Model-Level Watermarking in Large Language Models: Anthropic's Claude, Keyed Sampling Provenance, and Watermark Propagation
In August 2026, Anthropic disclosed how its Claude models watermark generated text. This paper explains the disclosed system, what it does and does not prove, and how to study it experimentally. The main points:
- Claude's watermark is a decoding-time, keyed pseudorandom sampling watermark. It is a variant of the SynthID-Text method published by Google DeepMind (Nature, 2024), in the family of approaches that began with Aaronson's 2022 proposal.
- The watermark changes only the source of randomness used to pick among candidate tokens during sampling. Nothing is added to the text, there are no hidden characters, no extra tokens are generated, and Anthropic reports no measurable impact on output quality, speed, or cost.
- Detection requires a secret key held by the provider, and it yields a likelihood that Claude was involved in producing a passage, not a binary auth