regression-testing skill

---
name: regression-testing
description: "Strategic regression testing with test selection, impact analysis, and continuous regression management. Use when verifying fixes don't break existing functionality, planning regression suites, or optimizing test execution for faster feedback."
category: specialized-testing
priority: high
tokenEstimate: 1000
agents: [qe-regression-risk-analyzer, qe-test-executor, qe-coverage-analyzer]
implementation_status: optimized
optimization_version: 1.0
last_optimized: 2025-12-02
dependencies: []
quick_reference_card: true
tags: [regression, test-selection, impact-analysis, ci-cd, change-based, risk-based]
---

# Regression Testing

<default_to_action>
When verifying changes don't break existing functionality:
1. ANALYZE what changed (git diff, impact analysis)
2. SELECT tests based on change + risk (not everything)
3. RUN in priority order (smoke → selective → full)
4. OPTIMIZE execution (parallel, sharding)
5. MONITOR suite health (flakiness, execution time)

**Quick Regression Strategy:**
- Per-commit: Smoke + changed code tests (5-10 min)
- Nightly: Extended regression (30-60 min)
- Pre-release: Full regression (2-4 hours)

**Critical Success Factors:**
- Smart selection catches 90% of regressions in 10% of time
- Flaky tests waste more time than they save
- Every production bug becomes a regression test
</default_to_action>

## Quick Reference Card

### When to Use
- After any code change
- Before release
- After dependency updates
- After environment changes

### Regression Types
| Type | When | Scope |
|------|------|-------|
| **Corrective** | No code change | Full suite |
| **Progressive** | New features | Existing + new |
| **Selective** | Specific changes | Changed + dependent |
| **Complete** | Major refactor | Everything |

### Test Selection Strategies
| Strategy | How | Reduction |
|----------|-----|-----------|
| **Change-based** | Git diff analysis | 70-90% |
| **Risk-based** | Priority by impact | 50-70% |
| **Historical** | Frequently failing | 40-60% |
| **Time-budget** | Fixed time window | Variable |

---

## Change-Based Test Selection

```typescript
// Analyze changed files and select impacted tests
function selectTests(changedFiles: string[]): string[] {
  const testsToRun = new Set<string>();

  for (const file of changedFiles) {
    // Direct tests
    testsToRun.add(`${file.replace('.ts', '.test.ts')}`);

    // Dependent tests (via coverage mapping)
    const dependentTests = testCoverage[file] || [];
    dependentTests.forEach(t => testsToRun.add(t));
  }

  return Array.from(testsToRun);
}

// Example: payment.ts changed
// Runs: payment.test.ts, checkout.integration.test.ts, e2e/purchase.test.ts
```

---

## Regression Suite Pyramid

```
         /\
        /  \  Full Regression (weekly)
       /    \  - All tests (2-4 hours)
      /------\
     /        \  Extended Regression (nightly)
    /          \  - Unit + integration + critical E2E (30-60 min)
   /------------\
  /              \  Quick Regression (per commit)
 /________________\  - Changed code + smoke tests (5-10 min)
```

---

## CI/CD Integration

```yaml
# .github/workflows/regression.yml
jobs:
  quick-regression:
    runs-on: ubuntu-latest
    timeout-minutes: 15
    steps:
      - name: Analyze changes
        id: changes
        uses: dorny/paths-filter@v2
        with:
          filters: |
            payment:
              - 'src/payment/**'
            auth:
              - 'src/auth/**'

      - name: Run affected tests
        run: npm run test:affected

      - name: Smoke tests (always)
        run: npm run test:smoke

  nightly-regression:
    if: github.event_name == 'schedule'
    timeout-minutes: 120
    steps:
      - run: npm test -- --coverage
```

---

## Optimization Techniques

```javascript
// 1. Parallel execution
module.exports = {
  maxWorkers: '50%', // Use half CPU cores
  testTimeout: 30000
};

// 2. Sharding across CI workers
// npm test -- --shard=1/4

// 3. Incremental testing (only changed since last run)
// Track last run state, skip passing unchanged tests

// 4. Fast-fail on smoke
// Run critical tests first, abort if they fail
```

---

## Agent-Driven Regression

```typescript
// Smart test selection
await Task("Regression Analysis", {
  pr: 1234,
  strategy: 'change-based-with-risk',
  timeBudget: '15min'
}, "qe-regression-risk-analyzer");

// Returns:
// {
//   mustRun: ['payment.test.ts', 'checkout.integration.test.ts'],
//   shouldRun: ['order.test.ts'],
//   canSkip: ['profile.test.ts', 'search.test.ts'],
//   estimatedTime: '12 min',
//   riskCoverage: 0.94
// }

// Generate regression test from production bug
await Task("Bug Regression Test", {
  bug: { id: 'BUG-567', description: 'Checkout fails > 100 items' },
  preventRecurrence: true
}, "qe-test-generator");
```

---

## Agent Coordination Hints

### Memory Namespace
```
aqe/regression-testing/
├── test-selection/*     - Impact analysis results
├── suite-health/*       - Flakiness, timing trends
├── coverage-maps/*      - Test-to-code mapping
└── bug-regressions/*    - Tests from production bugs
```

### Fleet Coordination
```typescript
const regressionFleet = await FleetManager.coordinate({
  strategy: 'comprehensive-regression',
  agents: [
    'qe-regression-risk-analyzer',  // Analyze changes, select tests
    'qe-test-executor',             // Execute selected tests
    'qe-coverage-analyzer',         // Analyze coverage gaps
    'qe-quality-gate'               // Go/no-go decision
  ],
  topology: 'sequential'
});
```

---

## Related Skills
- [risk-based-testing](../risk-based-testing/) - Risk-based prioritization
- [test-automation-strategy](../test-automation-strategy/) - Automation pyramid
- [continuous-testing-shift-left](../continuous-testing-shift-left/) - CI/CD integration

---

## Remember

**Regression testing is insurance against change.** Every code change is a risk. Smart regression testing mitigates that risk by testing what matters based on what changed.

**Good regression testing is strategic, not exhaustive.** You cannot test everything, every time. Select based on changes, risk, and time budget.

**With Agents:** `qe-regression-risk-analyzer` provides intelligent test selection achieving 90% defect detection in 10% of execution time. Agents generate regression tests from production bugs automatically.