Harper brings grammar and spell checking to Joplin's Markdown editor - fully offline. It is powered by Harper, the open-source grammar checker from Automattic: the whole engine ships inside the plugin as WebAssembly, so there is no cloud service, no account, and no network request. Your notes never leave your machine.
I wrote it because I already use Harper in my code editor and my browser, and I wanted the same checker - with the same personal dictionary - inside Joplin, where most of my actual writing happens.
As you type, issues get a coloured squiggle - a different colour per issue type, using Harper's own palette, so a misspelling looks different from a style nudge at a glance. Hovering an underline opens a small card with the explanation and the fixes:
What the card can do:
- Apply a fix in one click - each suggestion is a pill; clicking it rewrites the text in place.
- Add to dictionary - names and terms you use stop being flagged.
- Dismiss - hide one specific finding, remembered between sessions.
- Disable a rule - the toggle in the card header turns that rule off everywhere.
- Dialects - American, British, Australian, or Canadian English.
One feature I care about especially: the external dictionary. You can point the plugin at a plain text file (one word per line). Every word in it is accepted, Add to dictionary appends to it, and the file is re-read automatically. Because it is just a flat file, you can sync it between machines with whatever you already use - and share it with other Harper installations (harper-ls in Zed, Neovim, VS Code…). One personal dictionary, everywhere.
Practical notes: the plugin is ~20 MB because the entire checker ships inside it (downloaded once, works offline forever). The first check after startup takes a second or two while the engine warms up; after that it is ~50 ms per pass. It needs Joplin desktop 3.1+, works in the Markdown editor only (the Rich Text editor offers no hook for this), works both on desktop and mobile.
Install: in Joplin open Settings → Plugins, search for Harper, and Install (the marketplace listing may take a day or two to appear after this release). Until then - or if you prefer manual installs - download the .jpl from the latest GitHub release and use Install from file.
Source: GitHub - pmslava/joplin-plugin-harper: Harper grammar checker plugin for Joplin desktop · GitHub
All credit for the grammar engine goes to the Harper project (Apache-2.0, by Automattic) - this plugin integrates it into Joplin.
Like Cockpit, this plugin is AI-written - and extensively machine-tested: a stubbed-API harness plus a Playwright suite that drives a real Joplin desktop, both running in CI on every change.
Write your thoughts and comments - I will read them all with interest.