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Your First API Call

Beginner

The API lets your program talk to Claude. By the end of this lesson you'll have run a real request and you'll understand exactly what each piece of it does.

What you'll learn
  • Create an API key and store it safely as an environment variable
  • Install the official SDK for Python or TypeScript (or use plain cURL)
  • Send your first messages request and read Claude's reply
  • Understand the four core fields: model, max_tokens, messages, and system

The whole journey in three steps​

Everything below fits into one simple sequence. Keep this map in mind as you go.

Guided walkthrough1 of 3
  1. Create a key in the Anthropic Console and export it as an environment variable so it never lives in your code.

1. Get an API key​

Create one in the Anthropic Console. Then set it as an environment variable so it never lives in your code:

Export your API key

export ANTHROPIC_API_KEY="sk-ant-..."
Watch out
  • Never commit your key. Keep keys in environment variables or a secrets manager, never in source control. See the Security page (/docs/security).

2. Install the SDK​

Pick your language. The SDK reads the key automatically from the environment variable you just set.

Install (Python)

pip install anthropic

3. Make the call​

Every request is a list of messages. The model replies with content. Run the snippet for your language and you'll see Claude answer the question.

import anthropic

client = anthropic.Anthropic()  # reads ANTHROPIC_API_KEY from the environment

message = client.messages.create(
    model="claude-sonnet-5",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "In one sentence, what is an API?"}
    ],
)

print(message.content[0].text)

What just happened​

You sent four pieces of information. Here's what each one controls:

  • model — which Claude to use. Don't hard-code blindly; see Choosing a Model.
  • max_tokens — a cap on the reply length (in tokens). It does not set the context window.
  • messages — the conversation so far. The API is stateless: to continue a chat, send the whole history back each time.
  • system (optional) — a top-level instruction that sets Claude's role for the call.
The four core fields
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1 / 4
Pro tip
  • Because the API is stateless, a multi-turn chat is just a growing messages array: append each reply and resend the full list on the next call.

Check yourself

0/3
  1. What does max_tokens control?
  2. How do you continue a multi-turn conversation with the API?
  3. Where should your API key live?
Key takeaways
  • Three steps: get a key, install the SDK, send a messages request.
  • Store the key in ANTHROPIC_API_KEY so the SDK reads it automatically and it never enters your code.
  • A request is a list of messages; the reply comes back in content.
  • The API is stateless: resend the full conversation history to continue a chat.
  • model, max_tokens, messages, and system are the fields you'll reach for first.

Next​