Merge branch 'spec-authoring-skills' into skills-setup-and-triage

2026-10-02 19:17:10 +00:00 · 2026-08-15 22:58:07 +05:30
commit ffc857f58f
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 ---
 name: sanderling-property-patterns
 description: Decide what a sanderling spec should assert. A catalogue of property shapes that are sound (cross-panel agreement, bounds on an effect, counting actions against effects, input and navigation invariants), each with the tempting unsound version beside it. Use when starting a spec, when adding a property to one, or when a property keeps convicting an app that behaved.
 ---
 # Choosing what to assert
 You have sanderling driving your app and now you have to say what must be true.
 This is the hard part, and it fails in two directions: you freeze, or you write
 six properties none of which can ever be false.
 One rule orders everything below. **Soundness outranks detection.** A property
 that convicts more often and is sometimes wrong is strictly worse than one that
 convicts less and is never wrong, because a false conviction costs someone a day
 and then costs the whole suite its credibility. When a property cannot establish
 what it needs, it declines.
 Each shape below gives the sound form and the tempting form next to it, because
 the tempting one is usually what gets written first. The examples are from the
 two specs in this repo: `replay-ui/sanderling/spec.ts` (sanderling fuzzing its
 own trace browser) and `examples/folio/sanderling/spec.ts` with
 `examples/folio/sanderling/predicates.ts` (a KMP finance app).
 Once you have written properties, run `sanderling-spec-review` over them. It
 audits what this file helps you build.
 ## 1. Two parts of the UI derive the same fact and must agree
 Reach for this first, always. If your app shows the same number in two places,
 or shows a thing and a count of that thing, or renders a list and a selection
 into that list, you have a property and you do not have to think about windows,
 calibration, or attribution to write it.
 It is the strongest shape available. It holds on any run against any data, so
 nothing needs recalibrating when a fixture changes; it needs no reasoning about
 which action caused what; and an app that drifted on one of the two paths cannot
 satisfy it. It is the backbone of `replay-ui/sanderling/spec.ts`, which states it
 three times over: the toolbar's step count against the number of rows the list
 renders, the toolbar's step against the step the screenshot panel built its URL
 from, and the tab badge's violation count against the number of rows the
 violations panel shows.
 ```ts
 const stepCountMatchesTheList = always(() => {
  const current = toolbar.current;
  const rows = stepRows.current;
  if (!current || current.stepCount === null || rows.length === 0) return true;
  return current.stepCount === rows.length;
 });
 ```
 **What goes wrong: reading the second value off the wrong element.** Scope each
 reading to the panel you mean, by name, not by position in the tree.
 ```ts
 // tempting: the first screenshot on the page
 s.ax.find({ "data-testid": "screenshot" })
 // sound: the before panel's screenshot
 s.ax.find([{ "data-testid": "state-before" }, { "data-testid": "screenshot" }])
 ```
 Both versions pass most of the time. The fuzzer put the before panel on another
 tab, which left the after panel's image first on the page, and the first version
 fired against a UI that was behaving correctly.
 **What else goes wrong: never getting both readings onto one step.** This
 shape's failure mode is vacuity, not false conviction, which makes it quiet. An
 undirected run over replay-ui went 40 steps without switching a single tab out
 of roughly 15 clickable elements, leaving both tab-facing properties vacuously
 true. The fix is in the action tree, not the property: give the action that
 brings the second reading into view its own weight.
 ```ts
 const switchATab = actions(() => {
  const tabs = tabElements.current;
  return tabs.length === 0 ? [] : [Tap({ on: from(tabs).generate() })];
 });
 export const actionsRoot = weighted(
  [25, switchATab],
  [20, showAViolatingStepWithItsPanel],
  [25, defaultActions],
 );
 ```
 Weighting one half is usually not enough, and this is the part that surprises
 people. `badgeCountMatchesThePanel` needs a badge, which a tab strip renders
 only for a step that has a violation, and a panel to compare it against, which
 exists only while a particular tab is selected. Undirected actions put both on
 the same step 0 times in the 80 steps of replay-ui's first dogfood run. Aiming
 at the violating step alone just moved the misses to the other side: still 0
 judged. `showAViolatingStepWithItsPanel` in that spec aims at both halves in
 sequence, selecting a violating row and then opening a panel if none is up.
 It also opens the *after* panel deliberately, because the before panel's
 screenshot is what `screenshotShowsTheSelectedStep` reads, and covering it up
 would buy one property's evidence with another's. When two properties read the
 same screen, an action tree can starve one to feed the other, and nothing in the
 run output will say so.
 ## 2. An effect must not exceed what the actions could have caused
 When the app has an effect you can measure (money moved, rows added, a counter
 climbed), state a bound on it rather than a prediction of it.
 **Prefer an upper bound to an equality.** This is the single most valuable
 sentence in this file.
 `examples/folio/sanderling/predicates.ts` states one rule about one app both
 ways, so the two are worth reading side by side. Each line is the last line of
 its predicate, after the guards, at a step where exactly one submit sits in the
 window:
 ```ts
 // sound, committedAmountExceedsOneSubmit: the violation is moving by MORE
 // than the one submit in this window could account for
 Math.abs(currAccountBalance - prevAccountBalance) > typedAmount
 // tempting, submitChangesBalanceByTypedAmount: it moved by exactly what I typed
 Math.abs(currTotalBalance - prevTotalBalance) === typedAmount
 ```
 Both catch the bug, because a double submit moves the balance by twice the typed
 amount. Only the second also convicts an app that behaved. A balance that has
 not moved is a commit still in flight (folio's `createTransaction` runs in a
 coroutine), a submit the app rejected, or a tap that never landed, and none of
 those is evidence of anything.
 The asymmetry is the point. Moving by more than one submit's worth is not
 something a correct app can do, so the bound needs no case for any of the three.
 The equality needs a case for each, and every one you forget is a false
 conviction. Compare the two guard stacks and the price is exactly legible: the
 equality declines on `confirmedApplied` and on `acrossRelaunch`, and the bound
 carries neither, because a submit that may not have landed and a restart that
 may have eaten the commit both leave the balance under the bound anyway. Those
 are the two facts the runner cannot promise (see below), and needing to guard
 against both is a cost of the equality, not of the app.
 You do give something up, so make the trade deliberately. A bound cannot see a
 balance that moved by *less* than the typed amount, and for a ledger that is a
 real bug. The question to settle before giving it up is whether your readings are
 tight enough to tell "moved by less" from "has not finished moving yet". If they
 are not, the equality was never detecting that bug either; it was reporting it at
 random.
 The bound has one precondition, and it is the same one as shape 3: it bounds the
 effect by what the actions in the window could have caused, so the window has to
 count every action that could cause the effect. Miss one and the bound is not a
 bound.
 ## 3. Count the actions, not the amounts
 The same bound, stated in counts. One action must not produce two effects.
 ```ts
 !committedTransactionsExceedSubmits({
  countsBefore: homeTxnCounts.previous ?? null,
  countsAfter: homeTxnCounts.current,
  submitsInWindow: submitsSinceCounts.current,
 })
 ```
 Reach for this whenever the effect is countable. No arithmetic on values the UI
 formatted and you parsed back, no float precision to reason about, and it stays
 sound however wide the window between two readings gets, since both sides
 accumulate over the same window.
 It has exactly two failure modes and both are about the window. Neither makes it
 unsound. Both make it useless, quietly.
 **The window has to close often enough to attribute anything.** The window opens
 when you last read the fact and closes when you read it again, so a run that
 wanders away from that screen accumulates budget on one side of the bound
 without accumulating evidence on the other. Measured on a real iOS run: it went
 from step 19 to step 136 without returning to the screen the property reads,
 giving a transaction rise of 15 against a window of 37 submits. 15 is not more
 than 37, so nothing was reported. The same run also gave 4 against 7, 6 against
 13, and 1 against 1. Sound throughout, detected nothing.
 The obvious fix is to read the fact somewhere the run visits often. The better
 one, when the wide window is the app's own shape rather than an accident, is to
 **state the same rule a second time over a narrower window**, which is what
 folio's spec now does:
 ```ts
 const submitCommitsOneTransactionPerAction = always(
  next(
    () =>
      !committedTransactionsExceedSubmits({ /* Home's counts, wide window */ }) &&
      !committedAmountExceedsOneSubmit({ /* this account's balance, narrow window */ }),
  ),
 );
 ```
 The counting form can only close its window on a Home reading, and a walk that
 stays inside the transaction flow leaves it hundreds of steps and dozens of
 submits wide. The second conjunct says the same thing in money about the one
 account whose screen the walk is already on, and the transaction flow redraws
 that balance on nearly every frame, so its window is usually a single action
 wide, narrow enough to tell one commit from two. One rule, two windows, and the
 narrow one is where the detection actually comes from.
 That only works because the two readings are kept from spanning two accounts:
 `readAccountBalance` drops its carrier on every route that is not the ledger or
 the transaction screen, transition frames included. A narrow window buys nothing
 if the pair it compares straddles two different subjects.
 **The window must not be spent on actions that provably could not cause the
 effect.** A bound inflated by taps that commit nothing is a bound the app can
 never exceed, which is slack a real double submit hides behind. Folio's
 transaction submit is `clickable(enabled = amount.isNotBlank())`, so a tap with
 an empty field never fires at all, and the app's own `parseCents` refuses
 anything outside `^\d+(\.\d{1,2})?$` or parsing to zero. Measured over four
 recorded Android runs, 19, 11, 25 and 25 of 35, 26, 42 and 42 submit taps landed
 with the amount field empty, which is roughly half the budget in every one.
 `submitCouldCommit` in `predicates.ts` is that rule, and note how narrowly it is
 drawn. It returns false only where folio's own code **must** have refused, and
 returns true for anything it cannot rule out, including an undefined reading and
 an amount too large for the app to hold. Over-counting costs a detection;
 under-counting convicts a healthy app.
 Establishing that an action could not have had an effect is app knowledge, not
 something the runner can tell you, and it has to come from the frame the tap
 read. Folio reads the amount field on the landing frame, which is sound because
 the tap changes nothing about it and one action runs per step.
 `element.enabled` is on every `AccessibilityElement` for the general case,
 though whether your platform populates it honestly is worth checking on a real
 tree rather than assuming.
 The mirror of this rule matters just as much: an action whose effect you cannot
 rule out **must** be counted. Leaving out submits whose dispatch the runner
 could not confirm is what once convicted a healthy app here, when the property
 saw a transaction rise of one against a window of zero.
 ## 4. A value the user can reach must stay inside its legal range
 Anything the user can type, or a URL can carry, or a deep link can set, is
 attacker-controlled input to your app even when the attacker is a fuzzer. The
 property is that it stays legal, and it is cheap: one reading, no window, no
 attribution.
 ```ts
 const selectedStepIsInRange = always(() => {
  const current = toolbar.current;
  if (!current || current.step === null || current.stepCount === null) return true;
  return current.step >= 1 && current.step <= current.stepCount;
 });
 ```
 Two things make that sound. The bound comes from the app's own reading of how
 many steps the run has, not from a number you typed after looking at a fixture.
 And it asserts legality rather than a prediction:
 ```ts
 // tempting: I tapped next, so it must now be on step n + 1
 toolbar.current.step === (toolbar.previous?.step ?? 0) + 1
 ```
 which is false at the end of the run, false when the tap did not land, and false
 whenever the app is within its rights to clamp. Assert what must not happen.
 ## 5. State machine and navigation invariants
 Every screen with a selection, a mode, or a route has invariants that are true
 by construction and therefore worth stating, because "by construction" is
 exactly what breaks.
 **Exactly one, not at least one.** The looser version is the tempting one and it
 gives up the interesting half of the bug.
 ```ts
 const exactlyOneStepIsSelected = always(() => {
  const rows = stepRows.current;
  if (rows.length === 0) return true;
  return rows.filter((row) => row.active).length === 1;
 });
 ```
 Two selected rows is a stuck selection. Zero is the toolbar showing a step the
 list has no row for, which is what an off-by-one or a failed clamp looks like
 from the list's side, and `>= 1` would never see it.
 **A view change must not be a navigation.** Switching a tab, opening a menu or
 toggling a theme must leave the app where it was.
 ```ts
 const switchingTabsKeepsTheStep = always(
  next(() => {
    const previousTabs = activeTabs.previous;
    const previousToolbar = toolbar.previous;
    const currentToolbar = toolbar.current;
    if (previousTabs === undefined || previousTabs === activeTabs.current) return true;
    if (!previousToolbar || !currentToolbar) return true;
    return previousToolbar.step === currentToolbar.step;
  }),
 );
 ```
 Note the guard: it declines unless the tab strip actually changed. A property
 about an event must first establish that the event happened.
 The tempting unsound version of a navigation property is asserting the route you
 were hoping for, `route.current === "home"` after tapping submit. The app is
 within its rights to show a validation error and stay, and folio does exactly
 that for an amount of zero. State what must not happen, not what you wanted to.
 Deriving the route at all deserves care, and folio's `routeOfFrame` is the
 pattern: it returns the screen only when exactly one screen marker is in the
 tree, and null otherwise. Android's hierarchy dump carries the outgoing and the
 incoming screen together on 425 of 1879 steps measured across 17 runs, better
 than one frame in five. Such a frame is evidence about neither screen, and
 ranking the markers to pick one is how a spec convicts itself on an animation.
 ## 6. The ones you get for free
 ```ts
 import { noUncaughtExceptions, noLogcatErrors } from "@sanderling/spec/defaults";
 export const properties = { noUncaughtExceptions, /* yours */ };
 ```
 Export `noUncaughtExceptions` before you write anything of your own. It costs a
 line, it needs no app knowledge, and a fuzzer typing `'; DROP TABLE--` and a
 4096-character string into every field it finds will surface real breakage
 through it. `noLogcatErrors` is stricter and Android-only; it holds trivially
 elsewhere, so it is worth turning on once you know your app's log hygiene can
 support it.
 They do not substitute for the shapes above. An app can be thoroughly wrong
 about money without throwing once.
 ## The rules that cut across all of them
 **Absence is unknown, never a default.** Extractors return null when the element
 is not there, and a property handed null declines. `0`, `""` and `[]` are the
 values that turn a property into one that fires on healthy runs: folio's
 balances once parsed as `0` on web, so the check became `|0 - 0| === typed` and
 was false at every healthy submit. An empty list has the same problem in the
 other direction, and it is worse because it looks reasonable. Android renders
 Home's own node a frame or two before its list, so `findAll` over the cards
 comes back empty while the screen already claims to be Home. That is unknown,
 not "no accounts", and reading it as zero accounts killed folio's counting
 invariant outright: `countsBefore` was `{}` at every evaluation point of all 17
 runs measured.
 **Attribution needs injective keys.** If two distinct objects can produce the
 same identity key, a value silently jumps between unrelated series. Merged UI
 text is the usual culprit: web collapses an account card into a single node
 whose text runs the name into the count, so an account named `Travel1` with 25
 transactions and one named `Travel12` with 5 both render `TRTravel125
 transactions`. No function of that string can separate them. Where a key can
 collide, drop the reading rather than guess: `homeTxnCountsOf` leaves out any
 name carried by more than one card, because subtracting two different accounts'
 counts convicts a healthy app of double-submitting.
 **Match whole keys, not endings.** `endsWith` attribution judges an older
 account named `Emergency Fund` when the user typed `Fund`: have the new card
 clipped out of the reading, the way a list clips any card, and the old account
 is convicted for money it has held all along. Substring matching is looser
 still. Build every form of the key the platforms can produce and compare each
 one whole, which is what `createdAccountHasNonZeroBalance` does with
 `account.name === typed || account.name === initialsOf(typed) + typed`. Note
 that this bought detections as well as soundness: under the suffix test, a name
 that two cards ended with was thrown away as unattributable rather than matched
 to the one card that actually carried it.
 **What the runner could not promise.** `state.lastAction` is
 `Action & { applied: true | null; relaunched: true | null }`, and collapsing any
 of its states is unsound:
 - `null`, the whole field, means no action ran
 - `applied: true` means the runner saw the dispatch succeed
 - `applied: null` means it was dispatched and nobody can find out whether it
  landed, because an RPC deadline can fire after the tap arrived
 - `relaunched: true` means the runner had to bring the app back to the
  foreground after this action, so the two readings straddle a restart
 One rule covers the last two, and it is the rule that decides shape 2 for you.
 An action the runner cannot fully vouch for **still counts toward a bound on
 what the app could have done**, and it **never licenses attributing an effect to
 it**. So a bound counts it and an equality has to decline on it. That is why
 `committedAmountExceedsOneSubmit` needs no `confirmedApplied` guard and no
 `acrossRelaunch` guard while `submitChangesBalanceByTypedAmount` needs both: a
 property demanding the effect of an action that may never have run, or that a
 restart may have swallowed, convicts the app of the runner's own uncertainty.
 `relaunched` is the same shape of fact as `applied`, applied to app state rather
 than to dispatch. The action itself did happen. What nobody can promise across
 it is that the process ran continuously, that the commit survived, or that the
 screen is showing the same slice of the same list it was. So a property assuming
 continuous state declines, via `acrossRelaunch(lastAction)`, and folio uses it
 in three places: `createdAccountHasNonZeroBalance` declines because Home redraws
 from the top and the card carrying the typed name may be an older account laid
 out where the new one used to be, the equality property declines because it
 demands an effect, and `countSubmitsInWindow` uses it to **stop trusting its own
 refusal evidence**, since a relaunch is the one thing that can put a form state
 on screen other than the one the tap read.
 Both fields are `true | null` rather than booleans, and that is deliberate: only
 the positive report is a fact the runner can vouch for, so `null` is "not
 reported" rather than "did not happen". `relaunched` shows why it has to be that
 way. Web and iOS cannot read the foreground at all, so they never relaunch the
 app and equally cannot promise it never restarted, and a `false` there would be
 a claim nobody is in a position to make. Read the absence as a guarantee and you
 have made the same mistake as reading a missing value as zero, one level up.
 **Testing a property means both directions, every time.**
 - it fires on the bug it exists to catch
 - it stays silent on a run where the app behaved
 The second is the one people skip and the one that catches unsoundness. Build
 the fixture where the effect happens legitimately, at the boundary the property
 draws, and assert silence: the commit that is still settling, the submit the app
 refused, the card that scrolled into view rather than being created, the pair of
 readings taken either side of a relaunch. A property you have only ever seen go
 red is a property you have half tested.
 Then hand it to `sanderling-spec-review`, which will ask how many steps it
 actually judged on a real run.
@@ -0,0 +1,353 @@
 ---
 name: sanderling-spec-authoring
 description: Write a sanderling spec for an app: test hooks, extractors, selectors, properties, and an action tree that reaches the states the properties read. Use when adopting sanderling for a new app, when adding a property to an existing spec, and when a run is green because it never reached the state the property was written for.
 ---
 # Writing a sanderling spec
 A spec is a TypeScript module the runner evaluates once per step. It exports
 `properties` and `actionsRoot`, plus an optional `setup` and `generator`.
 Writing one is easy. Writing one that would catch a real bug is not, because a
 spec that checks nothing looks exactly like a spec that checks everything: both
 are a green run.
 So the order below is arranged around getting evidence early that each piece
 reads what you think it reads. When the spec is written, audit it with
 `sanderling-spec-review` before trusting a green run from it.
 ## Write it in this order
 Hooks, then extractors, then **one** property, then run it and read the witness,
 then everything else. Writing six properties before the first run is how people
 end up with six that cannot fire, and nothing in the output tells you which.
 ## Hooks first
 Your app needs stable handles or nothing can name what it is asserting on. The
 hooks the replay UI's spec drives (`data-testid`, `data-step`) were added to the
 UI for that spec, and its header says why: a UI with no stable handles is a UI
 nothing can assert on, and that is as true for a person writing a test as it is
 for a fuzzer.
 Put a hook on the screen or route markers, on every container you will scope a
 lookup to, and on every fact you will read. `testTag` is the portable one: it
 surfaces as resource-id on Android and accessibilityIdentifier on iOS, and on
 web it resolves to `data-testid` or `id`.
 ## Extractors
 `extract(name, fn)` reads one fact off `state.ax` per step. `.current` is this
 step's value, `.previous` the last step's, `undefined` on the first step.
 Name every one. The name is what you get back later: `extractor_changes` in
 `trace.jsonl` carries the prev/curr pair for each extractor whose value moved,
 and the witness recorded at a violation carries the extractor values behind it,
 by name. An unnamed extractor shows up as `extractor_3`, which tells you nothing
 at the point you most need to know what the property was looking at.
 The rule that decides whether the spec is worth anything:
 > **Return `null` when the element is absent. Never `0`, `""`, or `[]`.**
 An unreadable fact is unknown, and a default turns unknown into a claim. Both
 directions bite. Folio parsed a missing balance as `0` and its property became
 `Math.abs(0 - 0) === typedAmount`, false at every healthy submit. Read a missing
 panel's row count as `0` and the property says the cart is empty when the truth
 is that the cart is not on screen. Where the ambiguity is real, call it unknown:
 an empty `findAll` is both "no rows" and "not drawn yet", and folio treats it as
 unknown, which costs the very first account of a run and buys back every card
 that arrived late.
 Extractors run before properties and action generators, and they may not read
 each other. If two readings must come off one parse, put the parse in a helper
 both call: `examples/folio/sanderling/predicates.ts` does this with
 `oncePerFrame`, keyed on the state object, since both hosts build a new state
 object per step.
 ## Selectors
 `ax.find` and `ax.findAll` take a string (`"id:CartBadge"`), an object
 (`{id: "CartBadge"}`), or an array of objects for a path. Element handles carry
 their own `.find` / `.findAll` scoped to their subtree.
 The two forms resolve identically: an object key is matched by the same rule its
 string form uses, so `{id: "X"}` and `"id:X"` can never pick different elements.
 What differs is the rule per key. Measured against an Android dump holding
 `com.app:id/CartBadge`, whose content-desc is `Cart, 3 items`, alongside
 `AddAccountSubmit`:
 | Selector | Resolves to |
 |---|---|
 | `{id: "CartBadge"}` | the badge: `id` matches the whole resource-id, or the part after `:id/` |
 | `{id: "Sub"}` | nothing: `id` wants a whole name, not a fragment |
 | `{testTag: "CartBadge"}` | the badge: `testTag` reaches resource-id on Android and accessibilityIdentifier on iOS |
 | `{testTag: "Sub"}` | `AddAccountSubmit`, because every key outside the `id` / `desc` / `descPrefix` special cases is a **substring** match |
 | `{desc: "Cart"}` | the badge: `desc` takes the whole description, or an iOS merged label starting `Cart, ` |
 `testTag` is the portable key and the one to reach for, but name the element in
 full. A substring match on `Sub` is not a match, it is a coincidence, and it
 will one day pick a different control.
 A selector that matches nothing makes every property over it vacuous, and
 nothing anywhere reports that. This is why the selector you verify is the one
 you found a real value for in a witness, not the one that looked right when you
 wrote it.
 **Scope the lookup to a container instead of taking the first match on the
 page.** From `replay-ui/sanderling/spec.ts`: an earlier draft of
 `screenshotShowsTheSelectedStep` took the first screenshot on the page, the
 fuzzer put the "before" panel on another tab, which left the "after" panel's
 image first, and the property fired against a UI that was behaving correctly. It
 now reads `s.ax.find([{ "data-testid": "state-before" }, { "data-testid": "screenshot" }])`
 and is scoped to the panel it means.
 A screen marker is not enough scope on its own during a navigation. Android's
 hierarchy dump carries the outgoing and the incoming screen together on 425 of
 1879 steps measured across 17 runs, better than one frame in five, so a find
 scoped to a screen the app has already left still resolves. Decide the route once
 per frame, return `null` when more than one screen marker is present, and have
 every reading take its answer from there. `routeOfFrame` in folio's
 `predicates.ts` is that rule and carries the measurements.
 ## Properties
 `always(f)` requires `f` at every step. `next(f)` inside it compares this step
 to the next, which is how you state "this action had that effect". `now(f)`
 evaluates at the current step inside a formula body.
 `eventually(f).within(n, "steps" | "seconds" | "milliseconds")` requires `f`
 before the window closes; unbounded, it never fails a finite run. At the top
 level an `eventually` is one goal for the whole run, armed once and discharged
 for good the first time it holds; written inside `always` it re-arms at every
 step, which asks for the window to be met from everywhere. Every formula has
 `.implies`, `.and`, `.or`, `.not`.
 The stock properties are in `@sanderling/spec/defaults`. Put
 `noUncaughtExceptions` in every spec: it fails when the run captures an uncaught
 throwable or a `Sanderling.reportError` call, it needs no hooks and no
 calibration, and it is free. `noLogcatErrors` (also exported from
 `@sanderling/spec/defaults/properties`) fails on any error-level logcat line and
 is Android-only, holding vacuously elsewhere.
 ## What makes a good first property
 Prefer a **cross-panel agreement**: two parts of the UI that derive the same fact
 by different paths must say the same thing. The toolbar prints a step count and
 the list renders rows; a badge counts violation records and the panel counts the
 rows it can show for them. Those hold on any run, so they never need
 recalibrating against a fixture, and an app that gets the fact wrong in one of
 the two places cannot satisfy them however it was driven there.
 `replay-ui/sanderling/spec.ts` is six of these plus `noUncaughtExceptions`, and
 its header explains the choice.
 Contrast a property that needs the fuzzer to reach a specific state, like
 folio's "a submit moves the balance by exactly the amount typed". That is where
 the real bugs are, and it is the harder thing to keep honest: it needs an action
 tree that reaches the state, a window that closes often enough to bound what
 happened inside it, and attribution that cannot blame the wrong action. Folio's
 counting form went 117 steps between two readings on one iOS run and gathered 37
 submits against a rise of 15 transactions, which is perfectly sound and says
 nothing at all; the fix was to state the same rule over a number the app redraws
 on nearly every frame, so the window is usually one action wide. Write these
 second, and read `sanderling-spec-review` before you believe one.
 Whichever you write, name the input that makes it return false before you move
 on. If you cannot, it is decoration.
 ## Actions
 `actions(() => Action[])` returns the candidate actions for this step and the
 picker chooses one. The verbs are `Tap`, `DoubleTap`, `LongPress`, `InputText`,
 `Scroll`, `Swipe`, `PressKey`, and `Wait`. The built-in generators are `taps`,
 `doubleTaps`, `longPresses`, `typing`, `scrolls`, `swipes`, `pressKeys`, and
 `waitOnce`; `defaultActions` bundles them at taps and typing 100, scrolls 50,
 swipes 25, double taps 10.
 `weighted([n, generator], ...)` composes them with relative weights.
 `whenRoute(routeExtractor, routes, body)` runs `body` only on the named screens.
 The optional `setup` export runs before `actionsRoot` for as long as it returns
 actions, which is where login and onboarding belong; it re-engages on its own if
 the app logs itself out mid-run. Values come from `from(items)`,
 `integers().between(min, max)`, `strings().length(min, max).alpha()`,
 `emails().domain(host)`, and `edgeCaseText()`, all drawn from the run's seeded
 PRNG so a seed replays exactly.
 **The default enumeration explores, but reaching a specific interesting state
 usually needs a weighted action of your own.** With about 15 clickable elements
 on the replay UI's page, an undirected run went 40 steps without switching a
 single tab, which left both tab-facing properties vacuously true. Its badge
 agreement is worse: it needs two readings on one step, a badge, which only a
 step that has a violation renders, and a violations panel to compare it against.
 Undirected actions put both on the same step **0 times in the 80 steps of the
 first dogfood run**. The property was reachable in principle and judged nothing
 in practice, and aiming at the step alone just moved the misses to the other
 side, 0 judged either way. It took an action that selects a violating step and
 then opens a panel if none is up. Folio weights its transaction chain at 45 for
 the same reason: both balance properties observe that flow and nothing else
 reaches it.
 So for every property, name the action in the tree that puts everything it reads
 on screen at the same step. If there is none, add one, and give it enough weight
 that a short run gets there.
 ## Soundness outranks everything else here
 A property must never convict an app that behaved correctly. A property that
 convicts more often and is sometimes wrong is strictly worse than one that
 convicts less and is never wrong, because a false conviction costs someone a day
 and then costs the whole suite its credibility. **When in doubt, a property
 should decline to judge.**
 Declining costs at most a detection. Convicting a healthy app costs the spec.
 Concretely that means unknown stays `null`, a bound is preferred to an equality
 where the window can hold more than one cause, and a value carried across a
 screen change is dropped rather than compared.
 It also means reading `state.lastAction` for what it actually promises, which is
 three different things and not one:
 - `state.lastAction === null`: no action ran.
 - `applied: true`: the runner saw the dispatch succeed.
 - `applied: null`: it was dispatched and nobody knows whether it landed.
 The rule is short. **An action of unknown fate still counts toward bounds on
 what the app could have done, but it never licenses attributing an effect to
 it.** Leave it out of the bound and you convict a healthy app: folio saw a
 transaction rise of one against a window of zero submits and called it a double
 submit. Demand its effect and you convict the app of the runner's own
 uncertainty.
 `relaunched: true` says the runner had to bring the app back to the foreground
 after the action, so the two readings straddle a restart. The action still
 happened and still counts toward the bound, but nothing about state running
 continuously between the two readings survives it, and a property demanding that
 action's effect has to decline. Like `applied`, its null is "not reported", not
 "the app never restarted": web and iOS cannot read the foreground at all, so only
 an explicit `true` licenses declining.
 ## Run it, then read the witness
 **Write one property, run it, and read the witness before you write the second.**
 This is the step that gets skipped and it is the one that pays. A spec that has
 never had its readings confirmed against a real run looks exactly like a spec
 that has, right up until you find out that an extractor reads `null` on the
 platform you care about, or that a selector matches nothing, or that the value
 being compared is not the value you thought.
 ```sh
 sanderling test --spec spec.ts --bundle-id com.example.app --platform android --duration 2m
 sanderling replay
 ```
 Confirm two things before adding anything. First, that each extractor holds a
 real value at some step, by finding it in `extractor_changes` in `trace.jsonl`;
 the replay UI's hierarchy panel separately tells you whether the element your
 selector names is in the tree at all. Second, that the property actually
 compared values on some step rather than short-circuiting on its own guard. A
 green run is evidence only if you can point at a step where a property fired.
 Only then write the next property.
 Once a property is worth keeping, its logic is worth testing away from the
 device. Folio keeps its predicates in a plain module and unit-tests them in
 `pkg/spec/test/folio-*.test.ts`, run by `make test-spec-api`; the app's own
 Kotlin tests are `make test-folio`. Those files are the model for testing a
 property in isolation, including the direction people skip: a fixture where the
 effect happens legitimately, asserting that the predicate stays silent.
 ## A spec to adapt
 Complete and self-contained: a storefront whose header badge and cart panel both
 know how many things are in the cart.
 ```ts
 import { InputText, Tap, actions, always, extract, from, integers, weighted } from "@sanderling/spec";
 import { defaultActions, noUncaughtExceptions } from "@sanderling/spec/defaults";
 function wholeNumber(text: string | undefined): number | null {
  if (!text) return null;
  const parsed = Number(text.trim());
  return Number.isInteger(parsed) ? parsed : null;
 }
 // The header badge: the app's own count of what is in the cart.
 const badgeCount = extract("badgeCount", s =>
  wholeNumber(s.ax.find({ testTag: "CartBadge" })?.text));
 // The same fact by another path: the rows the cart panel renders. No panel is
 // null rather than 0, because nothing on screen is a fact we do not have, and
 // 0 would claim the cart is empty.
 const cartRowCount = extract("cartRowCount", s => {
  const panel = s.ax.find({ testTag: "CartPanel" });
  return panel ? panel.findAll({ testTag: "CartRow" }).length : null;
 });
 const checkoutEnabled = extract("checkoutEnabled", s => {
  const button = s.ax.find({ testTag: "CheckoutButton" });
  return button ? button.enabled === true : null;
 });
 // Two parts of the UI count the cart by different routes through the app's own
 // state, so they cannot disagree about how many things are in it.
 const badgeMatchesTheCart = always(() => {
  const badge = badgeCount.current;
  const rows = cartRowCount.current;
  if (badge === null || rows === null) return true;
  return badge === rows;
 });
 // Checkout is offered exactly when there is something to check out.
 const emptyCartCannotCheckOut = always(() => {
  const rows = cartRowCount.current;
  const enabled = checkoutEnabled.current;
  if (rows === null || enabled === null) return true;
  return rows > 0 || !enabled;
 });
 export const properties = {
  noUncaughtExceptions,
  badgeMatchesTheCart,
  emptyCartCannotCheckOut,
 };
 const productCards = extract("productCards", s => s.ax.findAll({ testTag: "ProductCard" }));
 const cartButton = extract("cartButton", s => s.ax.find({ testTag: "CartButton" }));
 const quantityField = extract("quantityField", s =>
  s.ax.find([{ testTag: "CartPanel" }, { testTag: "QuantityField" }]));
 const addAProduct = actions(() => {
  const cards = productCards.current;
  return cards.length === 0 ? [] : [Tap({ on: from(cards).generate() })];
 });
 const openTheCart = actions(() => {
  const button = cartButton.current;
  return button ? [Tap({ on: button })] : [];
 });
 const quantities = integers().between(1, 5);
 const changeAQuantity = actions(() => {
  const field = quantityField.current;
  return field ? [InputText({ into: field, text: String(quantities.generate()) })] : [];
 });
 // Both properties read the cart panel, so a run that never opens it judges
 // nothing. defaultActions carries the rest of the app.
 export const actionsRoot = weighted(
  [35, addAProduct],
  [25, openTheCart],
  [15, changeAQuantity],
  [25, defaultActions],
 );
 ```
 Both properties here decline whenever the panel is off screen, which is honest
 and also the thing to measure first: if `openTheCart` never wins the draw, they
 judge nothing, exactly like the replay UI's badge property did for 80 steps.
 The two real specs in the repo are the fuller references.
 `replay-ui/sanderling/spec.ts` is the cross-panel spec written the way this page
 recommends. `examples/folio/sanderling/spec.ts` with its `predicates.ts` is the
 harder kind, a spec that attributes effects to actions across screens, and every
 comment in it records a way it was once wrong. `docs/manual/spec-language.md` is
 the lookup reference for anything not covered here.
@@ -84,9 +84,14 @@ one named `Travel12` with 5 can both render `TRTravel125 transactions`. No
 function of that string can separate them.
 **Match whole keys, not endings or substrings.** `endsWith` attribution judges an
-older account named `Emergency Fund` when the user typed `Fund`. Substring
+older account named `Emergency Fund` when the user typed `Fund`.
-matching is looser still: `{id: "Sub"}` matching `AddAccountSubmit` is not a
+
-match, it is a coincidence.
+Selector matching has the same trap and it is easy to miss which keys carry it.
 `id`, `text`, `desc` and `descPrefix` resolve by their own rules, so `{id: "Sub"}`
 correctly matches nothing. Every other key, `testTag` included, falls through to a
 substring compare, so `{testTag: "Sub"}` matches `AddAccountSubmit`. That is not a
 match, it is a coincidence, and a property built on it judges whichever element
 happens to contain the fragment.
 **Drop the carrier when the screen changes.** A value carried across a route
 change is a value read from a screen that is no longer there.