characterization-testing

name	characterization-testing
description	Pin a legacy system's current behavior with characterization / golden-master / approval tests before changing it, so a refactor that changes behavior fails loudly. Reach for this before any edit to untested legacy code.

Skill: Characterization testing

A legacy system's behavior is its spec — bugs included. Capture it before you touch it (§2 #1).

Step 1 — Find the seam

Locate a substitution point where you can drive the code and observe its output without editing in place. If there isn't one, get codebase-archaeologist to find a seam.

Step 2 — Capture current behavior (not desired behavior)

Write tests that assert what the code does today, even if it's wrong. The point is a tripwire for unintended change, not correctness.

Step 3 — Use approval/golden-master where outputs are large

For code with no specified expected value, snapshot the output (an approval test) and lock it as the golden master. Diffs against the snapshot surface behavior change.

Step 4 — Maximize coverage of the change area

Drive the inputs that exercise the branches you're about to touch. Coverage of the blast radius, not the whole system.

Step 5 — Refactor against the net

Now hand off to refactoring-engineer: every refactor stays green; any red is an unintended behavior change caught before it ships.