Anthropic Expands Watermark to Older Claude Models, But Developers Can Bypass It via Sampling Parameters
Anthropic added text watermarks to four older Claude models to meet EU AI Act deadlines, but API users can easily remove the mark from Opus 4.6 and Sonnet 4.6 by tweaking sampling settings.
US-based AI company Anthropic has announced the expansion of its imperceptible text watermark feature to four additional models: Claude Mythos 5, Claude Opus 4.7, Claude Opus 4.6, and Claude Sonnet 4.6. This move aims to comply with the December 2026 deadline set by the European Union's Artificial Intelligence Act (EU AI Act). According to a report by technology outlet The Decoder, Anthropic stated in an email to API customers that the watermark is an "imperceptible pattern in word choice" that does not alter meaning, quality, or readability, nor does it add extra characters or tokens.
However, this compliance mechanism contains a notable technical loophole in two of the older models. For Claude Opus 4.6 and Claude Sonnet 4.6, Anthropic indicated that the watermark is applied only when developers leave the three sampling parameters—temperature, top_p, and top_k—at their default values. A minor tweak to any one of these randomness-controlling parameters is sufficient to bypass the watermark entirely. Anthropic warned in the email, "If you rely on the watermark for your own compliance, keep these settings at their defaults." In contrast, no such caveat was mentioned for Claude Mythos 5 or Claude Opus 4.7, suggesting the watermark applies to those models regardless of sampling settings.
Since Claude Opus 4.6 remains one of Anthropic's strongest writing models, even surpassing the newer Opus 5.5 in some aspects, this vulnerability poses practical risks for third-party organizations relying on automated tools to verify AI-generated content. Anthropic is currently testing a detection API in private preview to verify the watermark, which uses a variant of Google DeepMind's open-source SynthID text method. While OpenAI also launched its own textGrain watermarking system in October 2026, it activates by default only for ChatGPT and Codex users in the EU, remaining optional for global API customers.