Claude starts watermarking text from four older models on November 3
Starting November 3, 2026, Anthropic adds its invisible text watermark to four older Claude models: Mythos 5, Opus 4.7, Opus 4.6 and Sonnet 4.6. The mark applies worldwide, users cannot switch it off, and only approved organizations can detect it. Four days later xAI turns on a similar watermark for seven Grok API models.