Anthropic expands text watermarking to older Claude versions beginning November 3

Written on 11/01/2026

This isn't the first time Anthropic has issued a watermark, but the latest iteration will apply to Mythos 5, Opus 4.7, Opus 4.6, and Sonnet 4.6.Abstract binary code. Blue background of technology patterns - stock photo

As we first reported in early August, Anthropic has bowed to demands from EU legislators that AI-generated text carry a hidden watermark, to help eventually distinguish between human- and AI-generated content. 

The initial watermark rollout only applied to the latest and greatest versions of Claude, but on Friday Anthropic alerted its users that an expansion of the watermark would soon roll out to Claude Mythos 5, Claude Opus 4.7, Claude Opus 4.6 and Claude Sonnet 4.6, starting on November 3. 

According to the AI giant, it will be the same watermarking technology already in use in Opus 5.5, based on "an imperceptible pattern in word choice" that won't change the accuracy or readability of its output, or contain any identifying information about the user or organization working with Claude. 

But it's one thing to worry about the function of artificial intelligence and another to worry about the human element. Lawyers have been caught using AI-generated citations, novelists have been caught relying on AI to write entire books, and scientific researchers have been caught using AI in the peer review process. 

For all the problems solved by AI use, and the mundane busywork it so ably processes in seconds rather than hours, it also creates a powerful incentive for humans to cut corners and lie about it, so it will be interesting to see if widespread watermarking helps keep the human element honest.