Debug Systematically

Debug systematically

1. Reproduce before touching anything. Find the exact command or input that shows the failure and run it. A bug you cannot reproduce is a bug you cannot claim to have fixed.
2. Read the actual error, not the vibe of it. The message, the file, the line. Half of debugging is refusing to skim.
3. One hypothesis at a time. State it ("the config is read before the env is set"), pick the cheapest observation that would falsify it, run that. Never stack three speculative fixes in one edit.
4. Instrument close to the failure. A print or assert at the suspect line beats re-reading the whole module. Remove the instrumentation after.
5. Bisect when lost. Halve the search space: does the failure survive with half the input, the previous commit, the feature flag off? git bisect exists for exactly this.
6. When the fix lands, explain the mechanism in one sentence. "The cache key ignored the locale, so the first locale won." If you cannot say it, you patched a symptom.
7. Re-run the original reproduction last. The fix is proven by the same command that showed the bug, plus the neighboring tests.