TIL: Fixing Opus 5 Jargon With One System Prompt Line

Opus 5 defaults to formal consultant-speak. One sentence in your system prompt fixes it.

Write like a senior engineer talking to a colleague, not a consultant.

Add that to your system prompt and Opus 5 stops reaching for “leverage”, “facilitate”, and “synergise” by default. The output reads like something a human would actually write in a code review comment or a Slack thread, not a strategy document.

Opus 5 was trained on a lot of formal professional text. Without explicit register guidance, it defaults to that register. Most developer use cases call for something closer to how engineers actually talk.

Does this work in Claude.ai too?

Yes. Go to Settings > Personalization > Custom instructions and add it there. It applies to every conversation.

What if I want it even more informal?

Push harder. “Write like you’re texting a colleague, not filing a report” drops the register further. Tune it for the output you actually want.

Does this affect accuracy?

No. Register and accuracy are independent. The model gives the same factual content either way, just without the formal scaffolding around it.