What's New in RubyGems/Bundler 4

Ruby 4.0.0 was released on December 25, 2025 , and RubyGems/Bundler 4.0.3 is now bundled with Ruby 4.0.0. Since my previous post focused on migration and compatibility concerns , I’d like to highlight some of the exciting new features in this release. Parallelization of C-extension Gem Builds Add MAKEFLAGS=-j by default before compiling When installing gems with C extensions (such as mysql2 or pg ), RubyGems now automatically adds MAKEFLAGS=-j to the make command for parallel execution.