Tool Name:", toolBlock.name);
  console.log("Inputs:", toolBlock.input);
}

Key Differences vs OpenAI

Feature	OpenAI	Anthropic
System Prompt	Inside messages array	Top-level system param
Max Tokens	Optional	Required
Messages Role	system, user, assistant, tool	user, assistant
Pricing	Input/Output tokens	Input/Output tokens
Context	128k	200k

Best Practices for Claude

1. Use XML Tags: Claude is trained to pay special attention to structure like <document>...</document> or <instructions>...</instructions> within prompts.
2. "Put words in Claude's mouth": You can pre-fill the assistant's response to guide the output.

   messages: [
     { role: "user", content: "Write a JSON object." },
     { role: "assistant", content: "{" } // Forces JSON start
   ]

3. Prompt Caching: Anthropic supports caching long prefixes (like huge docs) to save up to 90% on costs.

Next Steps

• Explore the Vercel AI SDK to use Claude and OpenAI interchangeably.