Harper is completely private.
Every check happens locally. No cloud round-trips, no telemetry, no LLM in the loop. That means certainty that we never train models on your writing.
Your device The cloud
The Free Grammar Checker That Respects Your Privacy
I make you look like a grammar genius.
Harper is a free, open-source grammar checker designed to be just right. Think of it as the private alternative to Grammarly, built after years of dealing with the shortcomings of the competition.
Harper catches the kinds of mistakes that matter: improper capitalization, misspelled words, awkward phrasing, and broken grammar. Your writing never leaves your computer.
Harper is available as a language server, a JavaScript library, a Rust crate, browser extensions, editor extensions, and native apps. Pick the integration that matches your workflow or build your own.
Every check happens locally. No cloud round-trips, no telemetry, no LLM in the loop. That means certainty that we never train models on your writing.
Harper runs locally and is built for speed. You get a feedback loop that keeps up with your typing, without waiting for a server.
“Written in Rust, everything is processed in an instant and I find it neat to see the browser extension highlight words as I type, effectively checking per letter. And no account is required, allowing me to get up and running in no time.”
“Obsidian is my favorite productivity app, and Harper is a grammar checking tool that works well with it.”
“Awesome extension! It’s privacy focused, that means that every check it done locally on your computer, there is no server where your data goes! And because of that it’s blazingly fast compared to Grammarly.”
“What I loved about this tool is that it’s private, and open source and really fast.”
“Harper is great: it is discreet, fast, powerful, and private.”
“I’ve been using Harper in Neovim for a long time and am glad to see it as an extension!”
“What a delightful way to check for flagrant spelling errors in markdown files. Thanks Harper authors!”
“Harper excels at catching the kinds of mistakes that matter in technical writing – improper capitalization, misspelled words, and awkward phrasing that can make documentation unclear.”
“I’ve been using Harper instead of Grammarly for a few months already, and I can’t be happier! I can’t wait to see the great improvement when this tool reaches version 1.0.0! Great job! I hope that, eventually, it will also support languages other than English.”
Don’t see yours? Ask on Discord
Yes. Harper is free in every sense of the word. You don’t need a credit card to start using Harper, and the source code is freely available under the Apache-2.0 license.
Harper watches your writing and provides instant suggestions when it notices a grammatical error. When you see an underline, it’s probably because Harper has something to say.
No. Harper will never intentionally suggest an edit that might change your meaning. Harper strives to never make it harder to express your creativity.
Harper is the only widespread and comprehensive grammar checker that is truly private. Your data never leaves your device. Your writing should remain just that: yours.
That depends on your use case. Do you want to use it within Obsidian? We have an Obsidian plugin. Do you want to use it within WordPress? We have a WordPress plugin. Do you want to use it within your Browser? We have a Chrome extension and a Firefox plugin. Do you want to use it within your code editor? We have documentation on how you can integrate with Visual Studio Code and its forks, Neovim, Helix, Emacs, Zed and Sublime Text. If you’re using a different code editor, then you can integrate directly with our language server, harper-ls. Do you want to integrate it in your web app or your JavaScript/TypeScript codebase? You can use harper.js. Do you want to integrate it in your Rust program or codebase? You can use harper-core.
We currently only support English and its dialects British, American, Canadian, Australian, and Indian. Other languages are on the horizon, but we want our English support to be truly amazing before we diversify.
For harper-ls and our code editor integrations, we support a wide variety of programming languages. You can view all of them over at the harper-ls documentation. We are entirely open to PRs that add support. If you just want to be able to run grammar checking on your code’s comments, you can use this PR as a model for what to do. For harper.js and those that use it under the hood like our Obsidian plugin, we support plaintext and/or Markdown.
See this blog post.
No. Harper runs on-device, no matter what. There are no special hardware requirements. No GPU, no additional memory, no fuss.
You can join our Discord and ask your questions there or you can start a discussion over at GitHub.
Harper will not run in Gmail unless the built-in grammar checker is disabled. If you wish to use Harper in Gmail, please disable the built-in grammar checker.
Fork it, file an issue, add a rule, port it to a new editor. Harper is free software, and we’d love your help.