The trap, in one line
A custom output style in Claude Code does more than change the tone. It replaces part of the system prompt. And by default, the part it drops is the built-in software-engineering instruction block.
So you set a nice clean reporting style, and Claude quietly gets worse at the coding work itself. Not louder. Not obviously broken. Just a bit less disciplined about the things it used to do without being asked.
Why this matters right now
A fortnight ago I published the ELI5 system — an output style that makes Claude report in Simplified Technical English so you stop reading essays. That style has one line in it that most people skim past:
---
name: ELI5
description: keep it simple pls
keep-coding-instructions: true
---
It's been a long day and my brain is fried, talk to me like I'm 5.keep-coding-instructions: true. That is the whole fix. It was in the published style on purpose. If you copied it, you are fine.
If you wrote your own style, or adapted someone else’s, check it. The default is false.
What actually gets dropped
Be precise about this, because the internet is not. Claude Code’s official output-styles documentation states that custom styles “leave out Claude Code’s built-in software engineering instructions” unless keep-coding-instructions is set to true.
That is the coding-instruction block. Not the entire system prompt. Your tools still work. Your permissions still work. Your CLAUDE.md still loads. What goes missing is the standing guidance about how Claude approaches engineering work — the conventions it follows when it is writing and changing code without you spelling them out.
Which is exactly the part you did not intend to touch when you asked for shorter sentences.
The symptom
This is a hard bug to notice, because nothing errors. You just get a slow drift:
- Slightly sloppier edits to existing files.
- Less instinct for following the patterns already in the repo.
- More “here is a new file” where you wanted a small change.
- A vague sense that Claude got worse after an update, when what changed was your style file.
Style is meant to be presentation. Treat it that way, and keep the engineering brief intact.
Audit yours in one paste
Paste this into Claude Code. It reads only, changes nothing, and tells you where you stand:
Read every file in my global output-styles folder and every output-styles folder in this project.
For each style file, tell me its name and whether keep-coding-instructions is set to true.
List the ones where it is missing or false.
Then tell me which style is currently active in my global settings and in this project's settings.
Do not change anything yet.If anything comes back missing or false, add the line to the frontmatter of that style file and restart Claude Code. That is the whole repair.
The general rule
Anthropic’s own guidance on steering Claude Code draws the boundary well: each steering surface does one job. Output styles change how Claude talks to you. They are not the place to change how Claude works.
So when you write a style, write it as a reporting instruction and nothing else — then keep the coding instructions switched on. You want the shorter answer. You do not want the cheaper engineer.
Sources
From idea to operation
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