The loop · Your first agent loop

why does the loop check stop_reason?

Every reply comes with a reason the model stopped talking. If it’s "tool_use", it wants you to run a tool and hand back the result. Anything else means it’s finished.

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

import { tools, runTool } from "./tools";

const client = new Anthropic();

type Msg = Anthropic.MessageParam;

export async function agent(task: string) {

const messages: Msg[] = [{ role: "user", content: task }];

// the loop: think → act → observe → repeat

while (true) {

const res = await client.messages.create({

model: "claude-sonnet-5-5",

max_tokens: 1024,

tools,

messages,

});

messages.push({ role: "assistant", content: res.content });

if (res.stop_reason !== "tool_use") {

return res; // the model is done talking

}

const results = await Promise.all(

res.content

.filter((b) => b.type === "tool_use")

.map(runTool),

);

messages.push({ role: "user", content: results });

}

TypeScriptUTF-8● lines 12–32: the loop

OUTPUT

npx tsx agent.ts "Weather in Chicago?"

turn 1 stop_reason="tool_use" → get_weather

turn 2 stop_reason="end_turn"

41°F and overcast in Chicago.

run it

OUTPUT

npx tsx agent.ts "Weather in Chicago?"

→ tool_use get_weather({ city: "Chicago" })

← tool_result { temp_f: 41, sky: "overcast" }

→ end_turn "41°F and overcast in Chicago."

✓ 2 turns · 1 tool call · 1.8s

Two turns: one to ask for the weather, one to answer. Now break it on purpose. Delete line 31 and run it again.