Claude prepares to leave its mark
Anthropic will invisibly watermark Claude's text to meet EU transparency rules, but has released no detector. Our experiment using Google's SynthID shows detection fading with editing and length, evidencing model involvement rather than authorship.
Joel Miller

This week Anthropic announced that future Claude models will invisibly watermark their text, although no model has been launched since the policy took effect on 2 August and Anthropic has not confirmed that any existing model has been updated.
The announcement responds to new transparency requirements under the EU AI Act. Providers of generative AI systems must make synthetic text, images, audio and video detectable in a machine-readable form where technically feasible. Anthropic has signed the supporting Code of Practice alongside around 190 other organisations.
Several companies already claim to remove Claude watermarks. These claims cannot be verified until Anthropic provides a working detector. Passing a conventional AI detector such as GPTZero does not prove that the watermark has gone. Those products estimate whether writing resembles model output. They do not have access to Anthropic’s marking system.
Google provides a useful comparison. Its SynthID Text technology includes tools for generating watermarked text and testing for the resulting mark. This gives us a practical way to explore the behaviour Anthropic is proposing without assuming that both companies use the same implementation.
Anthropic has not released its detector, API or technical documentation. We cannot currently take a Claude response and establish whether it contains the mark. We also cannot independently assess how well the system survives editing, copying, translation or paraphrasing.
Most watermarking methods alter the choices made while a model writes. There is nothing visible in the finished text.
We ran some experiments with a small AI model and two watermarking methods, one based on a published green-list technique and the other using Google’s SynthID Text technology to explore how these systems behave.
With the green-list technique, at each point in a sentence, a secret rule divided the words the model might choose into two arbitrary groups. The model gave one group a small advantage. Those words were not better, clearer or more accurate. They were preferred solely to create a detectable pattern.
Google’s SynthID uses a more complex version of the same principle. It applies several secret scoring rules and uses a tournament process to influence which word is selected. Its detector reconstructs those scores across the finished passage and looks for a consistent imbalance.
We tested both methods using the same prompts, sampling settings and editing operations. The chart shows the percentage of passages detected after different types of subsequent edits:
What we found:
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Light editing leaves the mark intact. Both watermarks survived sentence reordering because most of the original wording remained unchanged.
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Rewriting weakens detection quickly. Both methods became unreliable when 30% of the words were replaced. Neither survived when half the words were deleted.
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Short extracts are difficult to judge. SynthID detected both 20-word fragments, while the green-list detector found one. Short passages contain fewer word choices for the detector to examine, so results are less reliable.
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Adding human writing dilutes the evidence. The green-list watermark survived when we added unmarked text equal to the original passage. SynthID fell below its whole-document threshold. Its mark may still have remained detectable within the original section, but it was lost when the document was assessed as one block.
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Detection does not establish authorship. Pasting one marked AI paragraph into a human document could cause the whole document to be flagged. The surrounding human writing has not acquired the watermark, but a whole-document result may create that impression.
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Human work can acquire an AI watermark. A model could revise writing created years before LLMs existed and return a marked version. The pattern would show that the model influenced the wording, not that it created the ideas or original argument.
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Writing quality remains an open question. Watermarking gives arbitrarily selected words a small advantage during generation. Some influence on the model’s choices is unavoidable, even if it is too subtle to notice in an individual response.
What is concerning is that schools, employers and publishers will treat watermark detection as a verdict, not just as evidence of model involvement. It should instead be one part of a wider provenance record.
The community reaction has been highly negative. Dozens of Claude users posted subscription cancellations, with writers and developers worried that clients, employers or schools could mistake evidence of AI assistance for proof of AI authorship. Others objected to Anthropic placing a persistent marker in private work without an opt-out, particularly while retaining sole control of the detector. Investor Bill Gurley described that position as making Anthropic “judge, jury and prosecutor”.
Businesses publishing synthetic media also have their own obligations. An AI-generated image that appears to document a real person, company, product, place or event may require a visible disclosure. AI-assisted text follows a more practical rule. The EU requirements target public-interest text published without substantive human review or accountable editorial control. They do not require every business to label material simply because AI helped with research, drafting, translation or editing.
This places the emphasis on the publishing process. A responsible organisation should know who checked the facts, who edited the copy and who approved publication. Automated articles published without meaningful review need disclosure. Work that has passed through a genuine editorial process does not need a warning merely because Claude contributed to it.
Takeaways: Our experiment shows that watermark detection is a sliding measure shaped by passage length and editing, not a permanent verdict attached to every word. The technology can provide evidence of model involvement and give AI labs a route to regulatory compliance, but it cannot determine authorship, accuracy or responsibility.
