Tool-Use Patterns

The Agentic Loop

You define tools with a name, description, and JSON schema. The model decides to call one, you execute it, you feed the result back, the model continues. Loop until it stops asking for tools.

import Anthropic from "@anthropic-ai/sdk"; const client = new Anthropic(); const tools: Anthropic.Tool[] = [ { name: "get_weather", description: "Return current temperature in Fahrenheit for a city.", input_schema: { type: "object", properties: { city: { type: "string" } }, required: ["city"], }, }, ]; function executeTool(name: string, input: any): string { if (name === "get_weather") { // real implementation would call an API return `72F and sunny in ${input.city}`; } throw new Error(`unknown tool ${name}`); }Tool definitions are JSON Schema. The model only sees what you describe — keep descriptions precise.

export async function runAgent(question: string) { const messages: Anthropic.MessageParam[] = [ { role: "user", content: question }, ]; for (let turn = 0; turn < 5; turn++) { const res = await client.messages.create({ model: "claude-opus-4-7", max_tokens: 1024, tools, messages, }); messages.push({ role: "assistant", content: res.content }); if (res.stop_reason !== "tool_use") { const text = res.content.find((b) => b.type === "text"); return text?.type === "text" ? text.text : ""; } const toolUses = res.content.filter((b) => b.type === "tool_use"); messages.push({ role: "user", content: toolUses.map((t) => ({ type: "tool_result" as const, tool_use_id: t.id, content: executeTool(t.name, t.input), })), }); } throw new Error("agent exceeded turn budget"); }Cap the loop at N turns. Pass tool_result blocks with the same tool_use_id so the model stitches them together.

The big idea: tools turn a chat model into a doer. Cap turns, parallelize when you can, and keep tool descriptions surgical.