I have this worded in my CLAUDE.md as "avoid counterfactuals". It still does it anyway, of course, but that language combined with a few rounds of review back-and-forth seems to work for me

That’s better but I’d argue it’s still not clear enough. If you told me that, I’d interpret that as meaning “don’t try to imagine things that never happened as a thought exercise”, which is what I normally understand “counterfactual” to mean.