Functions — browser-sandbox

defineFunctions is the only way a capability enters the sandbox. Each entry is the real function, which runs in your page, plus its policy.

import { defineFunctions, fromObject, describe } from '@faustine-ai/browser-sandbox';

const functions = defineFunctions({
  'orders.list':   (input) => api.orders.list(input),                   // plain
  'orders.cancel': { call: (input) => api.orders.cancel(input),          // with a policy
                     approval: true, description: 'Cancels an order' },
  'util.sum':      { call: (...nums) => nums.reduce((a, b) => a + b, 0),
                     positional: true },                                 // code calls sum(1, 2, 3)
  help:            (topic) => docs.lookup(topic),
});

A dotted name becomes a path in the tree: sdk.orders.list(input). Segments are identifiers; __proto__, constructor and prototype are refused.
A function receives one argument (whatever the code passed) unless positional: true, in which case it receives the arguments spread.
A name can be both a leaf and a branch (help and help.topics): it is a function with properties.
approval: true holds the call for a human (see Approval).
description and input (a JSON Schema) are used only by describe.

fromObject builds the map from an existing object by listing names, never by enumerating the object:

const functions = fromObject(client, {
  names: ['orders.list', 'orders.cancel', 'customers.get'],
  approval: (name) => name.endsWith('.cancel'),
  descriptions: (name) => catalog[name]?.summary,
  extra: { help: (topic) => docs.lookup(topic) },
});

Paths are resolved at call time with the right this, so lazily built clients work; a missing path throws UnknownFunction into the code.

For the system prompt, describe prints the names exactly as the code will use them:

describe(functions);                       // - sdk.orders.list(input)
                                           // - sdk.orders.cancel(input) (requires approval) — Cancels an order
describe(functions, { format: 'ts' });     // declare const sdk: { orders: { list(input?: unknown): Promise<unknown>; … } };