Lists multiple specific concrete actions and capabilities: writing/reviewing/debugging Ruby code, pattern matching, Data.define, endless methods, error handling conventions (raise vs fail, result objects), memoization, and performance idioms.

Explicitly answers both 'what' (idiomatic Ruby patterns, modern features, error handling, memoization, performance idioms) and 'when' ('Use when writing, reviewing, or debugging pure Ruby code'). Also provides clear boundary guidance by directing to other skills for Rails, testing, and code style.

Includes strong natural keywords users would say: 'Ruby code', 'pattern matching', 'error handling', 'memoization', 'performance', 'idiomatic'. Also includes version-specific terms like '3.x+' and concrete features like 'Data.define' and 'endless methods' that Ruby developers would reference.

Highly distinctive by scoping to 'pure Ruby code' and explicitly delineating boundaries — directing Rails to rails-guides, testing to minitest, and style to sandi-metz-rules. This cross-referencing significantly reduces conflict risk with related skills.

The content is lean and efficient throughout. It avoids explaining what Ruby is or how basic concepts work, instead focusing on specific patterns and idioms that Claude might not default to (Weirich convention, nil-safe memoization, etc.). Every section earns its place with concrete, non-obvious guidance.

Every pattern is illustrated with fully executable Ruby code examples that are copy-paste ready. The Result object class, pattern matching examples, memoization patterns, and string building comparisons are all concrete and immediately usable.

This is primarily a reference/idiom skill rather than a multi-step workflow skill. For its purpose, the organization is clear and unambiguous — each section presents a specific pattern with when/how to use it. The Common Mistakes table provides clear problem→fix mappings. No destructive or batch operations require validation checkpoints.

The skill references four files in a `references/` directory (modern_ruby.md, error_handling.md, performance.md, ood-philosophy.md) with clear signaling, which is good structure. However, no bundle files were provided, so we cannot verify these references exist. The inline content is well-sized for a SKILL.md overview, but the references section at the bottom partially duplicates the inline 'See references/...' links scattered throughout.