How I fixed Opus 5's writing style
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A developer shared a method for fixing Opus 5's writing style by modifying the CLAUDE.md file. The key is to stop putting style rules directly into CLAUDE.md and instead use frontmatter with keep-coding-instructions: true. This ensures that the desired brief, declarative prose style, which avoids personal pronouns, unsolicited additions, and em dashes, layers on top of existing coding instructions rather than replacing them, prioritizing perspicuity.
Stop putting style rules in CLAUDE.md
They're context, not instruction, so the model often treats them as optional.
Use output style files instead. They live in the system prompt and get reinforced every turn, so nothing drifts.
Make a file in ~/.claude/output-styles/, call it whatever, e.g. clean.md:
--- name: Clean description: Doesn't matter keep-coding-instructions: true --- You write brief, declarative prose, without personal pronouns, unsolicited additions, or em dashes. You should prioritize perspicuity. The answer goes in the final sentence.
Restart Claude Code and select it in your config.
Keep keep-coding-instructions: true in the frontmatter or your style replaces the coding instructions instead of layering on top.