Context7
MCP server by Upstash (52K+ stars) that injects up-to-date, version-specific documentation for any library directly into Claude Code's context. Solves the hallucinated API and outdated example problem, the LLM uses real docs instead of its training memory.
Strengths
- Versioned documentation, no mixing between React 18 and React 19 APIs
- Eliminates function signature and config option hallucinations
- Configurable token budget, 5K default, adjustable based on question complexity
- Topic filtering, ask for "routing" and get only routing docs
Limitations
- Depends on hosted Upstash service, if their API is down, no docs
- Uneven coverage, popular libraries are well covered, niche ones less so
Best for
- Any project using recent or rapidly evolving libraries (React 19, Tailwind 4, Next.js 16)
- Developers wanting Claude Code to generate correct code without debug iterations on changed APIs
- Long work sessions where generated code quality far outweighs the token cost