Harper – an open-source alternative to Grammarly

LanguageTool (a Grammarly competitor) is also open source and can be managed locally:

https://github.com/languagetool-org/languagetool

I generally run it in a Docker container on my local machine:

https://hub.docker.com/r/erikvl87/languagetool

I haven't messed with Harper closely but I am aware of its existence. It's nice to have options, though.

It would sure be nice if the Harper website made clear that one of the two competitors it compares itself to can also be run locally.

There are two versions of the LanguageTool: open source and cloud-based. Open source checks the individual words in the dictionary just like the system's spell checker. Maybe there is something more to it, but in my tests, it did not fix even obvious errors. It's not an alternative to Grammarly or this tool.

There is. It can be heavily customized to your needs and built to leverage a large ngram data set:

https://dev.languagetool.org/finding-errors-using-n-gram-dat...

I would suggest diving into it more because it seems like you missed how customizable it is.

This is a really nice app to use LanguageTool, it runs the server in the flatpak: https://flathub.org/apps/re.sonny.Eloquent