Validate a tool call before it runs

WHAT YOU WILL BUILD

One permitted read operation and two rejected requests with no side effects.

A model may propose a tool name and arguments. The application decides whether that proposal is allowed, validates it, performs the operation, and returns its result. A function schema helps describe the interface but does not replace those checks.

FOLLOW ALONG

Work through the example.

1. 01

   Inspect the permitted operation

   lookup_policy accepts one category: returns or delivery. Read the dispatcher and identify the checks that happen before the tool function is called.

2. 02

   Run the rejected requests

   The unknown tool and the extra admin argument both throw. The calls counter remains zero, demonstrating that rejected input never reaches this tool body.

3. 03

   Run the permitted lookup

   The final request returns the example policy and increases calls to one. Keep a structured result in a real application so the controller can distinguish success from failure.

4. 04

   Add another negative case

   Test a missing category, an array, and a category outside the allowlist. Verify the call count does not increase. Add authentication and per-resource authorization before adapting this pattern to user data.

THE COMPLETE LAB

Run it locally.

Run this command from the folder containing your downloaded file:

node agent-tool-validation.mjs

// SaveMyToken local lab. Run with Node.js 22 or newer.
import assert from "node:assert/strict";

let calls = 0;
const tools = new Map([["lookup_policy", ({ category }) => {
  calls++;
  return category === "returns" ? "Unused items: 30 days." : "Delivery: 3 to 5 working days.";
}]]);
function dispatch(name, args) {
  if (!tools.has(name)) throw new Error("Unknown tool");
  if (!args || typeof args !== "object" || Array.isArray(args)
    || Object.keys(args).length !== 1
    || !["returns", "delivery"].includes(args.category)) throw new Error("Invalid arguments");
  return tools.get(name)(args);
}
assert.throws(() => dispatch("delete_data", {}), /Unknown tool/);
assert.throws(() => dispatch("lookup_policy", { category: "returns", admin: true }), /Invalid arguments/);
assert.equal(calls, 0);
console.log(dispatch("lookup_policy", { category: "returns" }));
console.log("Executed tool calls:", calls);
assert.equal(calls, 1);

Expected output for the unchanged example

Unused items: 30 days.
Executed tool calls: 1

The assertions also check the baseline behavior. After changing an input, predict the result and update the relevant assertion.

EXPLAIN WHAT YOU LEARNED

Interview practice

Try answering aloud before opening the reference answer. These are original learning questions, not a record of any employer's interviews.