Delegate by checkability

She gives the apprentice the wall tiling: a crooked row or the wrong grout colour shows the moment she looks at the wall. She keeps the waterproof membrane under the shower tray. A membrane that will leak looks exactly like one that will hold, and it fails only when water runs for days behind finished tiles. A flood test, the tray plugged and filled to a marked line before any tile goes on, fails a leaking membrane; with that test, the membrane can go to the apprentice too.

In the bathroom	With a generated output
Looking at the tiled wall	A check that shows a mistake in one run (step 3)
A membrane that looks the same, leaking or not	Code that fails only under a condition (step 4)
Water running for days behind the tiles	Another caller, concurrent requests, production data
The flood test before tiling	A test that creates the condition and fails a wrong copy (step 6)
"What would show me this is wrong?"	Name the check (step 2)
Her sense that the apprentice saved the afternoon	A self-reported speed-up (steps 8–9)
The start and finish times on the job sheets	Measured times on comparable tasks (steps 10, 12)

Breaks down when: a flood test runs once and the tray either holds or leaks, while concurrent requests may never overlap in a test run; the test must force the overlap, not hope for it (step 6). And an apprentice learns from each job, while an assistant's next draft carries no memory of the last correction.

Contrasting case: deciding what to build, the scope and the design, is not work to delegate by checkability at all: those calls stay yours (aiassist-failure-modes step 8).

In real terms: before handing work over, ask which check a wrong output would fail; hand over what such a check covers, and write the check first for the rest.

4.1 EXH-2470: an admin-only guard and an admin-only suite

ExamHall's admin console lets an admin re-open a closed exam. The assistant wrote the guard:

// reopen.mjs (generated)
export function reopenExam(user, exam) {
  if (user.role !== 'admin') return { status: 403 };
  return { status: 200, exam: { ...exam, state: 'open' } };
}

Then it adds one test, MR !2770's: a proctor asks to re-open, and expects 403.

4.1 EXH-2470 existing tests (18, each as an admin) · generated guard: 18 of 18 pass · copy with the role check deleted: 18 of 18 pass
4.1 MR !2770 adds "a proctor is refused" · generated guard: 19 of 19 pass · copy with the role check deleted: 18 of 19 pass, failing: a proctor is refused

Every existing test calls as an admin, so the suite passes the copy that lets anyone re-open (steps 4–5). The added test creates the missing condition, another caller, and only the copy fails it (step 6).

4.2 Payments ledger: currency minor units, checked in one run

The ledger stores amounts in minor units, so each currency needs its number of decimal places. The assistant drafted the table; one run compares it with the currency data in the project's Node.

4.2 assistant's table: GBP 2 · EUR 2 · USD 2 · JPY 2 · KWD 2 · CHF 2
4.2 Intl.NumberFormat on node 24.19.0: GBP 2 · EUR 2 · USD 2 · JPY 0 · KWD 3 · CHF 2
4.2 rows that differ: 2 of 6 (JPY, KWD)

The run names the two wrong rows at once. A draft whose mistakes show against known values in one run is cheap to check (step 3).