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.
More from Models
- Cohere's Embed 5 Fast beats Google's EmbeddingGemma 2 on 95% of 163 benchmark datasets — krypticmouse · 2026-10-18
- MOSS SFT vs Flux 3 Klein vs SD 3.0: Same-Prompt Comparison Exposes SD3's Age — ZootAllures9111 · 2026-10-18
- Quick take: GPT-6 matches Opus 5.5 at coding, far ahead at computer use — _vztu · 2026-10-18
- Open models now above Sol-level, outpacing expectations as community awaits V4.1 Pro — yacineMTB · 2026-10-18
- Codex took 3 days, Opus 5.5 nailed it in an afternoon — banned user's video protest — dotey · 2026-10-18
- Yacine MTB: Users will remember how Anthropic treated them once intelligence proliferates — yacineMTB · 2026-10-18