---

name: ruby-quality description: Interpretive guidance for Ruby code quality using RuboCop (configurable) or StandardRB (zero-config). Use when linting Ruby files, configuring Ruby tools, troubleshooting lint errors, or understanding tool selection.

Ruby Quality Skill

This skill teaches how to apply Ruby linting and formatting tools effectively using mr-sparkle's tool selection system. It provides guidance on what the tools do, when each tool group is used, and how our configuration balances configurability with zero-config simplicity.

Official Documentation

Claude knows how to use RuboCop and StandardRB. Fetch these docs only when you need:

• Specific cop (rule) codes or error messages you don't recognize
• Advanced configuration options
• Recent feature changes

Reference URLs:

• https://docs.rubocop.org/rubocop/ - RuboCop cops and configuration
• https://github.com/standardrb/standard - StandardRB philosophy and usage

Core Understanding

Tool Selection Philosophy

Key principle: Prefer zero-config tooling (StandardRB) when project has it configured; fall back to configurable tool (RuboCop) when configured; default to StandardRB if no configuration exists.

What this means:

• StandardRB preferred - Zero-config, opinionated, removes decision fatigue
• Project config wins - Respects existing project tooling choices
• Smart fallback - Uses RuboCop if project has it configured
• Zero-config default - Falls back to StandardRB with sensible defaults

Decision test: Does the project have explicit tool configuration? Use configured tools. Otherwise use StandardRB.

How Tool Selection Works

The linting system uses group-based priority selection for Ruby files:

Priority 1: standardrb (if project has standard config)
    ↓
Priority 2: rubocop (if project has rubocop config)
    ↓
Fallback: standardrb (with default config)

Detection logic:

1. Find project root (Gemfile, .git, or other markers)
2. Check for StandardRB configuration (first group)
3. Check for RuboCop configuration (second group)
4. If no config found, use StandardRB with defaults

Tool in winning group runs (only one tool needed for Ruby).

StandardRB: Zero-Config Tool (Official Specification)

From official docs:

• Purpose: Ruby style guide, linter, and formatter with zero configuration
• Philosophy: "No configuration, just use it"
• Based on: RuboCop engine with curated rule set
• Capabilities: Linting + auto-fixing in a single tool
• Commands: standardrb (check), standardrb --fix (auto-correct)

Configuration locations:

• .standard.yml (optional overrides, but goes against philosophy)
• Generally: no config needed or wanted

See standardrb.md for detailed overview.

StandardRB: Zero-Config Tool (Best Practices)

When StandardRB shines:

• ✅ New projects (no legacy style preferences)
• ✅ Want to eliminate bikeshedding about style
• ✅ Prefer "just make it consistent" over customization
• ✅ Teams that value simplicity
• ✅ Want fast onboarding (no config decisions)

Philosophy:

StandardRB removes decision fatigue by providing a curated, opinionated rule set. You don't configure it; you use it. This is intentional.

Basic usage:

# Check for issues
standardrb

# Auto-fix issues
standardrb --fix

# Check specific files
standardrb app/models/**/*.rb

Rare configuration (discouraged but possible):

# .standard.yml - only if absolutely necessary
ignore:
  - 'vendor/**/*'
  - 'db/schema.rb'

See configs/standard.yml for minimal example and standardrb.md for philosophy details.

RuboCop: Configurable Tool (Official Specification)

From official docs:

• Purpose: Ruby static code analyzer and formatter
• Philosophy: Highly configurable, community-driven style enforcement
• Capabilities: 500+ cops (rules) covering style, performance, security
• Commands: rubocop (check), rubocop -a (safe auto-correct), rubocop -A (all auto-correct)

Configuration locations:

• .rubocop.yml (standard config file)
• Can inherit from shared configs (rubocop-rails, rubocop-rspec, etc.)

See rubocop.md for detailed overview.

RuboCop: Configurable Tool (Best Practices)

When RuboCop shines:

• ✅ Existing projects with established style preferences
• ✅ Teams with specific coding standards
• ✅ Need fine-grained control over rules
• ✅ Want to use community extensions (rubocop-rails, rubocop-rspec, etc.)
• ✅ Projects that haven't migrated to StandardRB

Philosophy:

RuboCop is powerful and flexible. You configure exactly which cops to enable, at what severity, with what parameters. This is ideal when you have strong opinions or legacy codebases.

Basic usage:

# Check for issues
rubocop

# Auto-fix safe issues
rubocop -a

# Auto-fix all issues (use with caution)
rubocop -A

# Check specific files
rubocop app/models/**/*.rb

Common configuration pattern:

# .rubocop.yml
AllCops:
  TargetRubyVersion: 3.2
  NewCops: enable

Style/StringLiterals:
  EnforcedStyle: single_quotes

Metrics/BlockLength:
  Exclude:
    - 'spec/**/*'

See configs/rubocop.yml for sensible defaults and rubocop.md for configuration guidance.

Tool Selection in Practice (Best Practices)

Scenario 1: New project, no config

$ lint.py file.rb
# Runs: standardrb --fix
# Uses: StandardRB defaults (zero config)

Scenario 2: Project with StandardRB config

# Project has .standard.yml or uses 'standard' gem
$ lint.py file.rb
# Runs: standardrb --fix
# Uses: project's .standard.yml (if exists)

Scenario 3: Project with RuboCop config

# Project has .rubocop.yml
$ lint.py file.rb
# Runs: rubocop -a
# Uses: project's .rubocop.yml config

Scenario 4: Mixed config (StandardRB wins)

# Project has both .standard.yml and .rubocop.yml
$ lint.py file.rb
# Runs: standardrb only (first group with config wins)

Common Pitfalls

Pitfall #1: Over-Configuring StandardRB

Problem: Trying to customize StandardRB extensively.

# ❌ Fighting StandardRB's philosophy
# .standard.yml
ignore:
  - 'app/**/*'
  - 'lib/**/*'

Why it fails: If you need this much customization, you should use RuboCop instead. StandardRB's value is zero-config consistency.

Better: Either accept StandardRB's defaults or switch to RuboCop for full control.

Pitfall #2: Using rubocop -A Without Review

Problem: Auto-correcting all issues without understanding changes.

Why it fails: -A (aggressive auto-correct) can make unsafe changes. Some fixes need human review.

Better:

# Safe auto-correct first
rubocop -a

# Review remaining issues
rubocop

# Only use -A for specific safe cops if needed
rubocop -A --only Style/StringLiterals

Pitfall #3: Not Using Community Extensions

Problem: Reinventing rules that exist in community gems.

# ❌ Custom rules for Rails-specific issues
# .rubocop.yml
Style/MyCustomRailsRule:
  Enabled: true

Why it fails: Community gems like rubocop-rails already have comprehensive Rails-specific cops.

Better:

# Gemfile
gem 'rubocop-rails', require: false

# .rubocop.yml
require:
  - rubocop-rails

# Now use built-in Rails cops
Rails/Validation:
  Enabled: true

Pitfall #4: Disabling Too Many Cops

Problem: Disabling cops because they're "annoying."

# ❌ Over-disabling
AllCops:
  DisabledByDefault: true

Style/StringLiterals:
  Enabled: false
Metrics/MethodLength:
  Enabled: false
Layout/LineLength:
  Enabled: false

Why it fails: Defeats the purpose of linting. Rules exist for good reasons.

Better: Understand why rules trigger, fix the code, or selectively disable with inline comments:

# Selective disable when truly needed
def legacy_method # rubocop:disable Metrics/MethodLength
  # ... complex legacy code that can't be refactored yet
end

Pitfall #5: Ignoring .rubocop_todo.yml

Problem: Committing a massive .rubocop_todo.yml and forgetting about it.

Why it fails: Todo file is meant to be temporary. It lets you enable RuboCop gradually but shouldn't be permanent.

Better:

# Generate todo file for existing codebase
rubocop --auto-gen-config

# Then incrementally fix issues
# After fixing some cops, regenerate:
rubocop --auto-gen-config --exclude-limit 0

# Goal: Eventually delete .rubocop_todo.yml entirely

Automatic Hook Behavior

The mr-sparkle plugin's linting hook:

1. Triggers after Write and Edit operations
2. Detects Ruby files (.rb, .rake, Rakefile, Gemfile)
3. Runs selected tool automatically (standardrb OR rubocop)
4. Applies auto-fixes where possible
5. Reports unfixable issues (non-blocking)
6. Silently skips if tools not installed

What this means: Most formatting issues auto-fix on save. Pay attention to reported unfixable issues.

Quality Checklist

Before finalizing Ruby code:

Auto-fixable (tools handle):

• ✓ String quote style consistency
• ✓ Indentation and spacing
• ✓ Line length and wrapping
• ✓ Trailing whitespace
• ✓ Method spacing
• ✓ Hash syntax (old vs new)

Manual attention required:

• ✓ Method complexity
• ✓ Undefined variables
• ✓ Security issues
• ✓ Performance anti-patterns
• ✓ Naming conventions
• ✓ Documentation completeness

CLI Tool Usage

The universal linting script handles Ruby files automatically:

# Lint Ruby file (applies fixes)
./plugins/mr-sparkle/skills/linting/scripts/lint.py file.rb

# JSON output for programmatic use
./plugins/mr-sparkle/skills/linting/scripts/lint.py file.rb --format json

Exit codes:

• 0 - Clean or successfully fixed
• 1 - Lint errors found (non-blocking)
• 2 - Tool execution error

See linting skill for complete CLI documentation.

Reference Documentation

Detailed guides (loaded on-demand for progressive disclosure):

• rubocop.md - RuboCop overview, philosophy, key features
• standardrb.md - StandardRB overview, zero-config philosophy
• configs/rubocop.yml - Sensible default RuboCop configuration
• configs/standard.yml - Minimal StandardRB configuration example

Official documentation to fetch:

• https://docs.rubocop.org/rubocop/ - RuboCop documentation and cops reference
• https://github.com/standardrb/standard - StandardRB documentation
• https://rubystyle.guide/ - Ruby Style Guide (RuboCop's basis)

Remember: This skill documents mr-sparkle's tool selection logic for Ruby. Fetch official docs when you need specific cop definitions or configuration syntax you're unsure about.