Analysis: The System Prompt Architecture Behind Anthropic's "Mythos" Tier (Claude Fable 5

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I’ve curated the leaked 120KB system prompt for Anthropic’s Fable 5 model. For those of us building open-weights agentic models or working on prompt engineering, there are some incredible architectural learnings here on how Anthropic handles tool orchestration:

  • Dual-Deployment Safety: Fable 5 shares weights with the unrestricted Mythos 5, but relies entirely on safety classifiers at inference.
  • The 22 JSON Tool Schemas: The prompt reveals exact JSON definitions for how Claude interfaces with external APIs, including its own internal Anthropic API (Claudeception).
  • Behavioral Refusal Softening: Explicit instructions not to use bullet points when refusing a prompt to “soften the blow” emotionally.

I’ve archived the raw prompt and broken down the tool definitions here for anyone researching frontier model alignment:

repo:GitHub - saynchowdhury/claude-fable-5-system-prompt: system prompt of claude fable 5 · GitHub

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