Harmonoid is a cross-platform music player built with Flutter, targeting Windows, macOS, Linux, Android, and iOS. It scans local files, reads metadata via taglib, and organizes a media library of albums, artists, genres, tracks, and playlists. Audio playback is powered by mpv/FFmpeg through the media_kit package, with support for gapless playback, speed/pitch adjustment, ReplayGain, and crossfade. The app integrates Last.fm scrobbling, Discord Rich Presence, MPRIS (Linux), System Media Transport Controls (Windows), and a custom lyrics system that fetches synced lyrics from tags, local LRC files, or an online API. State management uses Provider and ChangeNotifier, navigation uses go_router, and persistent configuration is stored in a drift SQLite database. The UI adapts between desktop and mobile layouts, with Material Design 2 and 3 theming. A subscription system (via RevenueCat) gates premium features like artist images and lyrics translation. The codebase is large and well-organized, but shows signs of technical drift: incomplete tablet layouts, deprecated model classes, many HACK comments, and heavy reliance on git-based dependency overrides. Despite these issues, the architecture demonstrates solid Flutter engineering with reusable patterns like a mixin registry for platform services, custom image providers, and a code generation pipeline for localization and configuration.