Diagnose

Discipline for hard bugs. Skip phase only if justify.

When explore codebase, use project domain glossary for mental model of relevant modules, check ADRs in area you touch.

Phase 1 -- Build a feedback loop

This the skill. Rest mechanical. Fast, deterministic, agent-runnable pass/fail signal for bug -> find cause; bisection, hypothesis-test, instrument just eat that signal. No loop -> no staring at code save you. Spend big effort here. Be aggressive. Be creative. No give up.

Strategies (try in roughly this order)