Claude API (Anthropic Messages API)

Claude API (Anthropic Messages API)

Status: Production Ready | SDK: @anthropic-ai/sdk@0.70.1

---

Quick Start (5 Minutes)

Node.js

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

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

const message = await client.messages.create({
  model: 'claude-sonnet-4-5-20250929',
  max_tokens: 1024,
  messages: [
    { role: 'user', content: 'Hello, Claude!' },
  ],
});

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

Cloudflare Workers

const response = await fetch('https://api.anthropic.com/v1/messages', {
  method: 'POST',
  headers: {
    'x-api-key': env.ANTHROPIC_API_KEY,
    'anthropic-version': '2023-06-01',
    'content-type': 'application/json',
  },
  body: JSON.stringify({
    model: 'claude-sonnet-4-5-20250929',
    max_tokens: 1024,
    messages: [{ role: 'user', content: 'Hello!' }],
  }),
});

const data = await response.json();
console.log(data.content[0].text);

Load references/setup-guide.md for complete setup with streaming, caching, and tools.

---

Critical Rules

Always Do ✅

1. Use environment variables for API keys (NEVER hardcode)
2. Set max_tokens explicitly (required parameter)
3. Pin model version (claude-sonnet-4-5-20250929, not claude-3-5-sonnet-latest)
4. Enable prompt caching for repeated content (90% cost savings)
5. Stream long responses (stream: true) for better UX
6. Handle errors - Implement retry logic for 429, 529 errors
7. Validate inputs - Sanitize user messages before sending
8. Monitor costs - Track token usage
9. Set timeouts - Prevent hanging requests
10. Use tool use properly - Return tool_result in follow-up message

Never Do ❌

1. Never expose API key in client-side code
2. Never skip max_tokens - API will error without it
3. Never ignore stop_reason - Check for tool_use, end_turn, max_tokens
4. Never assume single content block - content is an array
5. Never use outdated models - Pin to specific version
6. Never skip error handling - API calls can fail
7. Never mix message roles - Alternate user/assistant correctly
8. Never ignore rate limits - Implement exponential backoff
9. Never store API keys in logs or databases
10. Never skip input validation - Prevent injection attacks

---

Top 3 Errors (Prevent 80% of Issues)

Error #1: Rate Limit 429

Symptom: 429 Too Many Requests: Number of request tokens has exceeded your per-minute rate limit

Solution: Implement exponential backoff with retry-after header

async function handleRateLimit(requestFn, maxRetries = 3) {
  for (let attempt = 0; attempt < maxRetries; attempt++) {
    try {
      return await requestFn();
    } catch (error) {
      if (error.status === 429) {
        const retryAfter = error.response?.headers?.['retry-after'];
        const delay = retryAfter ? parseInt(retryAfter) * 1000 : 1000 * Math.pow(2, attempt);
        await new Promise(resolve => setTimeout(resolve, delay));
      } else {
        throw error;
      }
    }
  }
}

Prevention: Monitor rate limit headers, upgrade tier, implement backoff

---

Error #2: Prompt Caching Not Activating

Symptom: High costs despite cache_control blocks, cache_read_input_tokens: 0

Solution: Place cache_control on LAST block with >= 1024 tokens

// ❌ Wrong - cache_control not at end
{
  type: 'text',
  text: DOCUMENT,
  cache_control: { type: 'ephemeral' },  // Wrong position
},
{
  type: 'text',
  text: 'Additional text',
}

// ✅ Correct - cache_control at end
{
  type: 'text',
  text: DOCUMENT + '\n\nAdditional text',
  cache_control: { type: 'ephemeral' },  // Correct position
}

Prevention: Ensure content >= 1024 tokens, keep cached content identical, monitor usage

Load references/prompt-caching-guide.md for complete caching strategy.

---

Error #3: Tool Use Response Format Errors

Symptom: invalid_request_error: tools[0].input_schema is invalid

Solution: Valid tool schema with proper JSON Schema

// ✅ Valid tool schema
{
  name: 'get_weather',
  description: 'Get current weather',
  input_schema: {
    type: 'object',           // Must be 'object'
    properties: {
      location: {
        type: 'string',       // Valid JSON Schema types
        description: 'City'   // Optional but recommended
      }
    },
    required: ['location']    // List required fields
  }
}

// ✅ Valid tool result
{
  type: 'tool_result',
  tool_use_id: block.id,      // Must match tool_use id
  content: JSON.stringify(result)  // Convert to string
}

Prevention: Validate schemas, match tool_use_id exactly, stringify results

Load references/tool-use-patterns.md + references/top-errors.md for all 12 errors.

---

Common Use Cases (Quick Patterns)

Streaming Responses

const stream = await client.messages.stream({
  model: 'claude-sonnet-4-5-20250929',
  max_tokens: 1024,
  messages: [{ role: 'user', content: 'Write a story.' }],
});

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

Load: templates/streaming-chat.ts

---

Prompt Caching (90% Cost Savings)

const message = await client.messages.create({
  model: 'claude-sonnet-4-5-20250929',
  max_tokens: 1024,
  system: [
    {
      type: 'text',
      text: 'Long system prompt...',
      cache_control: { type: 'ephemeral' },
    },
  ],
  messages: [{ role: 'user', content: 'Question?' }],
});

Cache lasts 5 minutes, 90% savings on cached tokens

Load: references/prompt-caching-guide.md + templates/prompt-caching.ts

---

Tool Use (Function Calling)

const message = await client.messages.create({
  model: 'claude-sonnet-4-5-20250929',
  max_tokens: 1024,
  tools: [{
    name: 'get_weather',
    description: 'Get weather for a location',
    input_schema: {
      type: 'object',
      properties: { location: { type: 'string' } },
      required: ['location'],
    },
  }],
  messages: [{ role: 'user', content: 'Weather in SF?' }],
});

if (message.stop_reason === 'tool_use') {
  const toolUse = message.content.find(b => b.type === 'tool_use');
  // Execute tool and send result back...
}

Load: references/tool-use-patterns.md + templates/tool-use-basic.ts

---

Vision (Image Understanding)

const message = await client.messages.create({
  model: 'claude-sonnet-4-5-20250929',
  max_tokens: 1024,
  messages: [{
    role: 'user',
    content: [
      {
        type: 'image',
        source: {
          type: 'base64',
          media_type: 'image/jpeg',
          data: base64Image,
        },
      },
      { type: 'text', text: 'What is in this image?' },
    ],
  }],
});

Supports: JPEG, PNG, WebP, GIF (max 5MB)

Load: references/vision-capabilities.md + templates/vision-image.ts

---

Extended Thinking Mode

const message = await client.messages.create({
  model: 'claude-sonnet-4-5-20250929',
  max_tokens: 4096,
  thinking: {
    type: 'enabled',
    budget_tokens: 2000,
  },
  messages: [{ role: 'user', content: 'Solve complex problem...' }],
});

const thinking = message.content.find(b => b.type === 'thinking')?.thinking;
const answer = message.content.find(b => b.type === 'text')?.text;

Load: templates/extended-thinking.ts

---

Model Versions (Current)

Latest models:

• claude-sonnet-4-5-20250929 - Recommended (best performance)
• claude-sonnet-4-20250514 - Stable version
• claude-3-7-sonnet-20250219 - Previous generation
• claude-3-5-sonnet-20241022 - Legacy

Always pin to specific version (not -latest suffix)

---

When to Load References

Load references/setup-guide.md when:

• First-time Claude API user needing complete setup walkthrough
• Setting up Node.js or Cloudflare Workers from scratch
• Need production deployment checklist and platform-specific tips
• Troubleshooting basic setup issues (API keys, authentication, environment)

Load references/prompt-caching-guide.md when:

• Want to reduce costs by 90% on repeated content
• Using large system prompts, documents, or codebases in context
• Need detailed caching strategy with benchmarks and cost calculations
• Optimizing multi-turn conversations with conversation history caching
• Troubleshooting cache activation issues or cache misses

Load references/tool-use-patterns.md when:

• Implementing function calling and tool definitions
• Need tool execution loop patterns and multi-turn tool use
• Want Zod validation examples for type-safe tools
• Troubleshooting tool use response format errors

Load references/vision-capabilities.md when:

• Processing images with Claude (OCR, analysis, description)
• Need image format requirements and validation logic
• Want vision use cases and best practices
• Combining vision with tools or prompt caching

Load references/api-reference.md when:

• Need complete API parameter reference and request/response schemas
• Looking up specific fields or content block types
• Want endpoint details, authentication headers, or model IDs
• Need streaming event types and SSE format details

Load references/top-errors.md when:

• Encountering API errors (all 12 documented errors with solutions)
• Need error code reference (400, 401, 429, 529, etc.)
• Want comprehensive prevention strategies
• Implementing robust error handling and retry logic

Load references/rate-limits.md when:

• Planning high-volume usage and scaling strategy
• Encountering 429 rate limit errors repeatedly
• Need tier limits information and upgrade path
• Optimizing request patterns to stay within limits

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Using Bundled Resources

References (references/)

• setup-guide.md - Complete setup (Node.js + Cloudflare Workers + Next.js)
• api-reference.md - Complete API parameter reference + schemas
• prompt-caching-guide.md - 90% cost savings guide + benchmarks
• tool-use-patterns.md - Function calling patterns + Zod examples
• vision-capabilities.md - Image understanding guide + validation
• top-errors.md - All 12 errors with solutions + prevention
• rate-limits.md - Limits by tier + best practices

Templates (templates/)

• basic-chat.ts - Simple chat example
• streaming-chat.ts - Streaming implementation with SSE
• prompt-caching.ts - Caching example with benchmarks
• tool-use-basic.ts - Basic tool use pattern
• tool-use-advanced.ts - Advanced multi-turn tool patterns
• vision-image.ts - Vision example with validation
• extended-thinking.ts - Extended thinking mode
• error-handling.ts - Complete error handling
• nodejs-example.ts - Complete Node.js app
• cloudflare-worker.ts - Complete CF Worker
• nextjs-api-route.ts - Next.js API route
• package.json - Dependencies

---

Platform Integration

Node.js: bun add @anthropic-ai/sdk → Use templates: nodejs-example.ts, basic-chat.ts

Cloudflare Workers: Use fetch API → Use templates: cloudflare-worker.ts

Next.js: bun add @anthropic-ai/sdk → Use templates: nextjs-api-route.ts

---

Production Checklist

• API key stored securely (environment variable)
• Error handling implemented (401, 429, 400, 529)
• Rate limiting with exponential backoff
• Prompt caching enabled for repeated content
• Streaming for long responses
• Input validation + output sanitization
• Monitoring and cost tracking
• Timeouts configured
• Model version pinned
• max_tokens set appropriately

---

Related Skills

• openai-api - OpenAI Chat Completions for comparison
• claude-agent-sdk - Higher-level Claude SDK
• ai-sdk-core - Vercel AI SDK (supports Claude)

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Official Documentation

• Messages API: https://docs.anthropic.com/en/api/messages
• Prompt Caching: https://docs.anthropic.com/en/docs/build-with-claude/prompt-caching
• Tool Use: https://docs.anthropic.com/en/docs/build-with-claude/tool-use
• Vision: https://docs.anthropic.com/en/docs/build-with-claude/vision
• Extended Thinking: https://docs.anthropic.com/en/docs/build-with-claude/extended-thinking

---

Questions? Issues?

1. Check references/top-errors.md for error solutions
2. Review references/setup-guide.md for complete setup
3. See references/prompt-caching-guide.md for cost optimization
4. Load templates from templates/ for working examples