A quick reference for making your first Claude API call, picking a model, and reading the response.
ANTHROPIC_API_KEY variable.client.messages.create() with a model, prompt, and max_tokens.stop_reason before reading content to handle responses safely.pip install anthropic Installs the Python SDK from PyPI so you can start calling Claude.pip install anthropicnpm install @anthropic-ai/sdk Installs the TypeScript and Node SDK for JavaScript projects.npm install @anthropic-ai/sdkANTHROPIC_API_KEY Stores your API key as an environment variable instead of hard-coding it.export ANTHROPIC_API_KEY="sk-ant-..."Anthropic() Creates a client that reads the API key from the environment automatically.import anthropic
client = anthropic.Anthropic()messages.create() Sends a prompt to Claude with a model, max_tokens, and a messages array.message = client.messages.create(
model="claude-opus-4-8",
max_tokens=1024,
messages=[{"role": "user", "content": "Explain closures in one paragraph."}]
)system parameter Gives Claude a persistent role or persona for the whole conversation.message = client.messages.create(
model="claude-opus-4-8",
max_tokens=1024,
system="You are a concise technical writer.",
messages=[{"role": "user", "content": "Explain closures."}]
)max_tokens Caps the response length; short answers need less, long docs need more.# Short answers: 256-512 | Long docs: 4096-8192 | Max: 128000content[0].text Reads the text of Claude's reply from the response object.print(message.content[0].text)stop_reason Explains why generation stopped: end_turn, max_tokens, tool_use, or refusal.if message.stop_reason == "end_turn":
print(message.content[0].text)usage Reports input and output token counts so you can track cost.print(message.usage.input_tokens)
print(message.usage.output_tokens)message.model Confirms which model actually served the request.print(message.model) # e.g. "claude-opus-4-8"Claude Opus 4.8 Best for hard reasoning and agentic tasks; the default for most new integrations.claude-opus-4-8Claude Sonnet 5 Near-Opus quality on coding and agentic work at a lower cost.claude-sonnet-5Claude Haiku 4.5 Fastest and cheapest option for high-volume tasks like tagging or routing.claude-haiku-4-5Claude Fable 5 Anthropic's most capable model for the hardest, longest-horizon tasks.claude-fable-5claude-opus-4-8 in production so a later model update never silently changes your app's behavior.max_tokens generously for long or open-ended replies, since Claude stops the moment it reaches that limit, even mid-sentence.max_tokens is required on every request — omitting it raises a validation error before Claude even sees your prompt.Fast, clear reference sheets for technology, finance, health, and everyday adulting.