Fetch up-to-date library documentation via Context7 API. Use PROACTIVELY when:
(1) Working with ANY external library (React, Next.js, Supabase, etc.)
(2) User asks about library APIs, patterns, or best practices
(3) Implementing features that rely on third-party packages
(4) Debugging library-specific issues
(5) Need current documentation beyond training data cutoff
Always prefer this over guessing library APIs or using outdated knowledge.