Fable 5.1's system prompt leaked: what you can learn from it
A commented reading of the model's internal instructions and what to copy for your agents.
The text examines the leaked system prompt of Claude Fable 5.1, published in the CL4R1T4S repository on GitHub. The document grew from 17,501 to 40,046 words. About 75% of that growth came from instructions about memory and tools.
The analysis highlights 28 new tools, bringing the total to 46. They include features for saving information across conversations, retrieving earlier chats, and using visual interfaces, plugins, and skills. For people who use AI, the central idea is to assess the whole system — model, memory, tools, interface, and policies — rather than the model alone.
When creating your own agents, pay attention to how they save and retrieve information and which tools they can use. The published guide brings together an analysis of these instructions and helps identify ideas that may inform your agents.
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