CotEditor is a native macOS text editor written in Swift, designed for code editing with syntax highlighting, file management, find/replace, and scripting support. It uses a hybrid AppKit/SwiftUI architecture, with core editing logic in the EditorCore package, syntax parsing in the Syntax package (supporting both regex-based and tree-sitter incremental parsing), and macOS-specific UI controls in the MacUI package. The application follows an MVVM pattern with @Observable models, async task-based background processing, and extensive localization across 19 languages. Key entry points include the DocumentWindow and DocumentViewController for the main editing interface, the SettingsWindow for configuration, and the command-line tool `cot`. Data flows involve file opening with encoding detection, syntax highlighting via incremental parsers, find/replace operations with progress tracking, and remote editing via the rmate protocol. External dependencies include tree-sitter grammars for 20+ languages, Sparkle for updates, and various macOS system APIs. The codebase demonstrates strong engineering practices: modular package design, protocol-oriented interfaces, reactive state management, and comprehensive test coverage. It is actively maintained with frequent releases and security fixes.