| name | nw-mutation-test |
| description | Run an explicit mutation probe over the validated delivery delta, or support the project-level nightly-delta policy. Disabled by default. |
| user-invocable | true |
| argument-hint | [--delivery-contract <path> | --nightly-delta] [--threshold 80] |
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: validate the named DeliveryContract, then intersect its
production targets with the terminal candidate 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 intersection 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
- Detect the project's language and existing mutation runner. Do not impose a
Python-specific tool on another language.
- Prove the target's ordinary tests pass with the project's declared runner.
- Snapshot the user's worktree status.
- Create a dedicated disposable worktree or copy and run all mutations there.
Never mutate, reset or restore the user's worktree.
- 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.
