Write a Tool extension

import { tool } from "@aexhq/brain";
import Client from "@vendor/orders";
import { z } from "zod";

const options = z.object({ endpoint: z.url() });
const input = z.object({ orderId: z.string() });
const output = z.object({ status: z.string() });

export const lookupOrder = tool(
  { description: "Look up an order.", options, input, output },
  (author) => {
    let client: Client;

    author.setup(async ({ options, signal }) => {
      client = new Client({ endpoint: options.endpoint });
      await client.ready({ signal });
    });

    author.run(async ({ orderId }, call) => client.lookup(orderId, { signal: call.signal }));
  },
);

Run npx brain build. The export name becomes the model-visible name, input and output cross the trust boundary through the Zod schemas, and reachable JavaScript dependencies are bundled into the Environment-side runtime. Native addons and install-time files are not portable and fail the build.

Application code chooses placement explicitly:

lookupOrder({ endpoint }).useIn(environment)

Tool handlers are async. Cancellation aborts call.signal; call.deadline, call.requestId, call.workspace, and call.progress() are available when the selected Environment supports them.