Anthropic deleted 80% of Claude Code's system prompt. No regression.

Anthropic removed over 80% of Claude Code's system prompt for Claude Opus 5 and Fable 5, finding no regression on coding evaluations. The company discovered that overconstraining the model with detailed rules and explicit instructions, which were necessary for weaker models, actually hinders performance on newer models. 'We found that we were overconstraining Claude Code, both through our system prompt and in our CLAUDE.md files and skills,' Anthropic stated.

Anthropic just published something that should make every developer building on Claude rethink their context engineering. For Claude Opus 5 and Fable 5, they removed over 80% of Claude Code's system prompt — and saw no measurable loss on coding evaluations. That's not a trim. That's a rewrite of the rules. "We found that we were overconstraining Claude Code, both through our system prompt and in our CLAUDE.md files and skills." The insight buried in this post is about what happens when your prompting habits are built for a weaker model. They don't just fail to help — they start actively getting in the way. /doctor is the new command to know.This is the core tension: the habits that made your prompts robust against GPT-3.5 or earlier Claude versions — detailed rules, explicit fallback instructions, long constraint lists — are exactly what slow down Claude 5. The model doesn't need to be told "don't do the obviously bad thing." It already knows. Every token you spend explaining the obvious is a token the model now has to interpret, reconcile with other instructions, and work around. Less context, better results. That's the new rule. /doctor to audit your CLAUDE.md and skills for over-specification.Source: The new rules of context engineering for Claude 5 generation models https://claude.com/blog/the-new-rules-of-context-engineering-for-claude-5-generation-models ✏️ Drafted with KewBot AI , edited and approved by Drew.