Claude API Quickstart Guide

Setup

1. Get an API Key

Sign up at console.anthropic.com and create an API key.

2. Install the SDK

# Python
pip install anthropic

# Node.js
npm install @anthropic-ai/sdk

3. Set Your API Key

export ANTHROPIC_API_KEY="your-key-here"

First Request

Python

import anthropic

client = anthropic.Anthropic()

message = client.messages.create(
    model="claude-sonnet-4-20250514",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "What is machine learning in one paragraph?"}
    ]
)

print(message.content[0].text)

Node.js

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

const client = new Anthropic();

const message = await client.messages.create({
  model: 'claude-sonnet-4-20250514',
  max_tokens: 1024,
  messages: [
    { role: 'user', content: 'What is machine learning in one paragraph?' }
  ]
});

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

Streaming

For real-time output:

with client.messages.stream(
    model="claude-sonnet-4-20250514",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Explain neural networks"}]
) as stream:
    for text in stream.text_stream:
        print(text, end="", flush=True)

System Prompts

Set behavior with system prompts:

message = client.messages.create(
    model="claude-sonnet-4-20250514",
    max_tokens=1024,
    system="You are a Python expert. Respond with code examples.",
    messages=[
        {"role": "user", "content": "How do I read a CSV file?"}
    ]
)

Tool Use

Give Claude access to functions:

tools = [{
    "name": "get_weather",
    "description": "Get current weather for a location",
    "input_schema": {
        "type": "object",
        "properties": {
            "location": {"type": "string", "description": "City name"}
        },
        "required": ["location"]
    }
}]

message = client.messages.create(
    model="claude-sonnet-4-20250514",
    max_tokens=1024,
    tools=tools,
    messages=[{"role": "user", "content": "What's the weather in Tokyo?"}]
)

Available Models

Model	Best For	Context
claude-opus-4-20250514	Most capable, complex tasks	200K
claude-sonnet-4-20250514	Best balance of speed and quality	200K
claude-haiku-3-5-20241022	Fastest, cheapest	200K

Rate Limits

• Free tier: 40 requests/minute
• Build tier: 1,000 requests/minute
• Scale tier: Custom limits

Next Steps

• Read the full API documentation
• Try extended thinking for complex reasoning
• Explore Claude Code for agentic coding