Anthropic TypeScript SDK tool use loop, tested with a mocked fetch: working versions (tested 2026-10-15)

{
  "name": "anthropic-sdk-mocked-tool-use",
  "private": true,
  "type": "module",
  "scripts": {
    "test": "node --import tsx --test test/*.test.ts"
  },
  "dependencies": {
    "@anthropic-ai/sdk": "0.134.0",
    "zod": "4.6.7"
  },
  "devDependencies": {
    "tsx": "4.23.15",
    "@types/node": "24.19.2"
  }
}

import Anthropic from "@anthropic-ai/sdk";
import { z } from "zod";

const LookupInput = z.object({ sku: z.string().regex(/^[A-Z]+-\d+$/) });
export const tools: Anthropic.Tool[] = [{
  name: "lookup_stock",
  description: "Units in stock for a SKU",
  input_schema: { type: "object", properties: { sku: { type: "string" } }, required: ["sku"] },
}];

export async function ask(client: Anthropic, question: string, stock: (sku: string) => number) {
  const messages: Anthropic.MessageParam[] = [{ role: "user", content: question }];
  for (let i = 0; i < 5; i++) {
    const res = await client.messages.create({ model: "claude-sonnet-4-5", max_tokens: 512, messages, tools });
    messages.push({ role: "assistant", content: res.content });
    if (res.stop_reason !== "tool_use") return res.content.filter((b) => b.type === "text").map((b) => b.text).join("");
    const results: Anthropic.ToolResultBlockParam[] = [];
    for (const block of res.content) {
      if (block.type !== "tool_use") continue;
      const p = LookupInput.safeParse(block.input);
      results.push(p.success
        ? { type: "tool_result", tool_use_id: block.id, content: String(stock(p.data.sku)) }
        : { type: "tool_result", tool_use_id: block.id, content: "invalid sku", is_error: true });
    }
    messages.push({ role: "user", content: results });
  }
  throw new Error("too many turns");
}

import { test } from "node:test";
import assert from "node:assert/strict";
import Anthropic from "@anthropic-ai/sdk";
import { ask } from "../src/agent.ts";

const msg = (content: object[], stop_reason: string) => ({ id: "m", type: "message", role: "assistant", model: "claude-sonnet-4-5", content, stop_reason, stop_sequence: null, usage: { input_tokens: 1, output_tokens: 1 } });

function fake(replies: object[], seen: any[]) {
  const fetch = async (_u: any, init: any) => {
    seen.push(JSON.parse(init.body));
    return new Response(JSON.stringify(replies.shift()), { status: 200, headers: { "content-type": "application/json" } });
  };
  return new Anthropic({ apiKey: "test", baseURL: "http://mock.local", fetch: fetch as any, maxRetries: 0 });
}

test("runs the tool and returns the final text", async () => {
  const seen: any[] = [];
  const c = fake([
    msg([{ type: "tool_use", id: "tu1", name: "lookup_stock", input: { sku: "MUG-1" } }], "tool_use"),
    msg([{ type: "text", text: "There are 7 mugs." }], "end_turn"),
  ], seen);
  assert.equal(await ask(c, "How many mugs?", () => 7), "There are 7 mugs.");
  assert.deepEqual(seen[1].messages[2].content[0], { type: "tool_result", tool_use_id: "tu1", content: "7" });
});

test("invalid tool input is returned as is_error", async () => {
  const seen: any[] = [];
  const c = fake([
    msg([{ type: "tool_use", id: "tu1", name: "lookup_stock", input: { sku: "bad" } }], "tool_use"),
    msg([{ type: "text", text: "Need a valid SKU." }], "end_turn"),
  ], seen);
  await ask(c, "?", () => 0);
  assert.equal(seen[1].messages[2].content[0].is_error, true);
});