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Root-cause interrogation: a hypothesis ladder from a bug report

Prod bug, vague repro, clock ticking. Structure the investigation before you start changing code at random.

The prompt — copy and run it

You are a senior engineer running a structured root-cause investigation. I will paste the symptoms and every piece of evidence I have. Produce:

A) HYPOTHESIS LADDER — 5 ranked hypotheses. For each: the mechanism, which pasted evidence supports or contradicts it, and the cheapest discriminating test (a log line to add, a git bisect range, a feature-flag toggle, a minimal repro).

B) EVIDENCE GAPS — what I have not given you that would most change the ranking, in priority order.

C) PREVENTION — run 5-whys on the top hypothesis and name the class-level fix (test, lint rule, alert, type change) that prevents the whole category from recurring.

Inputs: [SYMPTOM + WHEN FIRST SEEN] · [PASTE LOGS / STACK TRACE / RECENT DIFFS] · [ENVIRONMENT + RECENT DEPLOYS]

Rules: Do not invent stack frames, log lines, or code behavior not in my paste — every hypothesis must cite the evidence line it rests on, and pure speculation must be labeled as such. Verify the discriminating test's result before acting on any hypothesis. Strip credentials and user PII from anything you echo back.

Why this prompt works

Ranked hypotheses with discriminating tests is how senior engineers actually debug — it converts panic into a binary-search plan. The prevention step turns one fix into a class fix, which is the difference between closing a ticket and improving the system.

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