Claude now watermarks its text invisibly – and coders are already cracking it

What's happening

Anthropic has signed the Code of Practice on the transparency of AI-generated content, which belongs to the EU AI Act (Article 50(2)). As a result, text and files generated by Claude will carry machine-readable markings. This doesn't apply only in the EU: according to Anthropic, it covers every region where Claude is offered. A Reddit post in r/ClaudeAI sparked discussion: from November 3, older models such as Mythos 5, Opus 4.7, Opus 4.6 and Sonnet 4.6 are also said to get text watermarks. Anthropic's help page doesn't say that paying users can opt out. The Reddit author claims there is no opt-out.

How the marking works

Anthropic uses two methods:

1. Text watermark: The model picks words based on a secret key and the preceding words. This creates a statistical pattern that people can't see, but a detector holding the key can evaluate. According to ExtremeTech, no hidden characters or metadata are inserted. The pattern lives solely in the word choice. It travels along when you copy the text, and according to Anthropic it can survive some edits.
2. Content Credentials (C2PA): When Claude creates files such as PNG or JPEG, it attaches signed provenance data. This follows the open standard of the Coalition for Content Provenance and Authenticity. The free "Claude Content Checker" tool tells you whether a file carries such a label.

According to Wired, the approach matches Google's SynthID-Text, which has been in use since 2023. Scott Aaronson proposed a similar method while at OpenAI, but according to Wired, OpenAI didn't deploy it, worried about scaring off customers.

According to ExtremeTech, the technique works best on natural language, because there are many ways to phrase things. In code, it only works in comments or variable names. Translations by Claude are fully marked, because Claude chooses every word. When Claude only fixes grammar, most words remain human, so detection may struggle. Longer passages can be detected more reliably.

Which models are affected

According to Anthropic's table, text watermarks apply to Fable 5 and 5.1, Mythos 5.1, Opus 4.8, 5 and 5.5, Sonnet 5 and 5.5, and Haiku 5.5, among others. Mythos 5, Opus 4.5 to 4.7, Sonnet 4.5/4.6 and Haiku 4.5 currently get only C2PA on files. The law provides a transition period for systems launched before August 2, 2026. Anthropic is working on retrofitting them. Wired names December as the deadline for existing models.

Detection for the few

Text detection is a private preview. Access is open to eligible organizations: regulators, law enforcement, media, fact-checkers, independent researchers, educational institutions, EU civil society and companies with their own verification obligations. Interested parties can fill out a form. Anthropic plans to expand the detection API later. The Reddit author criticizes that the very people being judged can't check anything themselves.

Limits, according to Anthropic

Anthropic acknowledges limitations:

• A detected mark isn't conclusive proof. Claude doesn't have to be the author, since many people use it for proofreading, translating or summarizing.
• Content may have been altered, shortened or mixed after generation.
• A missing mark doesn't mean AI wasn't used. Reasons include older models, heavy rewording, translation, very short texts, metadata stripped by screenshots or conversion, and unsupported platforms.

Companies that build Claude into their own products should check for themselves what Article 50 requires of them. Anthropic plans to provide technical guidance.

Reactions and workarounds

According to Wired, developer Guillaume Meyer released watermark-removal code within four hours of the confirmation. It was bookmarked more than 20,000 times on X and has over 100 contributors. His method has a different, non-watermarking language model generate several rewrites, using synonyms and slightly altered structure. Meyer says he isn't against transparency but considers watermarks a poor solution. He fears false positives and a failure to distinguish between light and heavy AI use. As a native French speaker, he uses Claude and Grammarly himself for revising.

Other approaches, according to Wired: Erik Hughes built a tool in 15 minutes that strips invisible characters, rearranges sentences and swaps words. Leon Chlon (Oxford) points to condensing text and round-trip translation through a language like Arabic. According to Wired, whether the tools work can only be verified once Anthropic releases the detector. Wayne Pan (Haimaker) thinks it's likely, though, because he knows the SynthID principle. He also says there's no such thing as a watermark that survives everything.

According to Wired, the rules only prohibit providers from marketing circumvention tools. Independent tools aren't legally restricted. Wired also reports that 190 organizations have signed the code, including OpenAI, Microsoft and Meta. How many will actually implement it remains unclear.

Our take

The watermark is a mandatory product of EU law, not a voluntary Anthropic idea. Anthropic says it changes neither meaning nor quality. The sources contain no independent evidence for that. The key problem is the asymmetry: if you only proofread or translate with Claude, your text may be marked, yet those affected can't check it themselves. Anyone who deliberately deceives gets around the mark with another model or by rewriting. Ars Technica notes that a fully generated article gets a watermark but no reader-facing label after editorial review, and that the EU largely leaves the societal benefit up to the companies. Several claims, such as those about opt-out and consequences for job applications, come from the Reddit author and are opinions, not established facts.