Working With Legacy Code

---
name: working-with-legacy-code
description: 'Safely change and test untested codebases using Feathers'' "Working Effectively with Legacy Code". Use when the user mentions "legacy code", "no tests", "untested codebase", "how do I test this", "seams", "characterization tests", "golden master", "sprout method", "afraid to change this code", "monster method", "dependency breaking", or "inherited a messy codebase". Also trigger when changing code without tests safely, getting a class under test when constructors, statics, or singletons block it, adding features to tangled modules, or planning incremental test coverage for an old codebase. Covers the legacy-code change algorithm, seams, characterization tests, sprout/wrap, and dependency-breaking techniques. For refactoring code that already has tests, see refactoring-patterns. For day-to-day code quality, see clean-code.'
license: MIT
metadata:
  author: wondelai
  version: "1.2.0"
---

# Working Effectively with Legacy Code

A field manual for changing code that has no tests, distilled from Michael C. Feathers' *Working Effectively with Legacy Code*. Use it to get untestable classes into a harness, pin down current behavior with characterization tests, and make changes one safe, verifiable step at a time — without resorting to a rewrite.

## Core Principle

**Legacy code is simply code without tests.** Not old code, not ugly code — untested code: without tests you cannot know whether a change preserves behavior, so every edit is a gamble. The craft is breaking dependencies just enough to get tests in place before changing anything — cover and modify, never edit and pray.

## Scoring

**Goal: 10/10.** Rate changes to untested code 0-10 against the principles below. Report the current score and the specific steps needed to reach 10/10.

- **9-10:** Change points covered by characterization tests before any edit; behavior changes and refactoring shipped as separate verified steps; dependencies broken with the least invasive technique
- **7-8:** Tests at most change points, but occasional mixed refactor-plus-behavior commits or heavier dependency surgery than needed
- **5-6:** Some characterization tests, yet key paths still changed on faith; sprouted code accumulating with no payback plan
- **3-4:** Edit-and-pray with manual verification; tests written after the change, asserting whatever the new code happens to do… install to load the full skill