yutto is a production-grade CLI application for downloading videos, subtitles, danmaku (bullet comments), and metadata from Bilibili. In production it resolves user-supplied URLs (UGC videos, bangumi anime, paid cheese courses, favourites, collections, watch-later lists) against Bilibili's private JSON APIs, normalizes responses into typed Pydantic download requests, and runs a planner/executor pipeline: immutable DownloadPlan construction (output paths, codec/quality selection), concurrent range-based downloading, FFmpeg muxing, and writing of NFO/SRT/ASS artifacts. It also ships an experimental WebSocket JSON-RPC server mode (`yutto serve`) for remote task submission, querying, cancellation, and live event streaming with token authentication.