Fable 5’s Leaked System Prompt, Run on Opus 4.8

A Threads thread by @artificiallyinfluenced.

FABLE 5 GOT SHUT DOWN. SOMEONE LEAKED ITS SYSTEM PROMPT. Then ran it on Opus 4.8 to see what actually changed.

• The setup: After the US government pulled Fable 5 on June 12, a developer extracted the leaked system prompt and ran it on Opus 4.8 head-to-head against stock Opus 4.8. The question: how much of Fable 5’s behavior comes from the system prompt vs the model weights?
• What it revealed: running Fable 5’s system prompt on Opus 4.8 produces a model that sounds more like Fable 5 — tone, phrasing, behavior. But reasoning quality, coding capability, and long-context performance do not transfer. “The prompt moves the surface.”
• Implication for builders: most of what you experience as a model’s personality is the system prompt; most of what you experience as its capability is the weights. When a new model feels dramatically different, ask which one actually changed.
• Deeper question: if a system prompt can meaningfully shift Opus 4.8’s behavior, how much of the gap between model generations is solvable with better prompting on older models? Answer: some of it — but not most.

(Commenters pushed for the actual prompt text, which the thread doesn’t include.)