YouCompleteMe (YCM) is a Vim plugin that provides as-you-type code completion, diagnostics, refactoring, and other IDE-like features for multiple programming languages. It uses a client-server architecture: a thin Vim client communicates via HTTP+JSON with a separate ycmd server, which in turn integrates with language-specific backends such as clangd, Jedi, OmniSharp-Roslyn, Gopls, TSServer, rust-analyzer, and jdt.ls. The plugin supports semantic completion, signature help, hover information, go-to-definition, find references, fixits, type/call hierarchy, inlay hints, and semantic highlighting. It is highly configurable through Vim global variables and per-project configuration files. The codebase is mature, with extensive Python and Vimscript test suites, Docker-based CI, and a well-documented help file. While primarily a Vim plugin, its client-server design and reusable components (e.g., async HTTP request pattern, composable diagnostic filters, thread pool) could be adapted for other editor integrations or standalone tools.