MR-21 — A claim that a guard bites cites the named assertion

Evidence that a check catches a defect records the mutation, the name of the assertion that failed, and the revert. A red exit status is not that evidence: a run that fails before reaching any assertion reads exactly like one that caught the mutation.

MR-20 asks whether a passing check looked at anything; MR-21 asks whether a failing run failed for the guard’s reason. Tooling that refuses a survived mutation, a red run naming no assertion, and a file left mutated satisfies the checkable half; whether the named assertion is the right one stays prose a human reads. (Trigger: an arno-host mutation probe that went red on a YAMLParseError from its own bad edit, with the guard never running. Ratified by parent-repo decision doc docs/decisions/2026-11-10-meta-rule-change-mutation-evidence-mr-21.md.)

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