Vaccinate — grade the grader

Twenty bugs were once planted in a working codebase and the review agents were asked to check it again. They reported everything was fine. Recall: 0/20.

A vaccine is a small, controlled dose of error that strengthens the whole system. This skill administers one.

The rule it enforces: an unqualified check is not weak evidence — it is none.

When to run it

• Before a referee simulation, reproducibility gate, or review agent is used to make a decision that matters (a submission, a release, a deposit).
• After changing a checker — a modified gate is unqualified until re-measured.
• On a schedule for gates that guard load-bearing claims. Detection decays as artifacts drift.

Protocol

1. Name the failure

State the defect class the check is supposed to catch. "Catches problems" is not a class. "Detects a coefficient in the text that no longer matches its table" is.

2. Build the seeded set + a clean control

Work on a copy, never the live artifact. Produce:

• N seeded variants, one defect each, drawn from references/defect-library.md.
• At least one clean control — an unmodified copy.

The control is not optional. Without it you measure recall and call it accuracy.

Verify each seed actually violates something. A seed that the artifact already permits creates no defect, and the checker correctly reporting "pass" will look like a broken gate. This is the most common way a qualification run produces a false alarm about itself.

3. Run the checker blind

Run the check or agent against each variant in a fresh context, one variant per run. It must not know which variant it has, how many defects exist, or that a qualification is underway. For an AI reviewer, spawn via the Agent tool with context: fork.

4. Score

Metric	Definition
Recall	seeded defects correctly identified / seeded defects planted
False-positive rate	findings on the clean control that are factually false / total findings on the control
Localization	did it name the right location, or just report unease?
Baseline delta	recall of a simpler alternative (a grep, a diff, a one-line assertion)

A finding on the clean control counts as a false positive only when it is factually wrong — not merely unwelcome. A reviewer prompted to find gaps will report some in sound work; that is expected behaviour, not a failure.

The baseline is load-bearing. A five-agent panel that scores no better than grep -n has not earned its cost.

5. Write the ledger row

Append to quality_reports/qualification/LEDGER.md:

| date | target | artifact | defect classes | N | recall | FPR | baseline | verdict |

Verdicts: PASS (detects its named class at an agreed threshold) · FAIL (misses it) · BLOCKED (could not be run — say why; do not record as PASS).

6. Act on the result

• FAIL → the check does not license its claim. Fix the check or stop citing it. Do not weaken the seed until it passes.
• PASS → record the threshold. A PASS at one difficulty is not a PASS at another.
• Either way, a checker with no ledger row is unqualified, and its green light means nothing.

Worked example

/vaccinate check-model-versions.sh

1. Failure class: "a superseded model presented as current".
2. Seed: append The newest model is Opus 4.8 and it is the default. to README.md. Control: unmodified README.md.
3. Run: bash scripts/check-model-versions.sh; echo $?
4. Score: seeded → exit 1 (detected). Control → exit 0 (no false alarm). Recall 1/1, FPR 0/0. Baseline: grep -c "Opus 4.8" README.md also detects — so the gate's value is its allow-marker logic, not raw detection.
5. Ledger: PASS.
6. Restore the artifact and re-run to confirm you are back to green.

Anti-patterns

• Seeding into an artifact that already permits the seed — measures nothing, looks like a broken gate.
• Telling the reviewer it is a test — it will look harder than it does in production.
• Counting any finding as a hit — a finding at the wrong location is not detection.
• One seed, one run — a single trial does not distinguish detection from luck. Use ≥2 replicates per class where cost allows.
• Weakening the seed until it passes — that is fitting the test to the checker.
• Skipping the clean control — the most common omission, and it hides the cost.

Reference files

File	Read when
references/defect-library.md	choosing what to seed — defect classes by artifact type
evals/README.md	the complementary question: does the skill produce better output than not having it?

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Doctrine: what qualification means

Do not assume more machinery is better

A second model, more agents, or a longer debate is not presumed to verify better. Before an elaborate procedure earns extra weight, show it outperforms a simpler check on the same prespecified seeded failures and valid cases, reporting both detection and false alarms. Complexity that has not beaten a baseline is cost, not assurance.

Treat AI verdicts as predictions, not facts

When a model grades, triages, or reviews at scale:

• keep a sampled set for qualified human review, and record how it was sampled (retain coverage of hard subgroups — do not sample only the easy middle);
• keep fitting/prompt-tuning cases separate from evaluation cases;
• report where AI and expert judgments diverge;
• remember a well-calibrated average score certifies no individual verdict;
• agreement between models is not independent evidence — they share failure modes and converge on the same wrong answer at a meaningful rate.

Any material change to the model, prompt, rubric, or target population requires fresh human labels and recalibration.

Requalify after material change

A check qualified against an old interface, schema, or scale may silently stop testing anything. Re-run the seeded-defect proof after material changes to the object under test or to the check itself.

Distinguish qualified checks from scientific judgments

• Qualified checks have a defensible reference answer: unique keys, units convert, a table regenerates, an estimator recovers an analytic special case, a seeded fault triggers a failure. These can be automated and rerun forever.
• Scientific judgments — whether a field measures the intended construct, whether an identifying assumption is plausible, whether a result deserves causal language — cannot be automated, and no volume of qualified checks substitutes for one.

Confirm the check actually ran

A missing, substituted, or degraded check is missing evidence, not a pass. Verify the run happened (log, exit status, artifact timestamp — not an assumption); that it ran on the current object, not a cached one; that nothing was skipped, filtered, or swallowed into a default; and that the tolerance was fixed before the comparison. A tolerance loosened after a failed comparison converts evidence into decoration. If it must be loosened, record it as an approved divergence with a reason.

Cross-references

• verification-ladder.md — rung 0; why this comes before everything
• Merged with the former /qualify-checks (2026-08-21): same goal — one skill, not two
• external-oracle-process.md — qualifying an external referee