It's done with a lot more subtlety and embedded directly into the content, no Unicode shenanigans. The basic idea is just to break token generations where the probability is nearly tied in favor of the side that matches the secret key. With a long enough text block you can be statistically certain if the generation was using the key. From another comment: https://johnjwang.com/post/2026/08/12/how-claude-watermarkin...