My comment was a bit tongue in cheek, I'm not actually convinced there was real degradation in opus 5 beyond a tendency to try to plough ahead without stopping to clarify things.

I don't really think 1 line in lengthy system prompt affects things that much, it'd just be an amusing form of emergent behaviour where we now have to massage the ego of something with no id.

For complex projects with lots of internal tools and strict requirements, I'm finding a fairly lengthy system prompt is quite worth it.

Start short or empty and watch where it makes mistakes then just keep tuning it so they're less frequent. That works for me.