Harper is an English grammar checker designed to be just right. I created it after years of dealing with the shortcomings of the competition.
Grammarly was too expensive and too overbearing. Its suggestions lacked context, and were often just plain wrong. Not to mention: it’s a privacy nightmare. Everything you write with Grammarly is sent to their servers. Their privacy policy claims they don’t sell the data, but that doesn’t mean they don’t use it to train large language models and god knows what else. Not only that, but the round-trip-time of the network request makes revising your work all the more tedious.
LanguageTool is great, if you have gigabytes of RAM to spare and are willing to download the ~16GB n-gram dataset. Besides the memory requirements, I found LanguageTool too slow: it would take several seconds to lint even a moderate-size document.
That’s why I created Harper: it is the grammar checker that fits my needs. Not only does it take milliseconds to lint a document, take less than 1/50th of LanguageTool’s memory footprint, but it is also completely private.
I’m used to LanguageTool, and at a glance it seems like Harper covers way fewer rules than LanguageTool does. Not sure if this is actually noticeable in practice, but I run my own LanguageTool server and am not too picky about the performance, so I’m not in a rush to move until someone figures out a good way to compare them. LanguageTool’s rules are all open source at least, so it’s only a matter of time before Harper gets anything it might be missing.