--- title: "Rails 8 upgrade story: duplicate keys sneaking into our JSON responses" url: https://daily.dev/posts/rails-8-upgrade-story-duplicate-keys-sneaking-into-our-json-responses-9mu6imbik source_url: https://blog.arkency.com/duplicate-keys-sneaking-into-our-JSON-responses/ type: article source: "arkency" published: 2025-09-29T09:58:28.936Z updated: 2026-08-24T07:00:46.163Z tags: ["rails"] reading_time: 4 upvotes: 0 comments: 0 language: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Rails 8 upgrade story: duplicate keys sneaking into our JSON responses **[arkency](https://daily.dev/sources/arkency)** · 4 min read · 0 upvotes · 0 comments ## Summary 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. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [This Week in Rails: February 13, 2026](https://daily.dev/posts/this-week-in-rails-february-13-2026-hnjprn3ns) · Rails · 3 upvotes · 0 comments - [Rails 7.2 to 8.1 Upgrade: What Actually Breaks and How to Fix It](https://daily.dev/posts/rails-7-2-to-8-1-upgrade-what-actually-breaks-and-how-to-fix-it-zwrjygtho) · Ruby Flow · 0 upvotes · 0 comments - [RFC 9110 Accept headers, dotenv fixes, and Ractor safety](https://daily.dev/posts/rfc-9110-accept-headers-dotenv-fixes-and-ractor-safety-fcju6cnjp) · This Week in Rails · 0 upvotes · 0 comments --- Tags: [#rails](https://daily.dev/tags/rails) [View this post on daily.dev](https://daily.dev/posts/rails-8-upgrade-story-duplicate-keys-sneaking-into-our-json-responses-9mu6imbik)