Claude's invisible watermark can flag involvement, but it cannot prove authorship
Anthropic says future Claude models will generate text with a statistical watermark as part of compliance with the European Union's transparency requirements. Its version of Google DeepMind's SynthID-Text changes the source of randomness when a model chooses among similarly suitable next words. It adds no characters, visible marks, extra tokens, user identifiers, organization data, or chat information, and Anthropic says internal testing found no practical quality effect. Detection is probabilistic. With Anthropic's key, a detector can estimate whether Claude was involved in writing a passage; it cannot establish human authorship, identify another model, or distinguish original generation from heavy editing. Confidence is weaker for short samples, factual passages, proofreading, and code because the model has fewer equally valid word choices. Light editing may preserve the signal, while a complete rewrite can remove it. Anthropic plans a detection API and says supported image files will use separate C2PA content credentials.