How to Reproduce a Bug

Reproduction is the skill that makes every fix possible: turn a vague report into a bug you can trigger on demand, shrink it to its essence, and tame the intermittent ones that won't show up when you're watching.

1. Why Reproduction Is the Whole Game You can't fix - or verify a fix for - a bug you can't trigger on demand. Reproduction converts a vague report into a controllable experiment. The whole skill is two moves: make it happen reliably, then shrink it.
2. Nailing It Down Four variables decide whether a bug reproduces: the exact steps, the environment, the input data, and the state/timing. Pin all four, then shrink to a minimal reproduction by removing everything that isn't load-bearing. 'Works on my machine' is one of these four differing.
3. When It Won't Reproduce (Heisenbugs) Intermittent bugs have a hidden input you haven't pinned: timing/races, uninitialized state, an external dependency, or caching. The tactics that make them reliable: log around it, force the timing, control randomness and the clock, and pin the dependency.