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Eight polls is not a budget. Each poll costs whatever the driver's idle wait happens to take, so the same launch cleared the gate on one device and exhausted it on another: across 80 runs of one app, the gate reported "app never reached foreground" on 38 of 40 Android 14 runs and 0 of 40 Android 16 runs, and it was wrong every time. On API 34 settleForForeground returned in ~100ms, so the eight polls gave up 1.2s into a launch whose window drew at ~1.9s; on API 36 the same eight polls spanned 3s and covered it. The Android 14 runs then spent their first step on the launch animation instead of the app, which is the one-step offset that came out of that campaign looking like a platform difference. The gate now polls for a fixed 15s at a 250ms floor, so its verdict is the same duration on every device, and a verdict of "not in front" ends the run instead of warning and carrying on: a run that never got its app on screen holds no evidence about the app, and the trace records why at step 0.
sanderling
Autonomous property-based testing for mobile and web apps.
You write rules that must always hold about your app. sanderling explores the app on its own for minutes or hours, performing thousands of taps, swipes, and inputs, and records every step where a rule breaks. No scripted test paths. One TypeScript spec runs against Android, iOS, and web builds of the same app.
import { extract, always } from "@sanderling/spec";
import { defaultActions } from "@sanderling/spec/defaults";
import { noUncaughtExceptions } from "@sanderling/spec/defaults/properties";
const balance = extract("balance", s =>
parseInt(s.ax.find({ testTag: "Balance" })?.text ?? "0", 10));
export const properties = {
noUncaughtExceptions,
balanceNeverNegative: always(() => balance.current >= 0),
};
export const actionsRoot = defaultActions;
Every run produces a trace: one JSON line and one screenshot per step. sanderling replay opens it in a web UI for stepping through actions, screenshots, property timelines, and violations.
Alpha. Android, iOS, and web (Chrome driver only). Full scope in the v0.1.0 roadmap.
Docs
- Introduction: what property-based testing is and how sanderling works
- Case study: Folio: sanderling finding a real bug in a mobile app
- Getting started: install the CLI and run it against Folio
- Spec language reference
- Examples: folio (KMP, Android/iOS/web), folio-web (React + Vite)
- Architecture for contributors
sanderling, a wading bird that probes the shoreline for bugs that lie beneath.