book-working-effectively-with-legacy-code-nano

name	book--working-effectively-with-legacy-code--nano
description	Working with Legacy Code (Michael Feathers) — Minimal rules — essential one-liners only. Use when asked to apply Working with Legacy Code principles or review code against Working with Legacy Code standards.
origin	github.com/ciembor/agent-rules-books (MIT)
license	MIT
version	1.0.0
compatibility	yana-ai >= 0.14.0

OBEY Working Effectively with Legacy Code by Michael Feathers

When to use

Use when changing poorly tested or poorly understood code under a tight context budget.

Primary bias to correct

Legacy work starts with control, not cleanup, rewrite, or elegance.

Decision rules

• Treat code without trustworthy tests as legacy code: state what changes and what must remain.
• Characterize uncertain current behavior before changing it, including ugly behavior consumers may rely on.
• Use the legacy loop: find the change point, find an observation point, create or exploit a seam, break the blocking dependency, test, change, then refactor locally.
• Prefer fast focused tests; use broader harnesses only as temporary first coverage when no narrow test point exists.
• Create the narrowest useful seam for sensing or separation, and break only dependencies that block feedback.
• Use sprout, wrap, parameterize, inject, extract, or override moves when direct edits would be unsafe.
• Keep behavior changes, structural refactorings, and cleanup separate and small.
• Do not leave test-only seams, hidden dependencies, wrappers, globals, subclass tricks, or link/preprocessor tricks without a cleanup plan.

Trigger rules

• When behavior is unclear, characterize first.
• When constructors, globals, statics, frameworks, I/O, clocks, randomness, environment, or deep object graphs block testing, break one dependency at the narrowest point.
• When a large method or class defeats local reasoning, sketch effects and create a seam before semantic edits.
• When rewrite or broad cleanup feels tempting, choose the next smaller verified move.

Final checklist

• Behavior characterized?
• Feedback fast enough?
• Dependency isolated?
• One kind of change?
• Safer and clearer now?