🔌What is an API?

The bridge between your code and Claude — explained simply

What is an API?

Think of an API as a waiter in a restaurant:

You (the customer) look at the menu and decide what you want
The waiter (API) takes your order to the kitchen
The kitchen (Claude's servers) prepares your food
The waiter (API) brings your food back to you

You never go into the kitchen yourself. The waiter handles all communication between you and the kitchen. That's exactly what an API does — it's the middleman between your code and Claude.

How it Works in Practice

Your Code  →  sends a request  →  Claude's API  →  Claude processes it  →  sends response back  →  Your Code

What's in an API Request?

When you talk to Claude's API, you send a structured message with these parts:

Part	What It Is	Example
Model	Which Claude to use	`claude-opus-4-7`
Messages	What you want to say/ask	`"Explain APIs to me"`
Max Tokens	Maximum length of response	`1024`
API Key	Your unique password	`sk-ant-api03-...`

What's in an API Response?

Claude sends back a structured response:

JSON
{
  "content": [
    {
      "type": "text",
      "text": "An API is like a waiter..."
    }
  ],
  "model": "claude-opus-4-7",
  "usage": {
    "input_tokens": 12,
    "output_tokens": 150
  }
}

What Are Tokens?

Tokens are how AI measures text. A token is roughly:

1 token ≈ ¾ of a word (English)
"Hello, world!" = ~3 tokens
100 tokens ≈ 75 words
1000 tokens ≈ 750 words

Why does this matter? You pay based on tokens used:

Input tokens — what you send to Claude
Output tokens — what Claude sends back
Output tokens cost more than input tokens

Your API Key — Your Secret Password

An API Key is like a password that identifies you:

It looks like: sk-ant-api03-abcdef123456...
You get it from the Anthropic Console (we'll set this up next)
NEVER share it — anyone with your key can use your account
NEVER put it in your code — use environment variables

REST API Basics

Claude uses a REST API, which means:

Communication happens over HTTPS (secure internet)
You send HTTP requests (like visiting a website, but from code)
Data is formatted as JSON (a way to structure data)

Don't worry if these terms are new — we'll learn by doing in the next lessons!

Next up: We'll set up your development environment and get your API key.

Module 1

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🧠 Meet Claude — Your AI Assistant

How LLMs Actually Work 🧬

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