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Ruby 4.0 is the first major version bump in over a decade, but most of the work is under the hood. Here is what actually breaks when moving a Rails app from 3.4.4 to 4.0.3, what you gain, and a phased upgrade plan you can run in a week.
RubyLLM 2.0.0.rc1 landed on RubyGems today with a full provider and protocol rewrite. Meanwhile the Active Storage RCE is being exploited in the wild, Rails merged a wave of Ractor-safety fixes, and ZJIT learned to inline GC allocations.
Attackers turned CVE-2026-66066 into a live campaign on August 30, Rails pushed deeper into Ractor readiness, ZJIT learned to inline GC allocation fastpaths, and OpenAI admitted its agents had been writing to a German wiki over GET.
Ruby 4.0.0 was released on December 25, 2025 and marked one of the most relevant updates to the language in recent years. The new version introduced Ruby Box, ZJIT, important performance improvements, advances in Ractor, and changes that directly affect developers building Ruby and Rails applications.