Claude Fable 5 System Prompt Clean — AI skill for Claude Code

Universal Fable 5 System Prompt

[](https://opensource.org/licenses/MIT) 

[English](README.md) | [简体中文](README.zh-CN.md) | [日本語](README.ja.md)

A vendor-neutral adaptation of Anthropic's publicly available **Claude Fable 5 / Mythos 5** system prompt.

The original prompt contains a large amount of infrastructure that exists specifically for Claude's production environment, including XML conventions, tool interfaces, product-specific behavior, and internal implementation guidance. While appropriate for Claude, much of that content provides little or no benefit when reused with other frontier LLMs.

This repository filters out those implementation details while retaining the behavioral guidance that generalizes well across modern models.

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Goals

• Reduce prompt size for better context efficiency.
• Remove Claude-specific implementation details.
• Preserve high-impact behavioral guidance.
• Maintain compatibility across major frontier LLMs.

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Versions

File	Purpose
universal/core.md	Core behavioral principles with minimal token usage.
universal/balanced.md	Recommended for most use cases. Balances prompt size with behavioral coverage.
universal/complete.md	Retains nearly all vendor-neutral behavioral guidance while remaining model-agnostic.

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What's Removed

This project intentionally excludes Claude-specific implementation such as:

• XML wrappers and formatting conventions
• MCP and tool-specific infrastructure
• Product-specific instructions
• Internal UI behavior