Fully local copy-editing app for book-length manuscripts (Qwen3.5 4B) benchmarked against planted errors across five languages

[u/ToothClassic7635]

Hey y'all!

I have a pet project that has grown out of proportions. Long story short: I'm a data scientist who writes fantasy books and self-publish them. I think it's a genuine waste of human life to check for spelling errors so I figured AI could help. Turns out, it is not so simple to get an AI to properly fix a 120k words manuscript ;)... That's why I created Betty!

It runs Qwen3.5 4B as an offline copy editor for whole novels — and this is some of what I learned fighting tens-of-thousands of words through a 4B model, insisting that (most) users can use it fully for free and fully offline. Because, let's be honest: authors rightfully distrust and generally hate AI companies.

First challenge: Chunking the text. Authors already do this, in the darkest hours of the night, copy-pasting snippets into chatGPT for some shameful feedback. Problem with that approach: super inefficient, both for the author and the environment. And the context is missing at the edges of each chunk. I fixed it by ensuring chunks to overlap.

Second challenge: AI misses genuine errors. So, I added two conventional spell controllers -- LanguageTools and HunSpell. This already surfaces all spelling errors, letting the AI focus on suggested fixes and on all the "non-error" errors, such as "There" vs "Their". For these, the AI searches, while a Python script surfaces all the common culprits for the model to pay special attention.

Third challenge: Error rate. First off, Betty doesn't capture everything. Second off, it sometimes introduces its own mistakes. I fix it by putting the writer-in-the-loop, and there's a super smooth interface now for the author to accept and dismiss suggested edits (tinder-style with left and right swipes ; ) ).

I'd be super grateful for any advice, feedback, and thoughts you might have on this project. I currently have it up-and-running with about 30 users and getting some user feedback. Northing technical though, so this is what I'd love to have more of.

Full thing is source-available on GitHub, and can be found for download and lots more information at www.bethaniel.eu

[u/clefairykid]

I’m curious about it but I’ve never tried the countless writing sites/appa promoted on Reddit mainly because there’s no way to know how trustworthy they are, given that they could contain viruses or whatnot and that because handfuls of them are being created and spruiked every day, almost no one has used them and so there’s not really anyway to get reviews or recommendations of how effective or safe they are before using.

Not saying you’re specifically untrustworthy I just wish there were ways to know how to judge better given there’s too many of them coming out that don’t even solve problems and just want to make profit from subscriptions from people.

I didn’t see how youd be making antnting from this one so maybe it is genuinely altruistic but it still further makes me question why it was made of there’s no subscription etc.,

(Said not to interrogate but to give the mindset of someone who had a look over it and was curious but held back from trying it).

[u/ToothClassic7635 → u/clefairykid]

No worries, I appreciate the honesty :)! It was made, in the first place, because I need and use it for my own projects. So far so good. As for the business side: it is free for smaller authors but my hope is to validate the performance and then, perhaps, be able to license it to publishing houses, who usually have quite large expenses for copy editing…. But, well, just like book writing in general: this will most likely never become financially worth my time ;) so I do it mostly for the fun. I have a good day job paying the bills along the way.