The new CLAUDE.md advice: delete, don't accumulate
Anthropic stripped more than 80% of Claude Code's system prompt for models like Opus 5 and Fable 5 and saw no measurable loss on coding evals, according to its context engineering post from July 24, 2026.
Guidance on CLAUDE.md has flipped in seven months. In January 2026, Boris Cherny recommended writing a new rule into the file every time the model got something wrong. After Opus 5 shipped on July 26, he told YC Startup School to delete CLAUDE.md, skills, and hooks every six months and add lines back only when a failure repeats.
The tooling already reflects that. In Claude Code 2.1.206, /doctor suggests cutting directory structure, dependency lists, and architecture overviews out of the file, leaving the gotchas and the deviations from defaults.
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