gating

gating

A gate is a check that blocks. Its only job is to go red when it should.

The characteristic failure is not a wrong check — a wrong check gets noticed. It is a check that cannot fail, which reports PASS forever and is indistinguishable from a working one from the outside. That is what makes this different from ordinary testing: the object under suspicion is the check.

When NOT to use this skill

Scope is ONE check and whether it can be made to fail.

Situation	Use
Sequence several steps with branches and retries	flowing
Run the repo's existing suite	run it
Decide what to test at all	this skill has no opinion; that is design

A gate is a thing that goes red. If nothing here can go red, there is no gate to audit.

The three obligations

Every gate owes these. A gate missing any of them is not yet a gate.

1. An anchor outside your own code. Something the check compares against that your implementation did not produce: a published constant, a closed-form answer, a conservation law, a degenerate case with a known result, an independent implementation. A check that compares this run to the last run only ever tells you the code still does what it did. See references/anchors.md.

2. A known-bad it demonstrably rejects. Break the subject the way it would plausibly break, run the gate, confirm red. Until you have done this you have not shown the gate works — you have shown it runs. This is the obligation people skip, because a passing gate feels like evidence.

Two things about known-bads that are easy to get wrong:

• Validate it at the configuration it will run in. A case tuned on a small or fast setting can stop being bad at full size. An "untrained" grid built from one Lloyd iteration was genuinely zero-gain at m=2/K=16 and earned a real +0.10 dB at m=8/K=65536, where one iteration relocates ~63,000 empty cells toward the mode. It passed the fast gate and certified nothing about the real one. This matters more than it sounds, because mutate.py needs a fast gate variant and it is tempting to validate everything there.
• Measure its reach. One known-bad is the floor, not the goal. Name which checks it exercises (known_bad(..., covers=(...))); the harness prints the checks no known-bad reaches. An audited gate had a single known-bad covering 1 of 8 checks — and the check its whole result rested on accepted the same bad case.