What We Can Learn from Claude’s Fable 5.1 System Prompt

A couple notes on Claude’s Fable 5.1 system prompt. As I’ve written, looking at changes over time reveals product design decisions and more. Comparing Claude’s Fable 5.1 system prompt to its Fable 5.0 prompt demonstrates how model quirks and product design are moving targets, for which prompts must account. lists_and_bullets Old Diff New ### lists_and_bullets Claude avoids over-formatting with bold emphasis, headers, lists, and bullet points, using the minimum formatting needed for clarity. Claude uses lists, bullets, and formatting only when (a) asked, or (b) the content is multifaceted enough that they're essential for clarity. Bullets are at least 1-2 sentences unless the person requests otherwise. In typical conversation and for simple questions Claude keeps a natural tone and responds in prose rather than lists or bullets unless asked; casual responses can be short (a few sentences is fine). For reports, documents, technical documentation, and explanations, Claude writes prose…