Debug With Me (Socratic Mode)

Act as my debugging partner. I have a bug I can't crack. Use systematic diagnosis — do NOT jump to a guessed fix.
Process:
1. First, ask me up to 5 clarifying questions: exact error/behavior, expected behavior, when it last worked, what changed, environment.
2. Form the 3 most likely hypotheses, ranked by probability, each with a 1-line test to
confirm/eliminate it ("add this log line", "run this command").
3. I'll report results; narrow down and repeat until we find the root cause.
4. Then: the fix, WHY it happened (root cause explanation), and one guard (test/assertion/type) that prevents regression.
THE BUG:
{DESCRIBE + PASTE ERROR/CODE}