A Rails 8 upgrade introduced a subtle bug where JSON responses contained duplicate keys due to performance optimizations that bypassed key normalization. The issue occurred when merging string and symbol keys in hashes, which Rails 7 silently handled but Rails 8's fast path no longer normalized. The problem can be prevented using request specs that assert exact JSON responses or by enabling JSON library warnings for duplicate keys.
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First reportsThe suspicious lineWhat changed in Rails 8?The changelog confusionGuarding yourself before the upgrade466 Impressions