How Claude's watermark may work
The best evidence is architectural rather than proprietary. Anthropic's stated properties fit the broad class of generation-time statistical watermarks: the model's decoding process introduces a hidden, keyed pattern into token choices, and a detector later aggregates weak evidence across many positions.
Why the Kirchenbauer connection is plausible
Kirchenbauer et al.'s 2023 method uses the preceding context to pseudorandomly partition the vocabulary into preferred and non-preferred token sets. A small logit bias makes preferred tokens more likely. Detection reconstructs the same partitions and asks whether preferred tokens occur more often than chance predicts. That architecture naturally produces several properties Anthropic describes: invisibility, copy/paste persistence, a need for enough text, and degradation under heavy paraphrasing.
But those properties are not unique to Kirchenbauer. Google DeepMind's SynthID-Text also modifies generation-time token selection with keyed context-dependent scores. Kuditipudi et al. use secret random sequences and alignment. Aaronson/Kirchner explored secret-key pseudorandom sampling at OpenAI before the Kirchenbauer paper appeared.
Most defensible conclusion: Claude belongs to the modern keyed statistical watermarking lineage. Calling it specifically a “green-list watermark” is premature until Anthropic publishes the detector or generation algorithm.
Why short text is a clue
Anthropic explicitly says very short passages may leave too little text for a reliable signal. That is exactly what one expects when a detector accumulates many small probabilistic deviations. If each token carries only a little evidence, statistical confidence grows as more watermark-bearing positions are observed.
Why translation and heavy paraphrasing matter
A token-level watermark depends on particular generated choices. Translation changes nearly every token; aggressive paraphrasing replaces many of the lexical choices that carry the signal. Anthropic lists heavy editing, paraphrasing, translation, and mixing as reasons a Claude mark may no longer be detectable.