Use when a bug report or failing test names a symptom and the code path producing it is not yet known — trace the call chain from symptom to entry point before proposing a fix. Trigger phrases: "why does this fail", "trace this bug", "find where this is called from", "what calls this function", "did a recent change cause this". Not a replacement for the general reproduce-isolate-fix discipline (a `systematic-debugging` skill, if present, governs that overall loop); reach for this skill specifically for the tracing step — once a symptom is located and the call chain to its entry point still needs walking before a hypothesis is formed.
