Debug: evidence-before-action investigation

The most expensive class of mistake in ML debugging is asserting a cause based on plausibility, then attempting a "fix" that masks the real problem. This skill enforces the discipline of probe → hypothesis → smoke → controls → claim, in that order.

The agentic Stop hook routes here from reason when an assistant claims a cause without backing tool output.

When to run

The user just said any of:

• "why is X failing / diverging / NaN / OOM / hung / slow / crashed"
• "the loss is going up", "metrics look weird", "GPU util is 0"
• "debug this", "diagnose", "troubleshoot", "investigate this run"
• pasted a log excerpt asking what's wrong

Five-step protocol

Step 1: cheap probes