The goal is an evidence-backed causal explanation, not a list of plausible fixes.
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Frame the problem. Record the observed and expected behavior, scope, impact, environment, and when it began. Keep observations separate from interpretations.
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Locate the divergence. Gather the smallest relevant evidence and trace the behavior toward the earliest point where reality differs from expectation. Preserve useful inputs, versions, timestamps, and environment details.
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Reproduce when safe. Find the smallest faithful check. For intermittent problems, repeat under controlled conditions and vary one factor at a time. If reproduction is not possible, state what the available evidence can and cannot establish.
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Test competing explanations. Keep only plausible hypotheses. For each, identify an observation it predicts; choose the next check for its ability to distinguish explanations, and look for evidence that could disprove the leading one.
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Establish the causal mechanism. Trace the evidence from symptom to trigger to responsible boundary and mechanism. Challenge the strongest alternative. Do not treat correlation or proximity as proof of cause.
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Report the result. State whether the cause is confirmed, likely, or unresolved; give the supporting evidence, material alternatives considered, checks performed, and remaining uncertainty. Include the smallest supported fix and how to verify it. If a fix is requested, use these findings as input to a separate implementation step following the project's conventions; this skill does not require another skill to be installed.