Anthropic says it cut 80 percent of Claude Code's system prompt because Fable 5 models "want a smaller system prompt"

Anthropic says it has cut 80 percent of the system prompt for Claude Code. Tariq Shihipar, a member of technical staff at Anthropic, describes a fundamental shift in how AI models are steered, tied to the new Fable 5 models, also known as the Mythos class. More instructions and more examples no longer automatically lead to better results. "Most recently we found this new class of models want a smaller system prompt," Shihipar says, and examples "tend to constrain it because it's actually more imaginative than the examples we give it." Instead of hard rules like "do not do this," Anthropic now tries to steer Fable models through context, he adds.

According to Shihipar, this change happened in stages. Early models needed short prompts with lots of examples and restrictive instructions. Prompts then grew longer as models got better at understanding them. Now they're getting shorter again.