fff (Fast File Finder) is a high-performance file search and content grep toolkit written in Rust, designed for long-running applications like code editors, AI agents, and file explorers. It provides fuzzy filename search with typo resistance, live content grep (plain text, regex, fuzzy), directory search, glob filtering, and git-aware frecency ranking. The core library maintains a persistent in-memory index with a bigram-based content prefilter, an LMDB-backed frecency and query history store, and a live filesystem watcher for incremental updates. The system is exposed through multiple interfaces: a Neovim plugin with a full TUI picker UI, an MCP server for AI assistants (Claude Code, Codex), language bindings for Python, Node.js, and Bun, and a C FFI for embedding in other languages. The architecture is a Rust core library with multiple frontends communicating via FFI or direct Rust API, all sharing the same `FilePicker` instance that manages indexing, searching, and watching. The codebase demonstrates exceptional performance engineering with SIMD-optimized string operations, compressed inverted indexes, arena-based path storage, and careful concurrency design with thread pools and lock-free snapshots.