Is Claude Fable 5.1 watermarked?
Yes. Claude Fable 5.1 launched 1 September 2026 and Anthropic explicitly lists it as a supported model whose text output is marked. The watermark is statistical: secret-keyed randomness influences choices among near-equivalent next words. Because the signal lives in word choice rather than hidden Unicode or metadata, copy-paste and invisible-character cleaners do not remove it. Anthropic's detector is in private preview for eligible organisations, so public checkers — including this one — cannot verify whether an individual passage carries the mark. Anthropic says light edits may preserve it, while a complete rewrite in which every word is replaced removes it. Our free checker finds concrete copy-paste artifacts. The optional rewrite resamples the passage through a non-Claude model, following the documented removal mechanism, but cannot yet be independently verified against Anthropic's private detector. This distinction matters because character-only removers cannot alter a word-choice signal. Last checked 2 September 2026.
Primary sources: Anthropic marking status · watermark mechanism and limitations
- Launched
- 1 September 2026
- Marking cutoff
- 2 August 2026
- Status
- Marked at launch
- Last checked
- 2026-09-02
Anthropic's first generally available model explicitly listed as marked at launch.
How does the Claude Fable 5.1 watermark work?
Anthropic’s mark is distributional. During generation a secret key changes the randomness used to choose among equivalent next words. The signal is therefore in the prose itself, not in metadata, HTML or invisible Unicode, and it travels with copied text.
Can a Claude Fable 5.1 watermark detector check it?
Anthropic's detector is in private preview for eligible organisations. There is no public Claude Fable 5.1 watermark detector, so public tools cannot verify whether an individual passage carries the mark.
Can the Claude Fable 5.1 watermark be removed?
Anthropic says light editing probably will not remove the watermark completely, while a complete rewrite in which every word is replaced will. Public tools cannot certify the result while detector access remains private.
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Check my text →Other models
- Is Claude Mythos 5.1 watermarked?Yes
- Is Claude Opus 5 watermarked?No
- Is Claude Sonnet 5 watermarked?No
- Is Claude Fable 5 watermarked?No
- Is Claude Mythos 5 watermarked?No
- Is Claude Opus 4.8 watermarked?No
- Is Claude Haiku 4.5 watermarked?No
