# Refactoring > Guide code refactoring using Martin Fowler's catalog of behavior-preserving transformations. Use during TDD REFACTOR phase, when code smells are detected (duplication, long methods, feature envy), when discussing structural improvements, or before behavioral changes (Tidy First). Use when this capability is needed. - Skill: `tomevault-io/refactoring` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/refactoring` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/refactoring/raw - Safety review: pending (external: skill-scanner PASS, skillspector PASS) - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/refactoring --- # Refactoring Skill Apply behavior-preserving transformations to improve quality of any codes, including tests. ## Core Principle > "Refactoring is a disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behavior." — Martin Fowler ## Structural Changes Only **Allowed:** - Renaming for clarity - Extracting functions/methods - Removing duplication - Simplifying expressions - Moving files/modules between directories - Reorganizing module boundaries **NOT allowed:** - Adding new functionality - Fixing bugs (if you find one, write a failing test first) - Changing observable behavior in any way See `library-application-criteria.md` for public API constraints (libraries vs applications). ## Refactoring Workflow 1. **Ensure tests pass** (you need a safety net) 2. **Identify ONE smell** to address 3. **Choose appropriate pattern** from `patterns.md` 4. **Apply mechanically** in small steps 5. **Run tests after each step** 6. **Commit when green** ## Safety Rules - **Never refactor with failing tests** - **One pattern at a time** - **Commit after each successful refactor** - **Revert immediately if tests fail** --- > Converted and distributed by [TomeVault](https://tomevault.io/claim/atusy) — claim your Tome and manage your conversions.