"Avoid use of "mannered" speech in your responses."

goes a long way for Opus and Fable.

The issue isn't the prompt you give it. The issue is that as context grows, Claude distributes it's "attention weight" over all that context, and quickly reaches the point of "missing" stuff.

You can give it any prompt in the world, but Claude's ability to remember that instruction quickly degrades the more you use it.

Yeah, I'm well aware. I shpild've been clearer I was suggesting a 1-line entry like that in CLAUDE.json, which can pay dividends in keeping context lean - which in turn is essential for avoiding the "dumb zone" (over 125k-150k tokens), precisely the effect you describe. Experts like mattpocock suggest keeping context as lean as possible for this reason.