ANTHROPIC'S CLAUDE FABLE 5 SYSTEM PROMPT JUST LEAKED
Someone extracted the full internal system prompt from claude fable 5 and it's a goldmine for anyone building with llms
here's what's actually inside:
▫️ fable 5 and mythos 5 share the same underlying model fable is the public version with extra safety guardrails, mythos goes to approved orgs only
▫️ model strings exposed: claude-fable-5, claude-opus-4-8, claude-sonnet-4-6, claude-haiku-4-5-20251001
▫️ knowledge cutoff is end of jan 2026 but claude is instructed to web search anything that might've changed
▫️ detailed instructions on when NOT to be sycophantic no excessive apologies, no self-abasement, maintain self-respect even when users are rude
▫️ artifacts get persistent key-value storage via
window.storage enabling journals, trackers, leaderboards across sessions
▫️ strict copyright rules baked in hard limit of 15 words per quote, one quote per source max, never reproduce lyrics or poems
▫️ memory system is opt-in claude only has memories if the user enables it in settings
▫️ claude can now end conversations with an end_conversation tool if users become abusive
▫️ full mcp app integration logic connectors need opt-in before claude calls third-party tools on your behalf
the most interesting part? how much of claude's "personality" is explicitly engineered in the prompt vs emergent from training.
github repo:
github.com/elder-plinius/CL4…