Nw Mutation Test Skill for Claude

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}.

Context to pass inline (agent has no Skill access):

• Project ID
• Implementation file list (from execution-log.json)
• Test scope path (e.g., tests/des/)
• Kill rate threshold (default: 80%)
• Language and tool selection

Configuration:

• threshold: 80 (percentage, minimum kill rate)
• approach: feature-scoped (one config per component, scoped test commands)
• config_generator: scripts/mutation/generate_scoped_configs.py (preferred over manual)

Output file: docs/feature/{feature-id}/deliver/mutation/mutation-report.md

Examples

Example 1: Python project with config generator

/nw-mutation-test des-hook-enforcement tests/des/

Reads the execution-log.json, runs generate_scoped_configs.py des-hook-enforcement, delegates to software-crafter with per-component configs. Agent runs cosmic-ray, produces mutation-report.md.

Example 2: Python project without config generator

/nw-mutation-test auth-upgrade tests/auth/

Extracts files manually from the execution-log.json, creates single cosmic-ray config with module-path = [file1, file2, ...] and test-command = "pytest -x tests/auth/", delegates to agent.

Example 3: Non-Python project

/nw-mutation-test payment-gateway tests/payment/

Detects package.json, selects Stryker, delegates with Stryker-specific instructions.

Success Criteria

• [ ] Implementation files extracted from execution-log.json
• [ ] All implementation files verified on disk
• [ ] Mutation testing executed without errors
• [ ] Per-file breakdown in mutation-report.md
• [ ] Kill rate meets threshold (>= 80% PASS, 70-80% WARN, < 70% FAIL)
• [ ] Source files restored to HEAD after mutation run (git checkout -- src/ tests/)

Post-Mutation Safety (mandatory)

After EVERY mutation run (success, failure, or interruption):

• Restore source files — Run git checkout -- src/ tests/. Gate: working tree clean (no mutations remain).
• Verify no corruption — Confirm test suite still passes after restore. Gate: pytest -x {test_scope} exits 0.

Mutation tools apply mutations directly to source files. An interrupted run can leave corrupted code (e.g. is not None -> is None). Agent MUST execute these steps even if the run errors out.

Quality Gate

Kill rate thresholds:

• >= 80% PASS — Proceed to next wave.
• 70-80% WARN — Review surviving mutants, document findings, proceed with caution.
• < 70% FAIL — Add tests targeting surviving mutants before proceeding.

Skip conditions (each requires documented justification in mutation-report.md):

• No tool for language — No mutation framework available for detected language.
• Project opt-out — .mutation-config.yaml has skip: true with justification.
• Broken test suite — Pre-invocation step 4 fails; fix tests before mutation testing.

Note: Python projects require mutation testing. All skips need documented justification.

Next Wave

Handoff To: Phase 8 - Finalize (orchestrator continues develop.md workflow)

Deliverables: docs/feature/{feature-id}/deliver/mutation/mutation-report.md

Expected Outputs

docs/feature/{feature-id}/deliver/mutation/
  mutation-report.md
  cosmic-ray-*.toml                (ephemeral)