When two or more implementations share one contract, a code read that concludes "they agree" is a hypothesis, not a result. Do not report "there's a test gap, but no defect" and stop. Write the executable comparison, run it, and let it decide.
If a gap is worth naming out loud, it is worth the ~30 minutes the battery costs.
One implementation is out of scope, and that is not a hedge. A promise with a single implementation is a definition, not a claim that can diverge — this rule does not license writing batteries for everything. Two is the threshold, and a fake plus the real thing it stands in for is the best case of two, because that pair is what everyone else's tests are silently trusting. Where a shared battery for the contract already exists, extend it rather than fork it.
The rule
Close a named test gap in a shared contract even when you believe there is no defect behind it. The battery is cheap; the reading is not proof.
The reason is not that code reads are usually wrong. It's the shape of the error they make: you generalize from the part you inspected to the whole. You read the method where the interesting promise lives, satisfy yourself that all implementations agree there, and conclude "this contract is consistent" — never putting the neighbouring method's behavior side by side, because you have already decided. A battery carries no such prior. It checks the axis you skipped.
Procedure
- Enumerate the promises. Read the contract's own docstrings/spec text and
list every testable guarantee. Grep for promise language across the interface:
idempotent,no-op,never,always,guaranteed,exactly once,at most one,must never,fails closed. Written promises with no test are the highest-yield place to look. - One shared module, one check per promise, parametrised over every implementation. Not per-backend test files — those are how the gap formed.
- Assert the discriminating detail, not the outcome (see below).
- Include a positive control so the battery cannot pass vacuously.
- Run it before deciding whether there was a defect. Then report what it actually said.
For the concrete file shape this converged on, see references/battery-template.md.
Three properties, or the battery proves nothing
A battery that runs green against every implementation may be measuring nothing at all. Each of these has been the difference, and all three are in references/battery-craft.md with the case that produced them:
- Assert the discriminating detail. Checking that both implementations raise is not conformance; checking that both raise the same kind, with the same message exposure and the same retryability, is.
- Exercise the discriminating state. A contract that differs only on the empty case, the concurrent case, or the second call is a contract whose battery must reach those states deliberately.
- Keep a positive control. One assertion that fails when the shared code is broken. Without it, "all backends agree" and "the battery never ran" are the same green.
Reporting
State three things: what the battery covers, what it found, and what you changed.
- Found a divergence → name the axis, both behaviors, and the user-visible consequence (status code, wrong value, duplicate execution).
- Found nothing → "battery green across N implementations; the gap was coverage, not correctness." That is a finished job, not an empty one.
Quick checklist
- Does this contract have ≥2 implementations?
- Am I about to report a gap without running anything?
- Is there one shared module, parametrised over all implementations?
- Does every check assert a discriminating detail (kind, state, value)?
- Does each check run in the state the production caller actually produces?
- Is there a positive control that makes the key check observable?
- Can each check fail for a reason I can name out loud?
- Did I run it before concluding whether a defect exists?
Related skills
fewer-tests-more-proof— the suite-wide economics: battery-ifying a multi-implementation contract is one of its consolidation moves; this skill owns the battery craft.self-audit— its verification-honesty pass is where this rule fires during a branch audit.readable-code— check names are the battery's documentation; each one states its claim. Absent it, name each check for the promise it tests, not for the function it calls.rfc-writer— when the battery surfaces a contract question too big to settle in the fix.