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anthropic-ts

purpose

Configuring and calling Anthropic Claude models from TypeScript using the official SDK. Covers direct API usage, tool use, streaming, vision, and integration patterns for bots.

rules

1. Use the official @anthropic-ai/sdk package. npm install @anthropic-ai/sdk. This is the only supported TypeScript/JavaScript SDK. docs.anthropic.com/en/docs/build-with-claude/getting-started

2. The SDK reads ANTHROPIC_API_KEY from the environment by default. You can also pass it explicitly: new Anthropic({ apiKey: process.env.ANTHROPIC_API_KEY }). docs.anthropic.com/en/api/client-sdks

3. Use the Messages API, not the legacy Completions API. All Claude models use client.messages.create(). The legacy client.completions.create() is deprecated. docs.anthropic.com/en/api/messages

4. Always specify max_tokens. Unlike OpenAI, Anthropic requires max_tokens on every request. There is no default. Omitting it throws an error. docs.anthropic.com/en/api/messages

5. System messages go in the system parameter, not in messages. Claude uses a top-level system string, not a { role: 'system', content: '...' } message. Putting system content in messages with role 'system' will error. docs.anthropic.com/en/docs/build-with-claude/prompt-caching

6. Use model IDs like claude-opus-4-6, claude-sonnet-4-6, claude-haiku-4-5-20251001. Check the model names page for the latest IDs. Aliases like claude-3-5-sonnet-latest are available but less predictable. docs.anthropic.com/en/docs/about-claude/models

7. Tool use follows the tools + tool_use / tool_result pattern. Define tools in the request, receive tool_use content blocks in the response, execute them, then send tool_result blocks back. docs.anthropic.com/en/docs/build-with-claude/tool-use

8. Streaming uses client.messages.stream(). Returns a MessageStream with event-driven or async iteration. Use stream.on('text', (text) => ...) for incremental text or for await (const event of stream) for full events. docs.anthropic.com/en/api/messages-streaming

9. Handle rate limits with exponential backoff. The SDK throws RateLimitError (HTTP 429). Implement retry logic with backoff, or use the SDK's built-in maxRetries option (defaults to 2). docs.anthropic.com/en/api/rate-limits

10. For Bedrock-hosted Claude, use the Bedrock expert instead. This expert covers direct Anthropic API access. If you're accessing Claude through AWS Bedrock, see bedrock-ts.md — the SDK and auth are completely different.

patterns

Basic chat completion

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

const client = new Anthropic({
  apiKey: process.env.ANTHROPIC_API_KEY,
  maxRetries: 3,
  timeout: 30000,
});

const response = await client.messages.create({
  model: 'claude-sonnet-4-6',
  max_tokens: 1024,
  system: 'You are a helpful assistant in a Slack workspace.',
  messages: [
    { role: 'user', content: userMessage },
  ],
});

const reply = response.content[0].type === 'text'
  ? response.content[0].text
  : '';

Streaming

const stream = client.messages.stream({
  model: 'claude-sonnet-4-6',
  max_tokens: 1024,
  system: 'You are a helpful assistant.',
  messages: [{ role: 'user', content: userMessage }],
});

stream.on('text', (text) => {
  process.stdout.write(text);
});

const finalMessage = await stream.finalMessage();

Tool use (function calling)

const response = await client.messages.create({
  model: 'claude-sonnet-4-6',
  max_tokens: 1024,
  tools: [{
    name: 'get_weather',
    description: 'Get current weather for a location',
    input_schema: {
      type: 'object',
      properties: {
        location: { type: 'string', description: 'City name' },
      },
      required: ['location'],
    },
  }],
  messages: [{ role: 'user', content: 'What is the weather in Seattle?' }],
});

// Check for tool use in the response
const toolUseBlock = response.content.find((b) => b.type === 'tool_use');
if (toolUseBlock && toolUseBlock.type === 'tool_use') {
  const result = await executeFunction(toolUseBlock.name, toolUseBlock.input);

  // Send tool result back
  const followUp = await client.messages.create({
    model: 'claude-sonnet-4-6',
    max_tokens: 1024,
    tools: [/* same tools */],
    messages: [
      { role: 'user', content: 'What is the weather in Seattle?' },
      { role: 'assistant', content: response.content },
      {
        role: 'user',
        content: [{
          type: 'tool_result',
          tool_use_id: toolUseBlock.id,
          content: JSON.stringify(result),
        }],
      },
    ],
  });
}

Multi-turn conversation

const conversationHistory: Anthropic.MessageParam[] = [];

async function chat(userInput: string): Promise<string> {
  conversationHistory.push({ role: 'user', content: userInput });

  const response = await client.messages.create({
    model: 'claude-sonnet-4-6',
    max_tokens: 1024,
    system: 'You are a helpful Slack bot.',
    messages: conversationHistory,
  });

  const assistantText = response.content
    .filter((b) => b.type === 'text')
    .map((b) => b.text)
    .join('');

  conversationHistory.push({ role: 'assistant', content: response.content });
  return assistantText;
}

pitfalls

• Putting system message in messages array. Claude requires system as a top-level parameter. { role: 'system', content: '...' } in messages will error.

• Forgetting max_tokens. Anthropic requires this on every request. Unlike OpenAI, there's no default.

• Response content is an array, not a string. response.content is ContentBlock[]. Each block has a type (text, tool_use). Don't treat it as a plain string.

• Tool result must reference tool_use_id. When sending tool_result blocks, you must include the exact tool_use_id from the model's response. Mismatches cause errors.

• stop_reason vs finish_reason. Anthropic uses stop_reason (values: end_turn, max_tokens, stop_sequence, tool_use). Don't confuse with OpenAI's finish_reason.

• Model ID format differs from OpenAI. Anthropic uses IDs like claude-sonnet-4-6, not gpt-4o. Check the models page for current IDs.

references

• Anthropic TypeScript SDK — GitHub

• Messages API Reference

• Tool Use Guide

• Streaming Guide

• Claude Models

instructions

This expert covers direct Anthropic API usage from TypeScript. Use it when the developer is calling Claude models via the Anthropic API (not through Bedrock). For Bedrock-hosted Claude, see bedrock-ts.md instead.

Pair with: bedrock-ts.md (if also using Bedrock), openai-azure-openai-ts.md (if mixing providers), ../security/secrets-ts.md (API key management).

research

Deep Research prompt:

"Write a micro expert on using the @anthropic-ai/sdk TypeScript package. Cover: client initialization, Messages API, system messages, max_tokens requirement, streaming with MessageStream, tool use (defining tools, handling tool_use blocks, sending tool_result), multi-turn conversations, vision/image input, prompt caching, rate limits and retries, error handling, and model selection guidance."