- Sources: Anthropic post, r/ClaudeAI
- Summary: Anthropic published a context-engineering post on 2026-07-24, written by Thariq Shihipar, stating it removed over 80% of Claude Code's system prompt for models such as Claude Opus 5 and Claude Fable 5 with no measurable loss on its own coding evaluations. The post names the specific reversals. Hard rules gave way to judgement: instructions such as defaulting to no comments and never writing multi-paragraph docstrings were replaced by a single line telling Claude to match the comment density, naming, and idiom of surrounding code. Tool-use examples gave way to expressive tool interfaces, on the finding that examples constrain newer models to the demonstrated space. Upfront instructions gave way to progressive disclosure, with code review and verification moved into separately invoked skills and some tools deferred behind a tool-search step so their definitions cost no context until needed. Repeated instructions moved out of the system prompt into tool descriptions, and memory written to
CLAUDE.md gave way to automatic memory. The guidance for users is to keep CLAUDE.md light and spend its tokens on codebase gotchas rather than what Claude can read from the repository, split long skills across files, and prefer references that are themselves code. Anthropic states a /doctor command applies these checks to a user's own skills and CLAUDE.md files. The evaluation claim carries no numbers. - Why it matters: Most of the prompt scaffolding teams have accumulated in
CLAUDE.md files is the vendor's own now-deprecated practice, and the stated finding is that deleting it costs nothing measurable. - Follow-up: Watch for the removal reproduced on evaluations Anthropic does not own.
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