What problem does it solve? Hard bugs and performance regressions often stall because developers jump straight to guessing fixes without a reliable reproduction. This Skill enforces a disciplined diagnosis loop — build a feedback loop, reproduce, hypothesise, instrument, fix, and regression-test — so root causes are found systematically instead of by luck. ## Core Features & Use Cases - Feedback loop construction: Ten ranked strategies for building an agent-runnable pass/fail signal, from failing tests and curl scripts to headless browser runs, fuzz loops, and git bisect harnesses. - Ranked falsifiable hypotheses: Generates 3-5 hypotheses with explicit predictions before testing, avoiding single-hypothesis anchoring. - Tagged instrumentation and cleanup: Every debug log gets a unique [DEBUG-...] prefix so cleanup is a single grep, plus a post-mortem checklist covering regression tests and architectural findings. - Use Case: A user reports that an export button intermittently throws an error. The Skill guides building a deterministic repro loop (or a HITL bash script via scripts/hitl-loop.template.sh if human clicks are required), ranking hypotheses, instrumenting the code path, and landing a fix with a regression test. ## Quick Start Ask the assistant to diagnose the bug you are experiencing and describe the exact symptom, error message, or performance regression you observed.