Pliny the Liberator posted on X that the Fable 5.1 system prompt had been obtained. The independent researcher, known for LLM jailbreaking and red-teaming work, uploaded the complete text to a public GitHub repository. The prompt runs longer than 270,000 characters and contains several edits and additions compared with the earlier Opus 5 version. The same repository hosts other leaked system prompts for models from Anthropic, OpenAI, Google, and additional providers. The announcement included a direct link to the Claude-Fable-5.1.md file.