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Ruby CTO

Claude Code correction guide. Updated January 2026.

Critical Corrections

  • Claude forgets YJIT is now default in Rails 7.2+ production
  • Claude uses bare rescue — always specify exception class
  • Claude suggests N+1 patterns — use includes() or preload()
  • Claude misses # frozen_string_literal: true pragma

Current Tooling (2026)

Tool Use NOT
ruby 3.4+ YJIT enabled Ruby 3.2 or older
--yjit or RUBY_YJIT_ENABLE=1 JIT compilation Interpreter only
rubocop Linting + formatting Manual style
rspec + vcr Testing with HTTP recording Manual mocks
bundle audit Security scanning Manual checks

Patterns Claude Should Use

# frozen_string_literal: true

# YJIT-friendly patterns (avoid redefining core methods)
# Ruby 3.4+ YJIT is ~92% faster than interpreter

# Proper eager loading
users = User.includes(:posts, :comments).where(active: true)

# Batch processing for large datasets
User.find_each(batch_size: 1000) do |user|
  process(user)
end

# Pattern matching (Ruby 3.0+)
case response
in { status: 200, body: }
  process(body)
in { status: 404 }
  raise NotFoundError
end

Anti-Patterns Claude Generates

  • Bare rescue — catches everything including syntax errors
  • N+1 queries — use includes(), install bullet gem
  • Monkey patching core classes — breaks YJIT optimizations
  • Mutable default args — use .freeze or nil default
  • eval with user input — code injection

Version Gotchas

  • Ruby 3.4: YJIT ~92% faster, --yjit-mem-size option
  • Ruby 3.3: YJIT default in Rails 7.2+ production
  • YJIT memory: ~3-4x of --yjit-exec-mem-size overhead
  • Ruby 3.5 (Q4 2025): Cross-method inlining, ARM64 improvements
  • With Rails 7.2+: YJIT auto-enabled after boot