FTS5 is SQLite's built-in full-text search engine. It indexes every word in your memory files and scores results using BM25, an algorithm that rewards rare terms and penalizes common ones.
When does keyword search beat vector search? When you need exact matches. If you search for a specific error code like ERR_SOCKET_TIMEOUT or a function name like parseConfig, FTS5 finds it directly while vector search might return vague semantic neighbors. OpenClaw builds the FTS5 index alongside the vector index so both are always up to date.