How to Use the Claude API: Complete Beginner's Guide (2026)

This guide walks you through using the Claude API from scratch. By the end, you'll have a working Python application that sends messages to Claude and handles responses — including streaming, system prompts, and tool use.

Prerequisites

• Python 3.9+ installed
• An Anthropic account (sign up at console.anthropic.com)
• Basic Python knowledge

Step 1: Get Your API Key

1. Go to console.anthropic.com
2. Click API Keys in the left sidebar
3. Click Create Key, give it a name
4. Copy the key — you'll only see it once

Store it as an environment variable:

export ANTHROPIC_API_KEY="sk-ant-your-key-here"

Step 2: Install the SDK

pip install anthropic

Step 3: Make Your First API Call

import anthropic

client = anthropic.Anthropic()  # Uses ANTHROPIC_API_KEY env var

message = client.messages.create(
    model="claude-sonnet-4-6",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "What is machine learning in one paragraph?"}
    ]
)

print(message.content[0].text)

That's it — you've made your first Claude API call.

Step 4: Add a System Prompt

System prompts set Claude's behavior and personality:

message = client.messages.create(
    model="claude-sonnet-4-6",
    max_tokens=1024,
    system="You are a senior Python developer. Give concise, production-ready code with brief explanations.",
    messages=[
        {"role": "user", "content": "Write a function to validate email addresses"}
    ]
)

Step 5: Multi-Turn Conversations

Build conversations by passing the full message history:

conversation = []

def chat(user_message):
    conversation.append({"role": "user", "content": user_message})

    response = client.messages.create(
        model="claude-sonnet-4-6",
        max_tokens=1024,
        messages=conversation
    )

    assistant_message = response.content[0].text
    conversation.append({"role": "assistant", "content": assistant_message})
    return assistant_message

print(chat("What's a decorator in Python?"))
print(chat("Show me a real-world example"))  # Claude remembers the context

Step 6: Streaming Responses

For real-time output (like typing), use streaming:

with client.messages.stream(
    model="claude-sonnet-4-6",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Explain how RAG works"}]
) as stream:
    for text in stream.text_stream:
        print(text, end="", flush=True)

Step 7: Tool Use (Function Calling)

Let Claude call your functions:

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

response = client.messages.create(
    model="claude-sonnet-4-6",
    max_tokens=1024,
    tools=tools,
    messages=[{"role": "user", "content": "What's the weather in London?"}]
)

# Claude will return a tool_use block — your code executes the function
# and sends the result back for Claude to formulate a natural response

Which Model to Use?

Start with Sonnet — it handles 95% of use cases well and costs 5x less than Opus.

Cost Estimation

A typical chatbot handling 1,000 conversations/day with Sonnet 4.6:

• Average conversation: ~2,000 tokens input + ~1,000 tokens output
• Daily cost: ~$6-9/day
• Monthly cost: ~$180-270/month

TL;DR

Install pip install anthropic, get an API key from console.anthropic.com, and you can make your first Claude API call in 5 lines of Python. Start with Sonnet 4.6 for the best price-performance ratio. Use streaming for real-time output and tools for function calling.

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Build more with our AI agents tutorial, explore the Claude API tool, and learn about RAG.