Anthropic Fable 5.1 System Prompt Leaked, Spanning 270k+ Characters
Scobleizer · x · 2026-09-02
The system prompt for Anthropic's Fable 5.1 model has been leaked, revealing a text exceeding 270,000 characters. The leak provides a detailed comparison with the previous Opus 5 version (dated July 24, 2026):
- Model Identity: Identifies as Claude Fable 5.1 (Mythos-class), sharing weights with Mythos 5.1. The export control suspension notice has been removed.
- Knowledge Cutoff: Updated to the end of June 2026 (previously May).
- Access Surfaces: "Claude Design" is no longer listed.
- Section Changes: Several older sections were removed, while detailed rules regarding safety, tool usage (e.g., <endconversation>), and addressing self-harm/violence were added or modified.
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