Harper is a grammar checker designed to run anywhere there is text (so really, anywhere). Most Harper users are catching their mistakes in Neovim, Obsidian, or Visual Studio Code.
Harper takes advantage of decades of natural language research to analyze exactly how your words come together. If something is off, Harper lets you know.
In a way, Harper is an error-tolerant parser for English.
After installing harper-ls, configure it with Eglot for text modes:
(with-eval-after-load 'eglot
(add-to-list 'eglot-server-programs
'((text-mode markdown-mode org-mode) . ("harper-ls" "--stdio"))))
(dolist (hook '(text-mode markdown-mode org-mode))
(add-hook hook #'eglot-ensure))