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Sangmin Lee
Sangmin Lee

Posted on • Originally published at claudeguide.io

Claude API for Beginners: Your First API Call in 10 Minutes

#beginners #tutorial #python #anthropic

Originally published at claudeguide.io/claude-api-beginners-guide

Claude API for Beginners: Your First API Call in 10 Minutes

The Claude API lets you build applications with Claude — chatbots, coding assistants, content tools, data extractors, and agents. Getting started takes 10 minutes: create an Anthropic account, get an API key, install the SDK, and send your first message. This guide walks through each step with complete code.

Step 1: Get an API key

  1. Go to console.anthropic.com
  2. Create an account (email + password, or Google)
  3. Add payment information (you won't be charged until you use the API)
  4. Navigate to API Keys → Create Key
  5. Copy the key immediately — you won't see it again

API key format: sk-ant-api03-... (starts with sk-ant-)

Security rule: never commit your API key to git. Never share it. Store it as an environment variable.

Step 2: Install the SDK

Python:

pip install anthropic
# or
pip install anthropic[bedrock,vertex]  # if using AWS or GCP
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Node.js / TypeScript:

npm install @anthropic-ai/sdk
# or
bun add @anthropic-ai/sdk
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Step 3: Set your API key

macOS/Linux (add to ~/.zshrc or ~/.bashrc for persistence):

export ANTHROPIC_API_KEY="sk-ant-..."
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Windows (Command Prompt):

set ANTHROPIC_API_KEY=sk-ant-...
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Python project (.env file):

ANTHROPIC_API_KEY=sk-ant-...
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Then load with python-dotenv:

from dotenv import load_dotenv
load_dotenv()
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Never put the key directly in your code — it's easy to accidentally commit to git.

Step 4: Your first API call

Python:

import anthropic

client = anthropic.Anthropic()
# The client automatically reads ANTHROPIC_API_KEY from your environment

message = client.messages.create(
    model="claude-haiku-4-5",  # Cheapest model for learning
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "Hello, Claude! What's 2 + 2?"}
    ]
)

print(message.content[0].text)
# Output: "2 + 2 equals 4."
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TypeScript:

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

const client = new Anthropic();

const message = await client.messages.create({
  model: "claude-haiku-4-5",
  max_tokens: 1024,
  messages: [
    { role: "user", content: "Hello, Claude! What's 2 + 2?" }
  ],
});

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

python first_call.py
# or
npx ts-node first_call.ts
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You should see Claude's response printed. If you see an AuthenticationError, your API key isn't set correctly.

Understanding the request structure

Every API call has the same structure:

client.messages.create(
    model="...",       # Which Claude model to use
    max_tokens=1024,   # Maximum length of Claude's response
    system="...",      # Optional: instructions for Claude's behavior (system prompt)
    messages=[         # The conversation history
        {"role": "user", "content": "..."},       # User messages
        {"role": "assistant", "content": "..."},  # Claude's previous responses
        {"role": "user", "content": "..."},       # Another user message
    ]
)
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model: which Claude to use. Start with claude-haiku-4-5 (cheapest, fastest).
max_tokens: maximum tokens in Claude's response. 1024 = ~750 words. Set higher for long responses.
system: tells Claude its role and behavior. Optional but powerful.
messages: the conversation. Always starts and ends with a user message.

The response structure 

message = client.messages.create(...)

print(message.content[0].text)   # The response text
print(message.model)              # Which model was used
print(message.stop_reason)        # Why Claude stopped: "end_turn" or "max_tokens"
print(message.usage.input_tokens) # Tokens in your request
print(message.usage.output_tokens) # Tokens in Claude's response
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Multi-turn conversations (chat)

To have a back-and-forth conversation, pass the full history each time:


python
import anthropic

client = anthropic.Anthropic()

conversation = []

def chat(user_message: str) -

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