How to Use the Claude APIComplete Guide 2026

import anthropic

client = anthropic.Anthropic(api_key="your-api-key-here")

message = client.messages.create(
    model="claude-sonnet-4-5-20251001",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "Explain what RAG is in 3 paragraphs."}
    ]
)

print(message.content[0].text)

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

const client = new Anthropic({ apiKey: process.env.ANTHROPIC_API_KEY });

const stream = client.messages.stream({
  model: "claude-sonnet-4-5-20251001",
  max_tokens: 1024,
  messages: [{ role: "user", content: "List 5 AI frameworks." }],
});

for await (const chunk of stream) {
  if (chunk.type === "content_block_delta") {
    process.stdout.write(chunk.delta.text);
  }
}

message = client.messages.create(
    model="claude-sonnet-4-5-20251001",
    max_tokens=2048,
    system="You are an assistant specialized in financial analysis. "
           "Always be objective and cite sources when relevant.",
    messages=[
        {"role": "user", "content": "What are the risks of investing in LLMs in 2026?"}
    ]
)

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-sonnet-4-5-20251001",
    "max_tokens": 1024,
    "messages": [{"role": "user", "content": "Hello!"}]
  }'