I used to think the same, that roasting burns off the caffeine so lighter roast = more caffeine.

But when I looked for a source on that one day it seems like that's a myth. There can be differences based on whether you measure by weight vs volume but that's about it. https://www.kickinghorsecoffee.com/en/blog/caffeine-myths-da...

So now I just say I like light roasts because I like to taste the coffee and not the roast.

This tends to be offset due to the density changes in the bean during roasting.

In short, a cup of light roast and a cup of dark roast have negligible differences in caffeine content.

Medium roasts have the most. With roast, both water evaporation and caffeine pyrolysis increase. Medium hits the maximum. I prefer very light roasts for the flavor.