diagnosing-test-failures

Diagnoses a failing test or a reported defect to its root cause: classifies the failure (application defect, test bug, environment or config, flaky, or test data), builds a tight reproduction loop that goes red on the exact symptom, tests ranked falsifiable hypotheses one variable at a time, and resolves it by class without weakening assertions. Use when a test fails on every run or since a merge, a CI job is red, someone asks "why is this failing", a bug is reported and needs a root cause, or a stack trace, error log, or trace file is pasted, even if the word debug is never used. When the failure is intermittent, hand over to stabilizing-flaky-tests; when the cause is an application defect, write it up with reporting-bugs.