Anthropic Claude Tool Use Pattern

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

const anthropic = new Anthropic();

const tools: Anthropic.Tool[] = [
  {
    name: 'get_weather',
    description: 'Get current weather for a location',
    input_schema: {
      type: 'object',
      properties: {
        location: { type: 'string', description: 'City name' },
        unit: { type: 'string', enum: ['celsius', 'fahrenheit'] },
      },
      required: ['location'],
    },
  },
];

async function executeTool(name: string, input: Record<string, unknown>): Promise<string> {
  switch (name) {
    case 'get_weather':
      return JSON.stringify({ temp: 22, condition: 'sunny', location: input.location });
    default:
      throw new Error(`Unknown tool: ${name}`);
  }
}

async function agentChat(userMessage: string): Promise<string> {
  const messages: Anthropic.MessageParam[] = [
    { role: 'user', content: userMessage },
  ];

  while (true) {
    const response = await anthropic.messages.create({
      model: 'claude-sonnet-4-20250514',
      max_tokens: 1024,
      tools,
      messages,
    });

    if (response.stop_reason === 'end_turn') {
      const textBlock = response.content.find((b) => b.type === 'text');
      return textBlock?.text ?? '';
    }

    messages.push({ role: 'assistant', content: response.content });
    const toolResults: Anthropic.ToolResultBlockParam[] = [];

    for (const block of response.content) {
      if (block.type === 'tool_use') {
        const result = await executeTool(block.name, block.input as Record<string, unknown>);
        toolResults.push({ type: 'tool_result', tool_use_id: block.id, content: result });
      }
    }
    messages.push({ role: 'user', content: toolResults });
  }
}

const answer = await agentChat('What is the weather in Tokyo?');