A roughly 120,000-character system prompt for Anthropic’s Claude Fable 5 model has been leaked, revealing detailed behavior instructions, product information, refusal rules, and formatting guidelines. The prompt outlines how Claude should handle user requests, safety measures, available features, and external documentation searches.

• A ~120,000-character leak allegedly exposes Anthropic's full system prompt for "Claude Fable 5," including model names like claude-opus-4-8 and claude-sonnet-4-6.
• Claude Fable 5 and Claude Mythos 5 reportedly share the same core architecture, but the public Fable 5 has extra safety checks that Mythos 5 lacks for approved partners.
• The prompt instructs Claude to never render antml:voice_note blocks and to search docs.claude.com or support.claude.com before answering questions about current features, specs, or pricing.
• Safety rules detailed include refusing weapons/drug synthesis instructions and malware creation, avoiding persuasive text impersonating real public figures, and giving factual (not advisory) answers on legal/financial topics.

Anthropic has confirmed its most powerful AI model, Claude Mythos, after a configuration error exposed details about it. The model is said to significantly outpace previous versions in reasoning and cybersecurity, but it also poses serious risks, with the potential for misuse in cyberattacks. Early access will be limited to cybersecurity-focused organizations due to these concerns.

• A configuration error accidentally leaked ~3,000 unpublished assets revealing Anthropic's next flagship model, internally called Mythos (or possibly Capybara)
• The model reportedly has advanced cyberattack capabilities that could outpace current defenses, so Anthropic plans to limit early access to cybersecurity organizations first
• This follows a real incident where a Chinese state-sponsored group already used Claude Code to breach about thirty organizations
• The model is described as highly resource-intensive, echoing GPT-4.5's cost/efficiency problems, with no confirmed release timeline or final name