Improve Test Quality Skill

A Claude Code slash command skill that automates mutation testing analysis and test quality improvement.

What It Does

This skill helps you improve test quality by:

1. Running Stryker mutation testing on a specific file
2. Analyzing survived mutants to identify weak spots in tests
3. Suggesting targeted test improvements
4. Implementing the improvements (with your approval)
5. Verifying improvements by re-running mutation testing
6. Iterating until target mutation score is reached

Prerequisites

• Stryker mutation testing must be configured (stryker.config.mjs)
• The file you want to test must have existing tests
• JSON reporter must be enabled in Stryker config (already configured)

Usage

Basic Usage

/improve-test-quality server/routes/projects.js

This will:

• Run mutation testing on server/routes/projects.js
• Analyze survived mutants
• Show recommendations
• Ask for your approval before making changes
• Target 85% mutation score by default

Auto Mode

/improve-test-quality server/routes/projects.js --auto

Automatically implements improvements without asking for approval at each step.

Custom Target Score

/improve-test-quality server/routes/projects.js --target 90

Set a custom target mutation score (default is 85%).

Limit Iterations

/improve-test-quality server/routes/projects.js --max-iterations 5

Set maximum number of improvement cycles (default is 3).

Dry Run

/improve-test-quality server/routes/projects.js --dry-run

Show recommendations without making any changes.

Combining Options

/improve-test-quality server/routes/projects.js --auto --target 90 --max-iterations 5

Workflow

1. You: Write initial tests to get ~60-70% coverage
2. You: Run /improve-test-quality <file-path>
3. Skill: Runs mutation testing and analyzes results
4. Skill: Shows you recommendations categorized by priority
5. You: Approve improvements (or skill auto-implements with --auto)
6. Skill: Adds test cases and strengthens assertions
7. Skill: Re-runs mutation testing to verify
8. Skill: Reports improvement (e.g., "76% → 89%")
9. Repeat if target not reached and iterations remaining

What Gets Improved

The skill targets common mutation testing issues:

High Priority

• Boundary conditions: Tests for exact threshold values (>= vs >)
• Boolean combinations: Tests for all true/false combinations (&& vs ||)
• Weak assertions: Strengthens status-only checks to verify response bodies

Medium Priority

• Arithmetic operations: Verifies calculation correctness (+ vs -)
• Return values: Checks specific return values, not just truthiness
• Block statements: Tests both branches of conditionals

Low Priority

• String literals: Often acceptable edge cases
• Optional fields: May not need testing if truly optional

Example Session

$ /improve-test-quality server/routes/projects.js

🔍 Analyzing test quality for server/routes/projects.js...

Current status:
- Test file: server/__tests__/routes/projects.test.js
- Target mutation score: 85%
- Max iterations: 3

Running mutation testing... (this may take 2-3 minutes)
✓ Mutation testing complete

Current mutation score: 76.84% (126 killed, 38 survived)

📊 Analysis Results:

Found 38 survived mutants across 4 categories:

High Priority (14 mutants):
  • Conditional boundaries (8 mutants)
    Lines: 145, 178, 203, 234, 267, 289, 312, 345
    Issue: Tests don't verify exact boundary conditions

  • Boolean operators (6 mutants)
    Lines: 156, 189, 221, 298
    Issue: Tests don't cover both true/false combinations

Medium Priority (12 mutants):
  • Arithmetic operations (5 mutants)
  • Return value mutations (7 mutants)

Low Priority (12 mutants):
  • String mutations (acceptable edge cases)

💡 Recommendations:

I can improve your test quality by:
1. Adding 8 boundary condition test cases
2. Adding 6 boolean combination tests
3. Strengthening 7 response assertions
4. Adding 5 arithmetic verification tests

Expected improvement: 76.84% → ~89% mutation score

Would you like me to:
1. Automatically implement all improvements
2. Implement only high-priority improvements
3. Let me review each category separately
4. Show me the specific changes first

Your choice (1-4): 1

Implementing improvements...
✓ Added 8 boundary condition tests
✓ Added 6 boolean combination tests
✓ Strengthened 7 assertions
✓ Added 5 arithmetic tests

Running tests to verify...
✓ All tests pass (40 tests, 0 failures)

Re-running mutation testing...
✓ Mutation testing complete

✅ Target reached! Final score: 89.47%

📈 Summary:
- Killed 22 additional mutants (76.84% → 89.47%)
- Added 26 new test assertions
- 12 mutants still surviving (low priority string mutations)

Changes made to: server/__tests__/routes/projects.test.js
Run `npm test` to verify all tests still pass.

Benefits

• Time savings: 1-2 hours of manual work → 10 minutes mostly automated
• Better insights: AI understands mutation patterns and suggests targeted fixes
• Higher quality: Systematic approach catches edge cases you might miss
• Learning tool: See patterns in mutation testing and improve your testing skills
• Consistency: Same high standard applied across all files

Limitations

• Requires Stryker mutation testing to be configured
• Works best with files that already have 60%+ test coverage
• Targets 85-90% mutation score (100% is rarely necessary or practical)
• Some survived mutants may be acceptable edge cases
• Mutation testing is slow (2-3 minutes per run)

Tips

1. Start with good coverage: Get to 60-70% line coverage before using this skill
2. Review changes: Even in auto mode, review the changes before committing
3. Iterate carefully: 85-90% is usually sufficient; don't chase 100%
4. Commit frequently: Commit after each successful improvement cycle
5. Learn patterns: Pay attention to what mutants survive to improve your testing habits

Troubleshooting

"File not in mutate array"

The skill will offer to add the file to stryker.config.mjs for you.

"Test file not found"

Make sure your test file follows conventions:

• server/routes/file.js → server/__tests__/routes/file.test.js

"Mutation testing failed"

Ensure all tests pass first: npm test

"JSON report not found"

The skill configuration should have already added the JSON reporter to stryker.config.mjs.

Configuration

The skill uses these defaults (overridable with flags):

• Target mutation score: 85%
• Max iterations: 3
• Mode: Interactive (ask before implementing)

Files Modified

This skill may modify:

• Your test file (adds tests, strengthens assertions)
• stryker.config.mjs (if file not in mutate array)

It will NOT:

• Modify your source code
• Commit changes automatically (you control commits)
• Run in CI/CD (it's a development tool)

Future Enhancements

Potential future features:

• Multi-file support
• Git auto-commit with descriptive messages
• Coverage improvement suggestions
• CI/CD integration recommendations
• Learning from past improvements

See Also

• Stryker Mutation Testing