Build Refactoring Test Suite

When to Use

You are about to refactor code — extracting methods, moving fields, changing conditionals — and one of these is true:

• The code has no tests at all
• The existing tests are incomplete, untargeted, or only cover happy paths
• A bug has been reported and you need to pin it down before fixing and refactoring around it
• You are inheriting code from someone else and want a safety net before touching anything

This is the Level 0 foundation skill: every other refactoring mechanic in Fowler's catalog assumes this suite exists. Without it, you are refactoring blind. With it, every subsequent step is reversible — if a test turns red, you revert and try smaller steps.

The core pattern: build a self-checking test suite that runs in seconds, covers the code you are about to change, and can answer one question without human inspection: "Did I break anything?"

Before starting, confirm you have:

• Read access to the class or module being refactored
• A test runner installed for the language (pytest, JUnit, RSpec, Vitest, go test, etc.)
• The ability to run the test suite from the command line