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Build an agent with tool use

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Tool use lets Claude call functions you define and act on the results. You describe each tool with a JSON schema, and when Claude decides to use one it returns a tool_use request instead of a final answer. You run the tool, send the result back, and repeat until Claude is done. This loop is the core of an agent. Set ANTHROPIC_API_KEY before running.

import Anthropic from "npm:@anthropic-ai/sdk";

const client = new Anthropic();
Describe the tool with a JSON schema. A clear description and prescriptive "when to call it" wording help Claude decide to use it at the right time.
const tools: Anthropic.Tool[] = [
  {
    name: "get_weather",
    description:
      "Get the current weather for a city. Call this whenever the user asks " +
      "about weather conditions.",
    input_schema: {
      type: "object",
      properties: {
        city: { type: "string", description: "City name, e.g. Tokyo" },
      },
      required: ["city"],
    },
  },
];
The actual implementation. In a real app this would call a weather API; here it returns canned data so the example runs without extra setup.
function getWeather(city: string): string {
  const data: Record<string, string> = {
    Tokyo: "18°C, clear",
    Paris: "12°C, light rain",
    Oslo: "4°C, snow",
  };
  return data[city] ?? `No data for ${city}`;
}
The API is stateless, so we carry the full conversation in `messages`.
const messages: Anthropic.MessageParam[] = [
  { role: "user", content: "Compare the weather in Tokyo and Oslo right now." },
];
Agent loop: keep calling the model until it stops requesting tools.
while (true) {
  const response = await client.messages.create({
    model: "claude-opus-4-8",
    max_tokens: 1024,
    tools,
    messages,
  });
No tool requested means Claude produced its final answer.
  if (response.stop_reason !== "tool_use") {
    for (const block of response.content) {
      if (block.type === "text") console.log(block.text);
    }
    break;
  }
Record the assistant turn; it holds the tool_use blocks we must answer.
  messages.push({ role: "assistant", content: response.content });
Run every requested tool and gather the results.
  const toolResults: Anthropic.ToolResultBlockParam[] = [];
  for (const block of response.content) {
    if (block.type === "tool_use" && block.name === "get_weather") {
      const { city } = block.input as { city: string };
      toolResults.push({
        type: "tool_result",
        tool_use_id: block.id,
        content: getWeather(city),
      });
    }
  }
Feed the results back so Claude can continue from where it left off.
  messages.push({ role: "user", content: toolResults });
}

Run this example locally using the Deno CLI:

deno run -N -E https://docs.deno.com/examples/scripts/anthropic_tool_use.ts

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