kfx_converter: EPUB to KFX with Word Wise, in pure Rust (no Kindle Previewer / Calibr

[hwanjongyu]

Hi all,

I built kfx_converter, a native EPUB to KFX converter in Rust, mainly to get Word Wise working on sideloaded books.

The problem: Word Wise works well on Amazon-purchased books, but not on personal EPUBs. MOBI is legacy, and AZW3/KF8 does not carry Word Wise data, so KFX was the only path. Until now, producing valid KFX usually meant relying on Kindle Previewer.

What this does:
- Converts DRM-free EPUB to KFX (Yellowjacket / Enhanced Typesetting)
- Generates a Word Wise sidecar (.kll) with a built-in CEFR B2-C2 lemma dictionary
- No Python, Calibre, or Kindle Previewer required
- CLI (kfx-conv) and GPUI desktop GUI (kfx-studio)
- Optional auto-sync to a connected Kindle

Word Wise data is stored in the local .kll sidecar, so it does not need to be fetched online after the book is on the device.

Build and use:
cargo build --release --bin kfx-conv
kfx-conv book.epub -o book.kfx -w
kfx-conv book.epub -w -s # convert with Word Wise and sync to Kindle

Source (MIT): https://github.com/hwanjongyu/kfx_converter

Current status: source build only, no prebuilt release yet. Tested on [Kindle model + firmware, e.g. Paperwhite / OS version].

Known limitations / feedback wanted:
- Which Kindle models and firmware versions show Word Wise correctly?
- Which EPUB structures break conversion or layout?
- Any books where the .kll glosses look wrong?

This is an independent project, not affiliated with Amazon. Feedback and bug reports are very welcome.