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How Rakuten's API hides the JAN, and the 3-tier cross-marketplace match
Rakuten's item search API never returns the JAN barcode, while Yahoo! Shopping looks it up directly via jan_code. Building a cross-marketplace price watcher on that asymmetry, I split product identity into three certainty tiers — JAN match, name fuzzy match, no match — with the scoring weights, thresholds, and the notification rule that keeps mis-merged items from firing alerts.
