Exercise 3: Why the Root Cause Is a Missing Check, Not "The Deploy Broke Search" — Possible Solution
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WHY "THE DEPLOY BROKE SEARCH" IS ONLY THE PROXIMATE TRIGGER
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Per Chapter 9, "the proximate trigger is the immediate, final action
that set the incident off." The v3.2 deploy shipping an incompatible
query format is exactly that - the specific, immediate event that
caused the incident. But per Chapter 9's own reasoning, stopping the
analysis there only explains what happened this one time, not why the
system allowed it to happen at all.

THE ACTUAL 5-WHYS CHAIN IN THE CAPSTONE
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Per the capstone: the new format returned empty results because the
index backend didn't support it; it was deployed anyway because no
compatibility test existed between the two services in the pipeline;
that test didn't exist because cross-service compatibility testing was
never set up for this specific service pair. The final answer - a
missing cross-service compatibility check - is the systemic condition
that made the incident possible, not just the specific action that
triggered it this time.

WHY THIS CHANGED WHAT THE ACTION ITEMS ADDRESSED
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If the review had stopped at "the deploy broke search," the natural
action item would be something like "be more careful with deploys" -
a vague aspiration Chapter 9 already warns against, and one that
wouldn't actually prevent the next incompatible deploy from shipping.
Because the review continued to the systemic gap, per the capstone the
actual action items were "add a cross-service compatibility test... to
the CI pipeline" and "document the query-format contract explicitly" -
concrete, owned, dated items that address the process gap directly,
meaning the next incompatible change would be caught automatically
rather than depending on someone being "more careful."

WHY THIS WORKS AS AN ANSWER
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It distinguishes the proximate trigger from the root cause using
Chapter 9's own definitions, walks through the capstone's specific
5-whys chain, and explains concretely how identifying the systemic
cause (rather than stopping at the immediate one) produced action items
that address recurrence rather than just today's specific incident.