Anthropic has cut the system prompt for Claude Code by 80 percent, reflecting a fundamental shift in how newer Fable 5 models respond to instructions. Rather than requiring lengthy, detailed rules and examples, these newer models perform better with shorter prompts and work more creatively within broader context-based guidance. This suggests that as AI models become more capable, the way companies steer and control them may need to change dramatically.
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Anthropic has reduced the system prompt for Claude Code by 80 percent. The shift reflects a change in how the newer Fable 5 models (also called the Mythos class) respond to instructions—they perform better with shorter prompts and fewer examples, according to Tariq Shihipar, a member of technical staff at Anthropic.
Why it matters
This represents a fundamental change in AI steering. Earlier models required lengthy, detailed instructions with many examples and hard rules. Now Anthropic is steering Fable models through context instead of explicit constraints, since the models are more capable of working imaginatively within broader guidance. This suggests how companies train and control next-generation AI systems is shifting.
What to watch
Anthropic describes this as one stage in an evolving pattern: early models needed short prompts with restrictive rules, then prompts grew longer as models improved at understanding them, and now they are becoming shorter again. The approach may indicate how instruction design will need to adapt for future model releases.
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