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I did search before posting. I've seen both of those, but they're just running the desktop files of the VN through a third-party kirikiri interpreter (Kirikiriroid2). I'm talking about the <em>official</em> mobile release.
I said the fate route not the whole VN. And hence Deen adaption.
Yup. Here's the link It's all in Japanese though and I think you have to pay to access the UBW/HF routes.
FSN got a mobile port back in 2014...
https://itunes.apple.com/jp/app/fate-stay-night-realta-nua/id967898020?l=en&mt=8
https://play.google.com/store/apps/details?id=com.typemoon.fsn2&hl=en
>prelude OVA
Or they could just adapt the existing UBW/HF route which is already written for them, compared to writing a prelude OVA which both takes time, and limit the option for the Grand Order writers to do one when they want, as well as limiting future events because it'll have to refer back to it to make sure they don't contradict themselves