How to use the Anthropic Claude API

Create an account at console.anthropic.com. Generate an API key and store it as `ANTHROPIC_API_KEY` in your environment. Install the SDK: `pip install anthropic` (Python) or `npm install @anthropic-ai/sdk` (Node.js).

The Anthropic API uses a slightly different format from OpenAI. The key difference: system prompt is a top-level parameter, not a message with role 'system'. Max tokens is required, not optional.

Python: `import anthropic; client = anthropic.Anthropic(); message = client.messages.create(model='claude-sonnet-4-5', max_tokens=1024, messages=[{'role': 'user', 'content': 'Hello'}]); print(message.content[0].text)`.

The response `content` is a list — it can contain text blocks and tool_use blocks. Always access the first text block with `message.content[0].text` for simple text responses.

Claude Sonnet and Opus support images in messages. Pass images as base64-encoded strings or URLs: `messages=[{'role': 'user', 'content': [{'type': 'image', 'source': {'type': 'url', 'url': 'https://...'}}, {'type': 'text', 'text': 'What is in this image?'}]}]`.

For base64, read the file and encode it: `import base64; with open('image.jpg', 'rb') as f: data = base64.standard_b64encode(f.read()).decode('utf-8')`. Use media_type `image/jpeg`, `image/png`, `image/gif`, or `image/webp`.

Anthropic's tool use format is clean and explicit. Define tools as a list of `{name, description, input_schema}` objects. Pass them to `client.messages.create(tools=[...])`. When the model wants to use a tool, it returns a message with `stop_reason: 'tool_use'` and a `tool_use` content block.

Extract the tool name and input: `tool_block = [b for b in message.content if b.type == 'tool_use'][0]; tool_name = tool_block.name; tool_input = tool_block.input`. Execute the tool, then send the result back as a `tool_result` content block in the next user message.