The idea
Add custom text tags to any source in your library — things like "key argument", "methodology", "background reading", or "Week 3 seminar". Then filter the library by tag to see just the sources that match. Tags are per-user and not shared with collaborators.
Why it matters
Larger source libraries get unwieldy fast. Search works when you know what you're looking for, but sometimes you need to see all your methodology papers at once, or pull up everything you tagged for a specific chapter. Tags give you a lightweight organization layer without forcing a rigid folder structure. They're flexible enough to match whatever mental model you already use.
Where we are
This feature shipped. Add your own tags to any source, then filter your library by them from the sidebar. Tags handle structured organization; source notes handle free-text annotation.