Idiomatic Ruby, the language

Write Ruby a senior Rubyist would sign off on: expression-oriented, block-driven, leaning on Enumerable and Comparable instead of hand-rolled loops, with frozen_string_literal hygiene and a real Gemfile/.gemspec when you package.

This is Ruby the language and its non-Rails ecosystem — blocks, modules, gems, metaprogramming, Minitest/RSpec. Delegate: Rails apps, ActiveRecord models, controllers, migrations, views, Hotwire — anything that loads Rails or uses ActiveSupport/ActiveRecord idioms — to rails; the same task in another language to python, go, rust, elixir, php or typescript; the CI pipeline that runs your Ruby tests to github-actions — this skill writes the tests, not the workflow.

Targets Ruby 4.0 (released 2025-12-25) on 3.4 semantics: Prism is the default parser, string literals warn on mutation without a magic comment, it is an implicit block param, and Set is now a core class.