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

hwanjongyu · 10-26-2026, 10:25 PM

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 Scribe and Paperwhite.

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.

DNSB · Yesterday, 04:42 AM

Compiled the CLI and GUI versions for Windows (rustc 1.99.1). Both seemed to work though I've only tried a few ePubs.

hwanjongyu · Yesterday, 03:38 PM

Thanks so much for testing, DNSB — I really appreciate you building both the CLI and the GUI on Windows and giving it an early run.

Any suggestions are very welcome, whether it's about conversion quality, Word Wise display, or EPUB structures that don't convert cleanly. Feedback of any kind is a big help at this stage.

hwanjongyu · Yesterday, 03:58 PM

Duplicated post - please ignore.

10-26-2026, 10:25 PM	#1
hwanjongyu Enthusiast Posts: 29 Karma: 10 Join Date: Aug 2015 Location: Canada Device: Kindle Scribe/Paperwhite and Kobo Libra H20 / Glo HD	kfx_converter: EPUB to KFX with Word Wise, in pure Rust (no Kindle Previewer / Calibr 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 Scribe and Paperwhite. 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. Last edited by hwanjongyu; Yesterday at 08:42 AM.

Yesterday, 03:58 PM	#4
hwanjongyu Enthusiast Posts: 29 Karma: 10 Join Date: Aug 2015 Location: Canada Device: Kindle Scribe/Paperwhite and Kobo Libra H20 / Glo HD	Duplicated post - please ignore. Last edited by hwanjongyu; Today at 10:29 AM.

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Yesterday, 04:42 AM	#2
DNSB Bibliophagist Posts: 55,432 Karma: 182544230 Join Date: Jul 2010 Location: Vancouver Device: Kobo Sage, Libra Colour, Lenovo M8 FHD, Paperwhite 4, Tolino epos	Compiled the CLI and GUI versions for Windows (rustc 1.99.1). Both seemed to work though I've only tried a few ePubs.

Yesterday, 03:38 PM	#3
hwanjongyu Enthusiast Posts: 29 Karma: 10 Join Date: Aug 2015 Location: Canada Device: Kindle Scribe/Paperwhite and Kobo Libra H20 / Glo HD	Thanks so much for testing, DNSB — I really appreciate you building both the CLI and the GUI on Windows and giving it an early run. Any suggestions are very welcome, whether it's about conversion quality, Word Wise display, or EPUB structures that don't convert cleanly. Feedback of any kind is a big help at this stage.

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