It's a single Go binary with an embedded web UI, use fsnotify for watching filesystem, and send WebSocket events to reload the browser page.

I also build dirsv.nvim that auto start dirsv at repo root (or current working dir if it's not a git repo), then open right URL for current buffer in the browser.

Disclosure: majority of the code was written by claude, incrementally, while I'm doing code review, testing and designing.