Harper is a free, open-source grammar checker that runs entirely on your own machine. It is written in Rust, maintained by Automattic, and installed as an extension inside the editors you already use rather than as a cloud service. If you found Harper while looking for something that is not Grammarly, the short answer is that it is genuinely good, genuinely free and genuinely private. The longer answer is where it fits, and where its scope ends.
What is Harper?
Harper is an offline, privacy-first grammar and spelling checker written in Rust. It reads your text, flags misspellings, awkward phrasing, capitalisation slips and broken grammar, and offers a fix, without sending anything to a server. There is no account, no subscription and no usage limit.
Technically it is a language server first and an app second. The core is a Rust crate, wrapped in harper-ls (a language server), harper.js (a JavaScript library) and a WebAssembly build. Everything else, from the Obsidian plugin to the browser extensions, is a front end onto that same engine. That architecture is why it feels instant: the project treats slow linting as a bug, reports checking a document in milliseconds, and claims under a fiftieth of LanguageTool's memory footprint. The whole thing is readable on Harper's GitHub repository.
Who makes Harper, and is it trustworthy?
Harper is maintained by Automattic, the company behind WordPress.com, Tumblr and WooCommerce. It started as a personal project by developer Elijah Potter and was acquired by Automattic in late 2024. That is unusual in this category, where most free grammar checkers are weekend projects that quietly stall after a year.
The backing shows up in the things that matter. The repository carries more than 11,000 stars and over a hundred releases, with an active issue tracker. The licence is Apache 2.0, so anyone can read, fork or audit the code. It also has real community standing: the Hacker News thread introducing it as an open-source Grammarly alternative reached roughly 645 points. When a tool sits between you and everything you write, being able to check who maintains it is a reasonable thing to want.
What does Harper actually catch?
Harper catches mechanical errors: misspellings, doubled words, missing or misplaced punctuation, capitalisation mistakes, subject-verb disagreement, and a large library of common phrasing slips such as "should of" in place of "should have". It works from rules and dictionaries rather than a language model, so its suggestions stay predictable and explainable.
That rule-based design is the honest trade-off. Harper is strong on things that are definitively wrong and deliberately quiet about things that are only a matter of taste. It will not restructure your paragraph or tell you a sentence is too passive. A hands-on review at It's FOSS noted that it misses some subtler constructions a heavier cloud tool would flag. For most writers, catching real errors without constant style nagging is the better bargain.
Where does Harper work, and where does it not?
Harper works wherever you install an integration for it. Supported front ends include Obsidian, VS Code, Neovim, Zed, a WordPress plugin, Chrome and Firefox extensions, and a macOS desktop app. If you write in one of those, Harper is a few clicks away.
What Harper is not is a system-wide corrector for the whole of macOS. It does not sit behind every text field the way a native menu bar utility does, so Mail, Messages, Slack, Notes and Pages fall outside its reach. It also has no next-word prediction, and it currently supports English only. None of that is a failing; it is a scope decision. Harper is an editor and browser tool, and inside that scope it is excellent.
Is Harper really private?
Yes. Harper runs entirely on your device, with no cloud calls, no telemetry and no account. Your writing is checked in the same process that displays it, so there is no upload step to trust.
It is also one of the few tools where the privacy claim is checkable rather than promised. Because the source is Apache 2.0 and public, anyone can read the code, and anyone can watch the network traffic and see nothing leave. That is a stronger position than a privacy policy, which can always be revised later. If privacy is why you are leaving a cloud grammar checker, Harper clears the bar. Our guide to what on-device AI privacy actually means explains why local processing is the part that counts.
How does Harper compare to Grammarly?
Harper and Grammarly are built for different jobs. Grammarly is a cloud service with a style and tone layer, a paid tier, and rewriting driven by large models. Harper is a local mechanical checker with no paid tier and no rewriting. If you want sentences reshaped, Harper will disappoint you. If you want your errors caught without your drafts leaving the machine, Harper is the better fit and costs nothing.
Cost is the other split. Grammarly's paid tier is roughly $12 a month billed annually, around $144 a year, every year. Harper asks nothing. Charm is a third model: $9.99 once, no subscription. Against a recurring plan that is a dramatic saving; against Harper it is not, because nothing beats free.
The point people usually miss is coverage. Grammarly reaches into more places on a Mac through its own integrations, while Harper reaches into the editors and browsers it supports. Neither one corrects every app on the system. We cover the wider field in our roundup of the best Grammarly alternatives for Mac, our guide to the best grammar checker for Mac, and a direct Charm versus Grammarly comparison.
| Feature | Harper | Grammarly | Charm |
|---|---|---|---|
| Price | Free forever, open source | Free tier, then a recurring subscription, about $12 a month billed annually | $9.99 one-time, no subscription |
| Where it works | Supported editors and browsers | Browser, desktop app, Office | Every Mac app |
| Runs on-device | Yes | No, cloud processing | Yes |
| Word prediction | No | No | Yes |
| Open source | Yes, Apache 2.0 | No | No |
What if you want corrections everywhere on your Mac?
If your writing happens in Obsidian, VS Code or the browser, Harper is likely all you need, and it costs nothing. The gap only opens when your writing is scattered across Mail, Messages, Slack, Notes and other native apps, outside what an editor-integrated tool covers.
Charm is a native answer to that particular problem. It is a macOS menu bar app that corrects spelling, fixes grammar and predicts your next word inline, in every Mac app, through the macOS Accessibility API. Everything runs on-device with local models, so like Harper there is no cloud, no account and no need to be online, and it skips password fields. In browsers and Electron apps it uses Fix Selection with Option-Command-V, and it corrects mechanics without rewording your voice. Charm is English only, needs macOS 14 or later, and covers up to 3 Macs.
To be plain, Charm is not the cheaper choice next to Harper, because Harper is free. What you pay for is coverage across every Mac app, word prediction, and no setup beyond two permissions. Against the subscription tools the maths flips: $9.99 once, no renewal, plus a 14-day money-back guarantee.
Frequently asked questions
Is Harper really free?
Yes. Harper is free with no paid tier, no account and no usage limit, and the source is published under the Apache 2.0 licence on GitHub. Automattic funds the work rather than charging users. Pricing and packaging can change over time, so check writewithharper.com before you rely on it.
Does Harper work on a Mac?
Yes. Harper runs on macOS through its editor integrations, its Chrome and Firefox extensions, and a macOS desktop app. It is not a system-wide corrector for every Mac app, so text you type in Mail, Messages or Notes sits outside what it covers.
Is Harper better than Grammarly?
For mechanical accuracy, privacy and cost, Harper wins: it is free, local and very fast. For style rewriting and tone suggestions, Grammarly does more, because that is a different job. If you want errors caught rather than sentences rewritten, Harper wins.
Does Harper have word prediction?
No. Harper checks text you have already written and does not predict your next word. If you want inline next-word prediction while typing on a Mac, that needs a separate tool built for it.
Can I use Harper alongside another corrector?
Yes. Harper covers the editors and browsers you install it in, and a system-wide Mac corrector such as Charm covers everything else you type in. Running both is a common setup, and neither interferes with the other. If privacy is what brought you here, our guide to protecting your writing privacy on a Mac is a good next read.