Imported from majiayu000/claude-skill-registry (skills/data/forensic-test-analysis/SKILL.md). Install upstream with npx skills add majiayu000/claude-skill-registry --skill forensic-test-analysis. Copyright stays with the author.

Forensic Test Analysis

🎯 When You Use This Skill

State explicitly: "Using forensic-test-analysis pattern"

Then follow these steps:

1. Calculate test change frequency vs production code changes
2. Identify brittle tests (coupling ratio >2x = test changes more than prod)
3. Find large test files (>500 LOC = maintenance burden)
4. Cite research when presenting findings (brittle tests = 2-3x maintenance cost)
5. Suggest integration with hotspot-finder and complexity-trends at end

Overview

Test analysis examines test code quality through git forensics. Unlike static test coverage tools, this reveals:

• Brittle tests - Change more frequently than production code
• Over-coupled tests - Break with every production change
• Test hotspots - High-churn test files requiring constant fixes
• Duplicate test logic - Copy-paste test code (maintenance burden)
• Large test files - Unmaintainable test suites
• Slow tests - Impact CI/CD cycle time

Core principle: Good tests are stable. If tests change more than production code (ratio >2x), they're brittle and expensive.

When to Use

• Investigating slow or flaky CI/CD pipelines
• Reducing test maintenance burden
• Before refactoring test suites
• Diagnosing "broken tests" tickets frequency
• Quarterly test health checks
• After major refactoring (did tests improve?)
• Justifying test refactoring investment

When NOT to Use

• Insufficient git history (<6 months unreliable)
• No test files (obviously)
• Greenfield projects (no patterns yet)
• When you need test coverage metrics (use coverage tools)
• When you need defect correlation (use hotspot analysis)

Core Pattern

⚡ THE TEST BRITTLENESS FORMULA (USE THIS)

This is the test health metric - don't create custom ratios:

Test Brittleness Ratio = test_changes / production_changes

Interpretation:
  - >2.0:  BRITTLE (test changes more than prod - expensive)
  - 1.0-2.0: NORMAL (tests evolve with production)
  - 0.5-1.0: GOOD (stable tests, well-designed)
  - <0.5:  UNDER-TESTED or integration tests (fewer changes expected)

Test File Size Risk:
  - >500 LOC:  CRITICAL (unmaintainable)
  - 300-500 LOC: HIGH (should split)
  - 150-300 LOC: MODERATE (monitor)
  - <150 LOC:  GOOD (focused tests)

Test Hotspot = Brittle (>2x) + High Changes (>20 commits/year)

Critical: Ratio >2x indicates tests are MORE expensive to maintain than production code.

📊 Research Benchmarks (CITE THESE)

Always reference research when presenting test findings:

Always cite the source when justifying test refactoring investment.

Quick Reference

Essential Git Commands

Test Health Classification

Common Test Anti-Patterns

Implementation

Step 1: Identify Test Files

Gather test file list:

# Find all test files (adapt patterns to your project)
test_files=$(find . -type f \
  -name "*.test.js" -o \
  -name "*.test.ts" -o \
  -name "*.spec.js" -o \
  -name "*_test.py" -o \
  -name "*Test.java")

# Get corresponding production files
# (remove .test/.spec from filename)

Step 2: Calculate Brittleness Ratio

For each test file:

# Pseudocode for brittleness calculation

def calculate_brittleness(test_file, production_file):
    # Count test file changes
    test_changes = git_log_count(test_file, since="12 months ago")

    # Count production file changes
    prod_changes = git_log_count(production_file, since="12 months ago")

    if prod_changes == 0:
        return None  # No production changes to compare

    # Calculate ratio
    brittleness_ratio = test_changes / prod_changes

    # Classify
    if brittleness_ratio > 2.0:
        classification = "BRITTLE"
        severity = "CRITICAL"
    elif brittleness_ratio > 1.5:
        classification = "BRITTLE"
        severity = "HIGH"
    elif brittleness_ratio > 1.0:
        classification = "MODERATE"
        severity = "MEDIUM"
    else:
        classification = "GOOD"
        severity = "LOW"

    return {
        'test_changes': test_changes,
        'prod_changes': prod_changes,
        'ratio': brittleness_ratio,
        'classification': classification,
        'severity': severity
    }

Step 3: Detect Test-Only Commits

Identify pure test maintenance:

def find_test_only_commits(since="12 months ago"):
    # Get all commits
    commits = git_log(since=since, name_only=True)

    test_only_commits = []
    for commit in commits:
        changed_files = commit.files

        # Check if only test files changed
        all_tests = all(is_test_file(f) for f in changed_files)

        # Check for brittle test keywords
        brittle_keywords = ['fix failing test', 'update snapshot',
                           'fix flaky', 'fix test', 'test fix']
        is_brittle = any(kw in commit.message.lower() for kw in brittle_keywords)

        if all_tests and is_brittle:
            test_only_commits.append({
                'hash': commit.hash,
                'message': commit.message,
                'files': changed_files,
                'category': 'BRITTLE_TEST_MAINTENANCE'
            })

    return test_only_commits

High count of test-only commits = brittle test suite

Step 4: Analyze Test File Size

Flag large test files:

def analyze_test_sizes():
    large_tests = []

    for test_file in find_test_files():
        loc = count_lines(test_file)

        if loc > 500:
            severity = "CRITICAL"
        elif loc > 300:
            severity = "HIGH"
        elif loc > 150:
            severity = "MODERATE"
        else:
            severity = "LOW"

        if severity in ["CRITICAL", "HIGH"]:
            large_tests.append({
                'file': test_file,
                'loc': loc,
                'severity': severity,
                'recommendation': 'Split into smaller test files'
            })

    return large_tests

Output Format

1. Executive Summary

Test Suite Health Assessment (forensic-test-analysis pattern)

Test Files: 247
Production Files: 312
Test-to-Production Ratio: 0.79:1

KEY FINDINGS:

Brittle Tests (>2x changes): 18 files (7%)
Large Test Files (>500 LOC): 12 files
Test-Only Commits: 89 commits (23% of test commits)
Test Hotspots (brittle + high-churn): 8 files

Research shows brittle tests cost 2-3x more to maintain (Google).

Estimated Annual Test Maintenance Cost: $45,000
  - Brittle test fixes: $28,000
  - Large file maintenance: $12,000
  - Duplicate code: $5,000

2. Test Hotspots (Brittle + High-Churn)

Rank | Test File                | Test Chg | Prod Chg | Ratio | LOC | Status
-----|--------------------------|----------|----------|-------|-----|----------
1    | auth/login.test.js      | 42       | 15       | 2.8x  | 687 | 🚨 CRITICAL
2    | api/users.spec.js       | 35       | 18       | 1.9x  | 523 | ❌ HIGH
3    | checkout.test.ts        | 48       | 22       | 2.2x  | 445 | ❌ HIGH
4    | Form.test.tsx           | 38       | 14       | 2.7x  | 392 | ❌ HIGH

3. Detailed Test Analysis

=== TEST HOTSPOT #1: auth/login.test.js ===

Brittleness Metrics:
  Test Changes (12mo): 42 commits
  Production Changes: 15 commits (login.js)
  Brittleness Ratio: 2.8x (CRITICAL - tests change faster than prod)
  Lines of Code: 687 (CRITICAL - unmaintainable size)

Research: Brittle tests cost 2-3x more to maintain (Google).

Change Pattern Analysis:
  - 14 commits: "fix failing test" (33% - pure maintenance)
  - 11 commits: "update snapshots" (26% - brittle snapshots)
  - 10 commits: aligned with production (24% - expected)
  - 7 commits: "refactor tests" (17%)

Issues Identified:
  ⚠️  Brittle: 2.8x change ratio (expected ~1.0x)
  ⚠️  Large: 687 LOC (expected <300 LOC)
  ⚠️  Snapshot-heavy: 26% of changes are snapshot updates
  ⚠️  Maintenance burden: 33% pure test fixes

RECOMMENDATIONS:
1. IMMEDIATE: Replace snapshots with semantic assertions
2. SHORT-TERM: Split into 3 smaller test files (~200 LOC each)
3. MEDIUM-TERM: Decouple tests from implementation details
4. PROCESS: Add test brittleness check to CI

Expected Impact: -60% maintenance cost, -70% brittleness ratio

4. Test-Only Commit Analysis

Brittle Test Maintenance (Test-Only Commits):

Total Test Commits: 387
Test-Only Commits: 89 (23% - maintenance overhead)

Top Brittle Tests (by fix commits):
  1. auth/login.test.js: 14 "fix" commits
  2. api/users.spec.js: 11 "fix" commits
  3. checkout.test.ts: 9 "fix" commits

Pattern: 23% of test effort is pure maintenance (not new tests)
Impact: Wasted effort, developer frustration

Research: Brittle tests cost 2-3x more to maintain (Google).

Common Mistakes

Mistake 1: Ignoring brittleness ratio

Problem: Only looking at test change count, not comparing to production.

# ❌ BAD: Just count test changes
high_churn_tests = tests with >20 changes

# ✅ GOOD: Calculate brittleness ratio
brittle_tests = tests where (test_changes / prod_changes) > 2.0

Fix: Always calculate ratio - 30 test changes with 30 prod changes is normal, not brittle.

Mistake 2: Treating all snapshot commits as bad

Problem: Flagging legitimate snapshot updates as brittle.

Fix: Distinguish between:

• Legitimate: Snapshot updates with corresponding UI changes
• Brittle: Frequent snapshot updates without meaningful prod changes (>5 per year)
• Always check: If "update snapshots" commit has NO production changes = brittle

Mistake 3: Not checking test file size

Problem: Focusing only on change frequency, missing unmaintainable large files.

# ❌ BAD: Only brittleness
flag tests with ratio > 2.0

# ✅ GOOD: Combine brittleness + size
flag tests where (ratio > 2.0 OR size > 500)

Fix: Always check file size - large files (>500 LOC) are maintenance burdens even if stable.

Mistake 4: Not estimating test maintenance cost

Problem: Identifying brittle tests without quantifying business impact.

Fix: Calculate cost:

• Average commit time: 30 minutes
• Brittle test commits: 89 per year
• Cost: 89 × 0.5 hours × $100/hour = $4,450/year per brittle test file
• Always translate to dollars for executive justification

⚡ After Running Test Analysis (DO THIS)

Immediately suggest these next steps to the user:

1. Correlate with production hotspots (use forensic-hotspot-finder)

   • Are brittle tests testing hotspot code?
   • Hotspot + brittle test = double maintenance burden
   • Prioritize refactoring both together

2. Check test complexity trends (use forensic-complexity-trends)

   • Are test files growing in complexity?
   • Track whether test refactoring is working
   • Set up monitoring for test file sizes

3. Calculate refactoring ROI (use forensic-refactoring-roi)

   • Test maintenance cost = annual waste
   • Test refactoring investment = effort estimation
   • ROI typically very high (brittle tests are expensive)

4. Track test health monthly

   • Re-run test analysis quarterly
   • Monitor brittleness ratio trends
   • Early warning for emerging brittle tests

Example: Complete Test Analysis Workflow

"Using forensic-test-analysis pattern, I analyzed 247 test files.

TEST HEALTH ASSESSMENT:

Brittle Tests: 18 files (7% of test suite)
  - Brittleness ratio >2.0x (tests change faster than production)
  - Research shows 2-3x higher maintenance cost (Google)

TOP BRITTLE TEST:

auth/login.test.js:
  - Ratio: 2.8x (42 test changes vs 15 prod changes)
  - Size: 687 LOC (CRITICAL)
  - Pattern: 33% "fix failing test" commits
  - Cost: ~$8,400/year in maintenance

ESTIMATED ANNUAL COST: $45,000 in brittle test maintenance

RECOMMENDATIONS:
1. Replace snapshot tests with semantic assertions
2. Split large test files (>500 LOC)
3. Decouple tests from implementation details

NEXT STEPS:
1. Check production hotspots (forensic-hotspot-finder) - Testing hotspot code?
2. Track complexity trends (forensic-complexity-trends) - Are tests growing?
3. Calculate ROI (forensic-refactoring-roi) - Business case for cleanup

Would you like me to proceed with hotspot correlation?"

Always provide this integration guidance - test issues often indicate production code quality problems.

Advanced Patterns

Test-Production Co-Change Analysis

Find which tests always change with production:

Co-Change Pattern:

login.test.js ↔ login.js:
  - 15 commits changed both together (expected)
  - 27 commits changed ONLY login.test.js (brittle!)

Ratio Analysis:
  - Expected: 1:1 co-change
  - Actual: 1:2.8 (test changes 2.8x more)

Conclusion: Tests over-coupled to implementation details

Test Refactoring Impact Validation

Measure before/after:

Before Refactoring (auth/login.test.js):
  - Brittleness: 2.8x
  - Size: 687 LOC
  - Maintenance commits: 14/year

After Refactoring (Q2 2024):
  - Brittleness: 1.1x (-61%)
  - Size: 245 LOC (-64%)
  - Maintenance commits: 2/year (-86%)

VALIDATION: ✅ Refactoring successful
Annual savings: $7,200 (from $8,400 to $1,200)

Flaky Test Detection

If test execution data available:

Flaky Tests (intermittent failures):

checkout.test.ts:
  - 12 "fix flaky test" commits
  - Pattern: Failures on CI but pass locally
  - Root cause: Race conditions, timing dependencies

Impact: Developer context switching, CI/CD unreliability
Fix: Condition-based waiting, not arbitrary timeouts

Research Background

Key studies:

1. Google Testing Blog (2017): Test brittleness cost

   • Brittle tests cost 2-3x more to maintain than stable tests
   • Snapshot tests are particularly brittle
   • Recommendation: Use semantic assertions, not snapshots

2. Microsoft DevOps (2019): Test duplication impact

   • 40-60% of test effort wasted on duplicate test logic
   • Copy-paste tests create maintenance burden
   • Recommendation: Extract test helpers, reduce duplication

3. Continuous Delivery (Humble & Farley): Slow test impact

   • Slow tests waste 20-30 minutes per developer per day
   • Developers skip running tests if they're too slow
   • Recommendation: Optimize test execution, parallelize

4. Test Maintenance Research (Garousi et al, 2013): Test code quality

   • Test code quality predicts test effectiveness
   • Large test files correlate with defects
   • Recommendation: Apply same quality standards to test code

Why test quality matters: Poor test quality wastes developer time, reduces confidence, and creates maintenance burden exceeding test value.

Integration with Other Techniques

Combine test analysis with:

• forensic-hotspot-finder: Brittle tests on hotspot code = double maintenance burden
• forensic-complexity-trends: Track test complexity over time
• forensic-refactoring-roi: Test refactoring typically has very high ROI
• forensic-debt-quantification: Test maintenance is quantifiable technical debt

Why: Test quality affects developer productivity - poor tests slow everyone down.