Rex provides sub-second Python symbol search across all installed .venv packages. Three tools — rex_find, rex_show, rex_members — let Claude look up function signatures, docstrings, class members, and file locations instantly, without web search or grepping through source files. Results are grounded in the actually installed version — no outdated docs, no hallucinated parameters. Auto-indexes on first use and re-indexes incrementally when packages change. Read-only by design.