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NW-MUTATION-TEST: Feature-Scoped Mutation Testing · nwave-ai | DecimalAI

testingby nwave-ai

NW-MUTATION-TEST: Feature-Scoped Mutation Testing

nwave-ai/nw-mutation-test

Source nwave-ai/nwave ↗License MIT

Runs feature-scoped mutation testing to validate test suite quality. Use after implementation to verify tests catch real bugs (kill rate >= 80%).

SkillScore82Maintained

Automatic

QA & Test Engineering pack

Last verified 8/17/2026

What it changes

Passed 15 of 18 test cases with the skill, and 8 of 18 without it.

Without skill45%

With skill82%

Checks it passed that the model alone missed

Hands off delivery to Phase 8 Finalize
Sets test command to pytest -x tests/session/
Requires documenting justification in mutation-report.md

and 8 more in the full report

8 fixed10 passed either way4 still failing0 broke

Pass-rate lift: +36 pp
Tokens: +54%
Agent turns: 0%
Response time: -38%

Model gemini-3.6-flashMeasured 8/17/2026v118 of 22 cases passed with skill · 18 comparable

Tokens, turns and time describe resource use, not a dollar price.

Use this skill

Save it into your project's skills folder. Claude Code picks it up in its next session.

curl -fsSL --create-dirs -o .claude/skills/nwave-ai-nw-mutation-test/SKILL.md https://app.decimal.ai/s/nwave-ai-nw-mutation-test/SKILL.md

Want it pinned and updatable? pip install "decimalai>=0.15", then decimalai skills add nwave-ai-nw-mutation-test, and start Claude Code with claude --plugin-dir "$(git rev-parse --show-toplevel)/.agents"

1 · One-time — install the CLI (docs ↗)

pip install "decimalai>=0.15"

2 · Get the skill — writes .agents/skills/nwave-ai-nw-mutation-test/ and pins it in .decimal/skills.lock

decimalai skills add nwave-ai-nw-mutation-test

No CLI? Get the same file — curl · raw file

curl -fsSL --create-dirs -o .claude/skills/nwave-ai-nw-mutation-test/SKILL.md https://app.decimal.ai/s/nwave-ai-nw-mutation-test/SKILL.md

Raw file: /s/nwave-ai-nw-mutation-test/SKILL.md ↗ · machine-readable summary at /s/nwave-ai-nw-mutation-test.json ↗

Safety checks: 3 of 3 passed

Code scan CleanIntent PassedContent Clean

See it in action

One of 7 test cases the skill fixed. Same model, same prompt, with and without the skill.

1 of 7

Prompt

Following a successful mutation test execution and report generation for `cache-layer`, which workflow phase should receive handoff?

What was checked, without and with the skill
Checked	Without skill	With skill
Hands off delivery to Phase 8 Finalize	Missed	Passed

case-19 · gemini-3.6-flash · 8/17/2026 · answers are capped and redacted

What's inside

The instructions your model reads when the skill loads.

SKILL.md120 linessingle file

NW-MUTATION-TEST: Feature-Scoped Mutation Testing

Wave: QUALITY_GATE Agent: Crafter (nw-software-crafter)

Overview

Run mutation testing against implementation files from the current feature. Extracts targets from the execution-log.json (see Target extraction)|generates feature-scoped configs|delegates to software-crafter. Uses cosmic-ray (Python)|PIT (Java)|Stryker (JS/TS/C#).

Mutation Testing Strategy

Projects declare a strategy via ## Mutation Testing Strategy in CLAUDE.md: per-feature | nightly-delta | pre-release | disabled.

Default (when unspecified): nightly-delta — the recommended mode. CI runs mutmut nightly against modules changed since the last run (the delta), keeping per-feature delivery gates fast. /nw-mutation-test performs an explicit, on-demand feature-scoped run regardless of strategy; under nightly-delta the in-wave Phase 5 gate is skipped and the work is handled by the CI nightly pipeline.

Target extraction

Implementation files are selected from the execution-log.json (completed_steps[].files_modified.implementation).

Context Files Required

docs/feature/{feature-id}/deliver/execution-log.json - Implementation file extraction
scripts/mutation/generate_scoped_configs.py - Automated config generation (if available)

Pre-Invocation

Orchestrator performs before delegating:

Extract files — Read execution-log.json and extract implementation files from completed_steps[].files_modified.implementation. Gate: file list non-empty.
Verify on disk — Check all extracted files exist on disk. Gate: zero missing files.
Detect language — Scan config files (pyproject.toml, pom.xml, package.json, etc.) to select tool. Gate: language identified.
Confirm tests pass — Run pytest -x {test_scope} (or equivalent). Gate: exit code 0, no failures.
Ensure mutation venv — For Python, verify .venv-mutation/ exists with cosmic-ray installed. Gate: cosmic-ray --version succeeds.

Agent Invocation

@nw-software-crafter

Execute mutation testing for project {feature-id}.

Track record

Benchmark

15 / 18

test cases passed, vs 8 / 18 without

Adoption

Users: 0
Times used: 0

Safety and source details

SkillSafety

✓ 3 of 3 passed

Code scan15 of 15 checks clean

0 critical · 0 warning · 0 info — scanner v4 — 8/3/2026

View 15 scanner checks

live_secretremote_code_executionreverse_shelldata_exfiltrationssrf_metadataagent_snoopinganti_refusaldestructive_commandsobfuscated_unicodeunicode_homoglyphopaque_blobsuspicious_urltool_overreachtrigger_abuseprompt_injection_phrasing

IntentSafe

LLM judge · does the skill do what it claims

ContentClean

LLM judge · policy & harmful-content review

Source

Imported

nwave-ai / nwave

nWave/skills/nw-mutation-test/SKILL.md

View on GitHub ↗

License MITImported 8/3/2026Version v1

More skills from the same source and nearby in the registry, with their headline benchmark lift.

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See it in action

What's inside

NW-MUTATION-TEST: Feature-Scoped Mutation Testing

Overview

Mutation Testing Strategy

Target extraction

Context Files Required

Pre-Invocation

Agent Invocation

Track record

Benchmark

Adoption

SkillSafety

Source

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