Thanks all for your answers.
The short answer:
After writing most of the email below I found the simple solution: if I switch to a US keyboard and type the answer, then Bunpro is transforming what I type in hiragana… it was not obvious to me. I think it solves my problem well enough. If the error message was different it might have hinted me at the solution. Maybe something like “Please use only Hiragana or Katakana in your answer. If you use a japanese keyboard, try switching to an Azerty or Qwerty keyboard and let Bunpro convert as you type in hiragana automatically.”
The long answer:
For reference, I am using the Japanese keyboard on mac. Maybe stuffs looks different on iOS or Windows, or Android. As mentioned in the “short answer” above I was using as japanese keyboard by default. All what is bellow is about supporting better typing with a native japanese keyboard setting in Bunpro.
I don’t know what would be best for vocabulary, or kanji practice, but at least for grammar I think that allowing submitting answers with Kanjis should be allowed, at least as an option.
There are many cases where there are actually kanji in the grammar. The problem that I had was with things like 上に, 上で, に限る, に違いない, etc. Looking at the list of grammar points, it is easy to see that there are many cases where the usual form of the grammar is using kanjis. When I type them with the Japanese keyboard, the kanji versions are automatically selected by default on the mac.
Also, in some ways, having to write the right kanji can be actually harder, since you have not just to remember the proper hiragana, but also selecting the right kanji in the list.
There is definitely cases where validation is going to be tricky. Some grammar form do have version in kanji that are not much used. Should it be allowed at all? with a warning? I have in mind things like にもかからわず that is actually written にも拘わらず and not にも関わらず.
Forcing to select the hiragana version in the list after I type with a japanese keyboard is not only slow, but since the system seems to remember that I prefer the hiragana writing like うえで, then I need to correct it again when I write a mail in Japanese with the same form.
All being said, I’d love to have this option and I am ready to help for it if needed, but I can understand that you might have a long list of more urgent feature to build.
Regards