Meet LEANN: An Open-Source Vector Index that Shrinks a 60M-Chunk RAG Index from 201 GB to 6 GB

UC Berkeley's Sky Computing Lab built LEANN, an MIT-licensed vector index on FAISS that stores a pruned graph and recomputes embeddings at query time instead of saving them, shrinking a 60M-chunk Wikipedia index from 201 GB to 6 GB in the repo benchmark. The paper, which won Best Research Paper at MLSys 2026, reports matching HNSW downstream accuracy (25.5% on NQ) at 90% recall using 4 GB instead of 188 GB, at the cost of slower retrieval (2.48 s vs 0.05 s on NQ). It ships apps for documents, Apple Mail, Chrome history, WeChat, iMessage and chat exports, plus an MCP server for Claude Code.