Refactoring

---
name: refactoring
description: Safe, behavior-preserving code transformation backed by tests. Refactor with evidence, not instinct.
category: review
applies-to: [claude, gemini, cursor, copilot, any]
version: 1.0.0
---

## Overview

Refactoring changes the internal structure of code without changing its external behavior. The keyword is "without" — if behavior changes, it's not refactoring, it's modification. This skill enforces safe refactoring with tests as the safety net.

## When to Use

- When code is hard to understand or extend
- When duplication makes maintenance risky
- Before adding a feature to a messy area of code
- Never: "while also adding feature X" — refactor separately

## Process

### Step 1: Establish a Safety Net

1. Before touching a single line: ensure there are tests covering the code to be refactored.
2. If tests are missing: add characterization tests first. These capture current behavior, not desired behavior.
3. Run the tests. They should all pass. This is your baseline.

**Verify:** Tests pass. They cover the code being refactored.

### Step 2: Refactor in Micro-Steps

4. Make the smallest meaningful change.
5. Run tests after EVERY change — not after 10 changes.
6. If tests break: revert immediately and take a smaller step.
7. Never batch multiple refactoring changes together.

**Verify:** Tests pass after every individual change.

### Step 3: One Thing at a Time

8. Refactoring types cannot be mixed in one step:
   - Extract method → separate commit
   - Rename → separate commit
   - Move → separate commit
9. "Refactor and also fix this" is not refactoring — it's two PRs.

**Verify:** This commit does exactly one type of refactoring.

### Step 4: Verify No Behavior Change

10. Run the full test suite.
11. If integration/E2E tests exist: run them too.
12. Compare external API responses before and after (if applicable).

**Verify:** All tests pass. No observable behavior change.

## Common Rationalizations (and Rebuttals)

| Excuse | Rebuttal |
|--------|----------|
| "I'll add tests after refactoring" | You can't verify a behavior-preserving refactor without tests before the refactor. |
| "This change is obviously safe" | Obvious safety is how production incidents happen. Run the tests. |
| "I'll just do a quick cleanup" | "Quick cleanup" that changes behavior is a bug, not a refactor. |

## Verification

- [ ] Tests existed before any code was changed
- [ ] Tests run after every individual change
- [ ] Only one type of refactoring per commit
- [ ] Full test suite passes at the end
- [ ] No behavior change observable externally

## References

- [test-driven-development skill](../test-driven-development/SKILL.md)
- [simplicity-first skill](../simplicity-first/SKILL.md)
- Martin Fowler, "Refactoring"