Harper
A free, open-source grammar checker that does its work offline in your browser.
Why it's a strong pick
Harper is a grammar checker that runs offline, per its docs. Its code is public on GitHub in the Automattic organization, and the docs say it is free to use. It uses Manifest V3. The latest repo release is v2.11.0, from September 2026. The store listing is named "Private Grammar Checker - Harper".
What it does
Harper checks your writing for grammar and spelling problems as you type on the web, such as in emails, messages and documents. The docs say it does all of its work offline. It runs through a content script on web pages, with a separate script for Google Docs.
Key features
- Offline checking. The docs say Harper does all of its work offline.
- Works across sites. A content script runs on pages, with extra support for Google Docs.
- Free to use. The docs say Harper is free to use.
- Open source. The code is public on GitHub.
- Other apps. It is also offered for Firefox and Edge, and as editor plugins.
Who it's for
Harper is for people who want a grammar checker that does not send text to a cloud service. Its README makes that claim, and it also claims low memory use compared with LanguageTool. Those are the developer's own claims. Its description says it is built for English.
What permissions does Harper ask for?
Its manifest asks for storage and tabs. It lists access to one host, writewithharper.com, and runs content scripts on all pages and on Google Docs documents. The README says no text goes to servers, which is the developer's own claim. We could not read the developer's privacy policy when we checked; read it before installing.
What licence does Harper use?
The repo shows two licences. The root LICENSE file and the extension's package file say Apache-2.0. The extension's own folder also carries a LICENSE file that reads MIT. We report both as the repo shows them. If the licence matters to you, check the repo.
Why does Harper need access to every page?
It injects a content script into every page so it can check text wherever you write. That is broad access, and it comes with the job: a grammar checker has to see the text you type on whatever page you are using. Review it in Chrome's extension settings before you install.
FAQ
Is it free? Yes. The docs say Harper is free to use.
Does it check other languages? Its description says English. We cannot say more.
Is the store version the newest? Maybe not. The docs say store updates can lag the repo by a few days.
Good to know
Harper needs broad site access to check text anywhere, and store updates can arrive a few days after a repo release. If you want a fix right away, check the release notes on GitHub first.
Other grammar checkers to compare
Harper is one of 8 picks in our grammar checkers guide:
- Grammarly: Grammar, spelling, punctuation and tone suggestions as you write in Chrome. A free plan and a paid Pro plan.
- LanguageTool: A grammar and style checker for 25+ languages. The browser extension needs a paid Premium plan.
- QuillBot: A grammar checker and paraphraser in one extension. Free plan, paid Premium. Read the privacy settings first.
- ProWritingAid: A grammar and style checker with 25+ writing reports for self-editing. Free plan, paid plans.
- Wordtune: A rewrite-first writing tool from AI21 Labs. Free Basic plan with daily limits, paid plans above it.
- Sapling: Grammar, rephrasing and autocomplete aimed at work messages. Free plan, paid Pro and team plans.
- Ginger: Checks spelling and grammar as you type. A free plan with limits, and a paid Premium plan.
This is a third-party extension, not built or maintained by Xplorekit. We link to the official Chrome Web Store listing — install and permissions are entirely between you and the extension's own publisher.