Fable 5.1 ships with Anthropic's statistical watermark on every platform plus cache-friendly API updates
Angaisb_ · x · 2026-09-02
Fable 5.1 is out with several API-level additions:
- Statistical watermark: text from Fable 5.1 and Mythos 5.1 carries Anthropic's statistical watermark on every platform, with no effect on output quality or token count
- Mid-conversation effort changes no longer invalidate the prompt cache
- Turn-scoped system messages allow per-turn reminders that stop rendering after the next user message, keeping the cache intact and costing no input tokens
- Progress updates between tool calls can now be surfaced as text via thinking-display "updates" while reasoning stays hidden
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