E999 error when send epub to Kindle

[s3rg3ysh]

I'm getting E999 error when trying to sent epub file to my Kindle via Amazon's Send to Kindle page

[kent-williams]

I just tried and experienced the same thing

[profvjreddi]

Thanks @kent-williams and @s3rg3ysh for reporting!

Quick question — is this the first time you're trying to send it to Kindle, or has it worked for you before?

I myself just use PDF, so honestly, the EPUB isn't something I have thoroughly debugged or made perfect. But I know it works on other readers for sure. I just haven't heard anyone mention Kindle before.

[s3rg3ysh]

It's first time to me, and this is only epub file (latest version) for this book I tried to upload via Send to Kindle page. I experienced the same error with other books as well, this for example victordibia/designing-multiagent-systems#8. Unfortunately I cannot upload epub to my Kindle directly via USB, that only works for mobi files. Thanks

[profvjreddi]

Ah, interesting — so it's not specific to our book. If you're hitting E999 on multiple EPUBs, you might have an older Kindle model? I should really get a Kindle, maybe I can get my kid one and tell her it's to debug our book 😅

Use Calibre (free) to convert the EPUB to MOBI yourself — would that work? When I get a chance I will try to play around with the Kindle side, but it's the start of the semester so things are hectic here :) Appreciate your patience!

[bl00dny]

Ah, interesting — so it's not specific to our book. If you're hitting E999 on multiple EPUBs, you might have an older Kindle model? I should really get a Kindle, maybe I can get my kid one and tell her it's to debug our book 😅

Use Calibre (free) to convert the EPUB to MOBI yourself — would that work? When I get a chance, I will try to play around with the Kindle side, but it's the start of the semester so things are hectic here :) Appreciate your patience!

I have latest Kindle Scribe device and about hundred other epub (even published in 2026) books which I have uploaded with no issues. It's still me (s3rg3ysh), just replied from another GH account

[panth]

I am also facing the same issue and yes other books are going in fine to my kindle. I like Kindle because doesnt play on the eyes and gives distraction free reading experience.. Lets see :-(

[profvjreddi]

Another user has reported the same issue in #1174 (now closed as a duplicate), indicating that this is affecting multiple Kindle users.

I acknowledge that this is a real issue — the EPUB I generate is not passing Kindle's validation. Based on @bl00dny's feedback (regarding Kindle Scribe), it appears that other EPUBs work fine, so the problem lies with what I am rendering and not with the device. The root cause is likely that our Quarto-generated EPUB contains elements that stricter validators reject. I have to figure this out. We have already encountered this issue with ClearView in #1052, including double hyphens in XML comments, CSS variables, and other factors. Still sifting through this in the midst of all the semester craziness here :) Kindle's "Send to Kindle" service conducts its own validation and is rejecting the file as a result. So if folks know how I can test this that would be helpful. Or if someone can help that'd be even more awesome!