Claude Fable 5 System Prompt Leaked: 120,000 Characters Exposed on GitHub
In mid‑June 2024 Anthropic released Claude Fable 5, and within 24 hours a hacker posted the model's full 120,000‑character system prompt—covering personas, safety rules, downgrade policies, and more—on the public GitHub repository elder‑plinius/CL4R1T4S, revealing its close relationship with Claude Mythos 5.
Event timeline
In mid‑June 2024 Anthropic released Claude Fable 5, the first model in its new Mythos‑level family. Within 24 hours a hacker using the alias “Pliny the Liberator” uploaded the complete system prompt to the public GitHub repository elder-plinius/CL4R1T4S.
Prompt details
The uploaded file is 120,000 characters long, spans 1,585 lines and contains over 27,000 tokens. It enumerates:
Persona definitions and role assignments
Safety‑classifier rules
Refusal logic and fallback mechanisms
Style guide and dialogue strategy
Model‑downgrade rules, e.g., automatic switch to Claude Opus 4.8 in high‑risk domains such as cybersecurity and biochemistry
The repository elder-plinius/CL4R1T4S is dedicated to collecting leaked system prompts from major AI providers and already hosts prompts from ChatGPT, Gemini, Grok, Perplexity and dozens of other systems.
Relationship between Fable 5 and Mythos 5
The leaked prompt states explicitly that Claude Fable 5 and Claude Mythos 5 share the same underlying model. Fable 5 is the public‑facing, safety‑enhanced version, while Mythos 5 removes those safety measures and is offered only to authorized institutions. Consequently, the two are essentially the same model differentiated solely by the strictness of safety guardrails.
Anthropic response and dissemination
Anthropic has repeatedly requested removal of the file, but the prompt has already propagated widely across the internet, making containment effectively impossible.
Source links
GitHub repository: https://github.com/elder-plinius/CL4R1T4S
Original tweet by @KanikaBK
Times of India report
Signed-in readers can open the original source through BestHub's protected redirect.
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