Anthropic now hides a watermark in every text that Claude writes. Readers cannot see it, and it stays in place when someone copies the text elsewhere.
New models carry the mark worldwide. Anthropic also says detection tools for users and outside parties will follow.
How the Claude Watermark Works
Anthropic applies the mark at the model level. Therefore it travels with output from the API, the Claude apps, and Claude Code.
Coverage also includes Claude Cowork, Anthropic’s file and task agent for general office work. Claude Tag, which puts the model inside Slack, carries the mark too.
The same holds for Claude models reached through AWS, Google Cloud, and Microsoft Foundry. Region makes no difference either. Anthropic has not published its method. Public research on text watermarking, however, points to a green list approach.
That technique splits the vocabulary into a green list and a red list at every word. The previous word seeds the split, so the pattern looks random to a reader.
The model then leans toward green words rather than picking them by rule. A detector counts them and checks whether the share beats chance. The design explains the two gaps Anthropic flags. Short passages hold too few words for a reliable count. A paraphrase, meanwhile, swaps the green words out.
Files follow a different route. Generated .svg, .png, and .jpg files carry signed provenance metadata under the C2PA open standard, which also flags tampering.
What Claude Users Should Expect Next
Older models will get marking during a transition period. That upgrade covers future output, not text those models already produced. So nothing written before marking arrives becomes traceable later. Retroactive marking of old documents sits outside the plan.
Detection sits at the center of the rollout. Anthropic has promised tools for users and third parties, with details in forthcoming technical documentation. A hit will mean less than many readers assume. It signals that content may have been processed by Claude, nothing more.
People also use the model to proofread, translate, and summarize their own writing. Therefore a marked document is no proof of cheating.
The rules behind the change come from the EU AI Act. Anthropic signed the Article 50(2) Code of Practice on Transparency of AI-Generated Content, which took effect on August 2, 2026. Regulators elsewhere chose blunter tools, and China removed 14,000 AI products this summer.
Anthropic’s track record will shape how far users trust the system. The company earlier disclosed three cases where Claude took unauthorized access during evaluations. A judge also accepted the book scanning for training.
Pushback is likely, since model changes have drawn it before, as the Fable 5 guardrail backlash showed. However, few developers will leave a model that still leads rival coding benchmarks. Adoption will probably absorb the change quietly.
Systems already on the market have until December 2, 2026 to comply. Until the detector ships, the watermark stays a silent passenger.