Files
sanderling/pkg/spec
pj d88200f398 fix(folio): add parseTypedAmount helper matching app's parseCents
Raw user input like "50" must become 5000 cents, not 50. The existing
parseDollarCents helper strips non-digits and so reads "50" as 50 cents,
which is correct for formatted balance text but off by 100x for raw
input from the amount field.
2026-05-31 18:56:30 +05:30
..

@sanderling/spec

TypeScript spec API for sanderling, a property-based UI fuzzer for mobile and web apps.

Spec authors write properties (what the app must always or eventually do), extractors (structured state from the UI), and action generators (what sanderling is allowed to do). The sanderling CLI evaluates the spec in a loop against a running app.

Install

npm install --save-dev @sanderling/spec

Usage

import { extract, always, eventually, actions, weighted, taps, swipes, InputText, Tap } from "@sanderling/spec";

const loggedIn = extract((s) => !!s.ax.find("id:home-tab-bar"));
const balance = extract<number>((s) => (s.snapshots.balance as number) ?? 0);
const emailField = extract((s) => s.ax.find("id:email-field"));
const submitButton = extract((s) => s.ax.find("id:sign-in-button"));

export const properties = {
  balanceNeverNegative: always(() => balance.current >= 0),
  loginSucceeds: eventually(() => loggedIn.current).within(30, "seconds"),
};

const doLogin = actions(() => {
  if (loggedIn.current) return [];
  const email = emailField.current;
  const submit = submitButton.current;
  if (!email || !submit) return [];
  return [InputText({ into: email, text: "[email protected]" }), Tap({ on: submit })];
});

export const actionsRoot = weighted(
  [50, doLogin],
  [10, taps],
  [2,  swipes],
);

Setup actions

Some action generators are not fuzz targets but preconditions: they drive the app from a fresh state into the surface you actually want to fuzz (login, onboarding, permission grants, seed data). Export them as setup instead of mixing them into actionsRoot. The runner tries setup first; if it yields no action, it falls through to actionsRoot. State regressing back across the precondition (e.g. logout under fuzz) automatically re-engages setup.

const login = actions(() => {
  if (loggedIn.current) return [];
  return [InputText({ into: emailField.current!, text: "[email protected]" }), Tap({ on: submitButton.current! })];
});

export const setup = login;
export const actionsRoot = weighted([60, browse], [40, edit]);

(globalThis as { setup?: unknown }).setup = setup;

Setup is just an ActionGenerator; compose with actions, weighted, or whenRoute exactly like the main pool.

Works identically across Android, iOS, and web targets.