Claude Fable 5 system prompt leaks on GitHub

// 2h agoNEWS

Claude Fable 5 system prompt leaks on GitHub

The complete system prompt for Anthropic's Claude Fable 5 has leaked on GitHub, revealing a new "Mythos-class" tier of models and detailed AI instructions. The leak also references unannounced products like Claude Cowork and integrations for Chrome, Excel, and PowerPoint.

// ANALYSIS

This leak is highly significant as it inadvertently announces several new Anthropic products and provides deep insights into how advanced AI assistants are prompted to use tools and handle safety.

–Exposes a new naming convention and tier ("Mythos-class") with Fable 5 and Mythos 5, indicating a shift from the current structure.
–Reveals "Claude Cowork," a new agentic desktop app for non-developers, alongside "Claude Code".
–Uncovers upcoming beta integrations: Claude in Chrome, Excel, and PowerPoint.
–Details a new key-value storage API for Artifacts, allowing data to persist across chat sessions.

// TAGS

anthropicclaudeclaude-fable-5system-promptleakaillm

DISCOVERED

2h ago

2026-06-12

PUBLISHED

2h ago

2026-06-12

RELEVANCE

9/ 10

AUTHOR

AlphaSignalAI

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