Claude API Deep Dive

Ready to build Claude-powered applications? This guide covers everything from authentication to advanced streaming patterns.

Authentication

Every API request requires an API key:

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: YOUR_API_KEY" \
  -H "content-type: application/json" \
  -H "anthropic-version: 2023-06-01"

Best Practices

• Store keys in environment variables, never in code
• Use different keys for development and production
• Rotate keys regularly
• Set usage limits to prevent surprises

Making Your First API Call

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

const client = new Anthropic();

const response = await client.messages.create({
  model: 'claude-sonnet-4-20250514',
  max_tokens: 1024,
  messages: [
    { role: 'user', content: 'Explain quantum computing in one paragraph.' }
  ],
});

console.log(response.content[0].text);

Streaming Responses

For real-time applications, stream Claude's responses token by token:

const stream = await client.messages.stream({
  model: 'claude-sonnet-4-20250514',
  max_tokens: 1024,
  messages: [
    { role: 'user', content: 'Write a short story about a robot.' }
  ],
});

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

Error Handling

Always handle these common errors:

try {
  const response = await client.messages.create({ ... });
} catch (error) {
  if (error.status === 429) {
    // Rate limited - implement exponential backoff
    await sleep(Math.pow(2, retryCount) * 1000);
  } else if (error.status === 529) {
    // API overloaded - retry with delay
  } else if (error.status === 400) {
    // Bad request - check your parameters
  }
}

System Prompts via API

const response = await client.messages.create({
  model: 'claude-sonnet-4-20250514',
  max_tokens: 1024,
  system: 'You are a helpful coding assistant. Always include code examples.',
  messages: [
    { role: 'user', content: 'How do I sort an array in Python?' }
  ],
});

Cost Optimization

• Use claude-haiku for simple tasks (10x cheaper)
• Set appropriate max_tokens limits
• Cache responses for identical queries
• Use streaming to improve perceived performance
• Batch similar requests when possible

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The API is your gateway to building AI-powered products. Start small, ship fast, iterate often.