Anthropic will watermark Claude's text, and has published how the watermark breaks
Anthropic described the mechanism behind the watermark it will apply to Claude's text output for EU AI Act compliance. It is not hidden characters or metadata. The pattern is created during generation, by using a watermark key plus the preceding context to bias which word gets selected where the model would otherwise choose at random. Detection requires the key. Anthropic states that light editing probably will not remove it, and that a complete rewrite in which every word is replaced will. Detection is weak on short samples and weak on factual writing, where there are fewer word choices to encode a pattern. The approach is built on Google DeepMind's SynthID-Text method from 2024.