Chases a bug or failure to its actual root cause through systematic evidence-gathering, rather than patching the first symptom. It is the diagnose-why-this-breaks step, distinct from reviewing a plan, reviewing a diff, or reporting overall code-quality metrics.
Situations it fits
Every morning at 3am the user-auth DB connection pool empties and the service sputters until the on-call engineer slaps on a quick hot-fix before breakfast.
Investigate. investigate-gstack is built to trace a live, recurring failure to its true underlying mechanism by collecting logs and repeated test failures rather than accepting quick hot-fixes or glancing at dashboards. Here the issue is a specific intermittent production crash that keeps reappearing at the same hour, which points to a need for deep forensic tracing; the team is not evaluating an unused design, not auditing a recently landed diff, and not summarizing system-wide health numbers, so only the forensic skill fits, though it's tempting to pick health-gstack because the crash recurs daily and looks like a trend.
At 2 a.m. the CI pipeline passes every test, yet the production app returns blank screens to shoppers at checkout. They rolled back once and the screens cleared, but the logs now show a customer-service thread hanging on a database call - so what do you do?
Investigate. the investigate-gstack skill exists to find the real reason a live system is misbehaving by gathering clues, not to grade a blueprint or a pre-merge pull request. In this scenario, a checkout thread stays frozen in production and a spin-down-and-up only hides the issue, so you need to figure out why that database call is stalling under load. The tempting distractor, review-gstack, would tempt you to look at the last diff that touched payment code, but that skill is meant to flag merge-blocking bugs in code about to ship, not to diagnose why a currently running service is broken in a live environment.
3 scenarios in the bank answer to Investigate. The rest are in the game.
Skills it gets confused with
These share a family with Investigate, which is another way of saying they are the ones you might reach for by mistake.