A fellow user replied below, but it refers to the software that uses the LLM (Claude Code, Opencode, pi.dev, etc.).

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Funny you mention that, because I started noticing the word 'harness' being used everywhere about a month ago, even though I hadn’t seen it before (in this context). As I don’t trust my memory, I assumed I had just been overlooking it and added it to my vocabulary. However, a Google Trends search does show increased usage since the end of March: https://trends.google.com.br/trends/explore?date=today%203-m...