claude-api

Claude API

Build applications with the Anthropic Claude API and SDKs.

When to Activate

• Building applications that call the Claude API
• Code imports anthropic (Python) or @anthropic-ai/sdk (TypeScript)
• User asks about Claude API patterns, tool use, streaming, or vision
• Implementing agent workflows with Claude Agent SDK
• Optimizing API costs, token usage, or latency

Model Selection

Model	ID	Best For
Opus 4.1	claude-opus-4-1	Complex reasoning, architecture, research
Sonnet 4	claude-sonnet-4-0	Balanced coding, most development tasks
Haiku 3.5	claude-3-5-haiku-latest	Fast responses, high-volume, cost-sensitive

Default to Sonnet 4 unless the task requires deep reasoning (Opus) or speed/cost optimization (Haiku). For production, prefer pinned snapshot IDs over aliases.

Python SDK

Installation

pip install anthropic

Basic Message

import anthropic

client = anthropic.Anthropic()  # reads ANTHROPIC_API_KEY from env

message = client.messages.create(
    model="claude-sonnet-4-0",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "Explain async/await in Python"}
    ]
)
print(message.content[0].text)

Streaming

with client.messages.stream(
    model="claude-sonnet-4-0",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Write a haiku about coding"}]
) as stream:
    for text in stream.text_stream:
        print(text, end="", flush=True)

System Prompt

message = client.messages.create(
    model="claude-sonnet-4-0",
    max_tokens=1024,
    system="You are a senior Python developer. Be concise.",
    messages=[{"role": "user", "content": "Review this function"}]
)

TypeScript SDK

Installation

npm install @anthropic-ai/sdk

Basic Message

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

const client = new Anthropic(); // reads ANTHROPIC_API_KEY from env

const message = await client.messages.create({
  model: "claude-sonnet-4-0",
  max_tokens: 1024,
  messages: [
    { role: "user", content: "Explain async/await in TypeScript" }
  ],
});
console.log(message.content[0].text);