To the OP: Proton is privacy first. I use their tools for a reason, trusting file transfer to a third party would never happen even with an open code base.

It's fine if the tool is for yourself, great for your CV and well done for shipping something. HN can at times be a cruel place, think of this as a learning experience.

e.g tar -cf - * | <some unwieldly proton drive cmd>

I guess this is not possible due to the way the encryption works?

"Sign-in happens through your browser — no password on the command line."

which I can confirm having tried it myself. It's hard to imagine what the proton devs were thinking. Did the case of using a command line tool on a headless or remote server never occur to them?