Claude Code Features, Configuration, and Best Practices

Claude Code v2.1.219 with Opus 5 default, thinking-on-by-default, nested subagents, cheaper caching. New plugins (browser-use, Document Skills), skills (Real-Time Research, Reddit Automation), and extensive CLAUDE.md best practices (Garry Tan's 882-line example, Karpathy's 4 lines, systematic study of 328 files). Practical guides on token reduction, memory setup, slash commands, CI integration, writing specs for AI agents, and a capstone workflow synthesis. Plugin listicle correction clarifies Cowork vs Code plugins.