Debug

1. Reproduce. Minimal deterministic repro. No repro -> no fix.
2. Root-cause. Ask "why" until one cause explains all symptoms. Symptoms != cause.
3. Regression test first (red), then fix at the root (green). Use tdd.

Don't

• Guess. Read the code and the full error.
• Change two things at once.
• Stop at the first plausible cause.

3 failed hypotheses -> re-read from scratch. Done = repro + regression test + root cause named + green.