源码解读：手写一个最小 ReAct Agent

循环 {
  向模型发送: system(规则+工具) + 历史(thought/action/observation)
  模型返回: 下一步的 thought 和 action(调哪个工具、传什么参数)
  如果模型说"完成了" → 结束
  否则: 执行 action(工具),把结果作为 observation 加进历史
  继续
}

const tools = [
  {
    type: "function" as const,
    function: {
      name: "search",
      description: "Search the web for factual information. Use when you don't know something.",
      parameters: {
        type: "object",
        properties: { query: { type: "string" } },
        required: ["query"],
      },
    },
  },
  {
    type: "function" as const,
    function: {
      name: "calculate",
      description: "Evaluate a math expression, e.g. '12 * (3+4)'. Use for exact arithmetic.",
      parameters: {
        type: "object",
        properties: { expr: { type: "string" } },
        required: ["expr"],
      },
    },
  },
];

// 工具的实际实现
const handlers: Record<string, (args: any) => Promise<string>> = {
  search: async ({ query }) => `[模拟搜索结果] 关于 "${query}" 的信息...`,
  calculate: async ({ expr }) => String(Function(`return (${expr})`)()),
};

import Anthropic from "@anthropic-ai/sdk";
const client = new Anthropic();

async function reactAgent(userMessage: string, maxSteps = 10) {
  const messages: any[] = [{ role: "user", content: userMessage }];

  for (let step = 0; step < maxSteps; step++) {
    // 1) 让模型想下一步
    const response = await client.messages.create({
      model: "claude-sonnet-5",
      max_tokens: 1024,
      system: "你是一个会使用工具的助手。一步步推理。需要信息就 search,需要计算就 calculate。完成就给出最终答案。",
      tools,
      messages,
    });

    // 2) 把模型这轮的输出加入历史
    messages.push({ role: "assistant", content: response.content });

    // 3) 模型没调工具 → 它给出最终答案,结束
    if (response.stop_reason === "end_turn") {
      const text = response.content.find((c: any) => c.type === "text");
      return text?.text ?? "(no answer)";
    }

    // 4) 模型调了工具 → 执行,把结果作为 observation 回灌
    if (response.stop_reason === "tool_use") {
      const toolUses = response.content.filter((c: any) => c.type === "tool_use");
      const toolResults = [];
      for (const tu of toolUses) {
        const handler = handlers[tu.name];
        const result = handler
          ? await handler(tu.input).catch((e) => `tool error: ${e.message}`)
          : `unknown tool: ${tu.name}`;
        toolResults.push({ type: "tool_result", tool_use_id: tu.id, content: result });
      }
      messages.push({ role: "user", content: toolResults });
    }
  }

  return "(达到最大步数,未完成)";
}

await reactAgent("2024 年中国 GDP 总量是多少万亿美元?换算成人民币是多少(按 7.2 汇率)?");
// 模型会: search(GDP) → observation → calculate(换算) → observation → 给最终答案