Text Claude writes carries a mark.
It isn't a visible mark.
On August 14, 2026, Anthropic published how its text watermarking works.
The key is not changing the sentences.
It changes the source of randomness in how words are chosen.
It Looks at the Pattern of Choices, Not the Meaning
A language model picks the next word.
At that point, there are often several candidates with nearly the same meaning.
Either "cloudy" or "dim" might fit the context.
The watermark leaves a pattern in these low-stakes choices.
Anthropic explained that it uses a key and the preceding words to set the randomness of the next choice.
A checker looks for whether the pattern matches under the same key.
The result is "the possibility that Claude was involved in writing some of it."
Users Reportedly Won't Notice Any Change
Anthropic said the watermark has no real effect on quality or content.
It doesn't need extra tokens either.
So it explained that speed and price don't change.
The important point is that the watermark contains no user information.
It is not a function that identifies a particular person, organization, or conversation.
It Weakens in Short Text and Proofread Text
A watermark shows up better when there are more choices.
Short sentences have few choices to observe in the first place.
So detection confidence can be low.
The same goes for proofreading that only fixes grammar and punctuation.
That's because Claude picks few new words.
In code it is even more limited.
Where exact tokens are required, there is almost no room for choice.
A Watermark Does Not Identify a Person
First, don't use a watermark as proof of authorship.
The official explanation also says it only assesses the possibility of Claude's involvement.
Second, short quotations or documents that have been edited are hard to use as sole evidence.
Third, don't write the equation "detection result = identifying the user" into internal policies.
This announcement is closer to a supporting signal that helps judge provenance than a device that settles at once whether AI was used.