Refactoring: change structure without changing behaviour

• Refactoring changes structure without changing behaviour. State the behaviour that must stay fixed and establish a test or characterization baseline first.
• Work in small, reviewable steps. Separate moves from behaviour changes so a regression has one obvious cause.
• Find seams: inject a dependency, extract a pure function, or isolate I/O behind an interface. Do not rewrite the world to test one branch.
• Keep the public interface stable unless the task explicitly changes it. Update every caller in the same change.
• Delete dead code only when you can prove it is unreachable; check reflection, configuration and external consumers first.
• Run the tests after each step. If behaviour changes, stop and decide whether that change belongs in this task.
• Report the structural change, the preserved behaviour and the tests that prove it.

NullByte skill pack. Original guidance for this tool; no third-party skill text is bundled.