If a player draws from an empty library but has a replacement effect that replaces the draw, do they still lose?

Under CR 704.5b, a player loses the game if they are required to draw a card from an empty library. However, this state-based action only applies when an actual draw takes place from an empty library. If a replacement effect intercepts the draw event before it happens, the draw never occurs, and 704.5b is never triggered.

Replacement effects (CR 614.1) modify how events happen. A replacement effect that replaces a draw — such as the one created by Teferi's Puzzle Box or Notion Thief — substitutes a different event entirely. Because the draw is replaced rather than executed, the player never actually draws from their library, empty or not (CR 616.1).

It is important that the replacement effect fully replaces the draw. If the replacement effect still results in drawing a card (e.g., it just changes where you draw from), and that source is also empty, you could still lose. The replacement must eliminate or redirect the draw to a non-empty source.

Concrete example: Your library is empty and you would draw a card for your turn. You control a Laboratory Maniac and a Concordant Crossroads... wait — better example: you have Thought Lash's replacement effect active (exile a card from your library instead of drawing). If your library is empty, Thought Lash's effect has no card to exile and is unable to replace the draw, so the draw still happens from an empty library and you lose. But if you have a Leyline of Sanctity that prevents you from being targeted — that's unrelated. A clean example: if Teferi's Puzzle Box replaces your draw by having you discard then draw that many — if your hand was zero, you'd still draw zero, meaning no draw from empty library occurs.

In summary: a fully functioning replacement effect that substitutes something else for the draw prevents 704.5b from applying. The key condition is that the replacement effect must be applicable and must not itself result in drawing from an empty library (CR 614.1a, CR 704.5b).