Nw Mutation Test

Mutation Test

Mutation testing is disabled by default. This skill runs only after an explicit on-demand invocation or when the project itself declares Mutation Testing Strategy: nightly-delta.

It is a standalone diagnostic outer loop, not a DELIVER phase, rigor knob, completion gate, workflow state or substitute for semantic acceptance tests.

Input authority

• Explicit probe: use the named production diff.
• Nightly delta: use production files changed since the last successful mutation run.
• Never infer targets from docs/feature/, feature-delta, a progress ledger, filenames alone or the implementation agent's claims.

If the diff is empty, report not-applicable and stop. If a target is missing, outside the repository or not production code, fail loudly before starting a mutation tool.

Execution

1. Detect the project's language and existing mutation runner. Do not impose a Python-specific tool on another language.
2. Prove the target's ordinary tests pass with the project's declared runner.
3. Snapshot the user's worktree status.
4. Create a dedicated disposable worktree or copy and run all mutations there. Never mutate, reset or restore the user's worktree.
5. Compare the user's status with the snapshot and discard only the disposable target.

No supported runner is an honest indeterminate, not a fabricated PASS.

Result

Default threshold: 80% killed mutants.

• pass: kill rate >= threshold.
• fail: kill rate < threshold; name surviving mutants by production file, line or symbol and the smallest missing observable.
• indeterminate: runner unavailable, baseline tests fail, execution is interrupted or evidence cannot be attributed to the validated delta.
• not-applicable: validated delta contains no mutable production target.

Keep the interactive report concise: verdict, numerator/denominator, per-file survivor count, and a bounded list of actionable survivors. Do not create a per-delivery markdown report or another persistent progress artifact.

For nightly-delta, CI owns persistence: create or update one deduplicated issue containing the same bounded report and the exact comparison range. A later green run closes that issue. The nightly job must not expand to the whole repository when its baseline is missing; establish a new baseline explicitly instead.