Pattern: Claude Code's Own System Prompt Shrank ~80% for Opus 5, Making Long Rules Files a Liability
paddo.dev·high signal
Simon Willison, relaying tips from the Claude Code team, reports that Claude Code's system prompt recently shrank by 80% and that it's time to stop overloading prompts with examples and lists of things not to do. paddo.dev corroborates the same figure with no measurable loss on Anthropic's coding evals, and cites one build dropping from 470k tokens ($1.29) on Opus 4.8 to 179k tokens ($0.33) on Opus 5. The instruction-heavy CLAUDE.md that was best practice six months ago is now the thing to audit.