Harper
Harper is an open-source English grammar checker under Apache-2.0 from Automattic, designed to be fast and private. Its README says it takes milliseconds to lint a document and uses less than a fiftieth of LanguageTool's memory, its own figures. It runs as a language server in VS Code, Neovim, Helix, Emacs and Zed, as an Obsidian plugin, as an npm package, or in the browser through WebAssembly, with nothing sent anywhere.
What does Harper do?
Harper is "an English grammar checker designed to be just right", from Automattic. Speed and size are the point: the README says it takes "milliseconds to lint a document" and uses "less than 1/50th of LanguageTool's memory footprint", both its own measurements, and that Harper "is even small enough to load via WebAssembly." It calls itself completely private, since the checking happens where you are writing.
It runs as harper-ls, a language server published on crates.io, which connects it to VS Code, Neovim, Helix, Emacs and Zed; as harper.js on npm for web applications; as an Obsidian plugin; and as a WebAssembly build in the browser. It checks English only, though the README says the core is extensible to other languages. The README also criticises other products by name, which this page does not repeat.
Key facts
- Licence: Apache-2.0, with no added conditions.
- English only; the README says the core is extensible to support other languages.
- Speed and memory, by the project's own figures: milliseconds to lint a document, and less than a fiftieth of LanguageTool's memory footprint.
- Runs as harper-ls, a language server on crates.io, for VS Code, Neovim, Helix, Emacs and Zed; as harper.js on npm; as an Obsidian plugin; and in the browser through WebAssembly.
- Private by design: the checking runs where you write, with nothing uploaded.
- Maintained by Automattic, the company behind WordPress.com.
- The README states "We consider long lint times bugs."
- Latest tagged release when read: v2.11.0 on 2026-09-16; 15,491 stars on 2026-09-17.
What does it replace, and where does it fall short?
Harper replaces Grammarly for English writing inside a code editor, Obsidian or a web application, where speed matters and nothing should leave the machine. Because it is small enough to run in the browser through WebAssembly, it can check text in a web app without a server. It is the pick for editors and speed on the open-source alternatives to Grammarly page.
Where it falls short: English only for now, and it checks grammar and style rather than rewriting text the way Grammarly's AI does. There is no browser extension for arbitrary websites in the README's list. For other languages, LanguageTool; for AI rewriting anywhere on your screen, Writing Tools.
How does Harper run?
Install harper-ls from crates.io and point your editor at it, add harper.js from npm to a web application, install the Obsidian plugin, or use the WebAssembly build on the project's website. The README lists no other requirements and describes no hosted or paid tier.
Who is Harper for?
A developer, note-taker or web developer who wants fast English checking inside the editor or app they already use, with no account. Someone writing in another language should use LanguageTool.
What limits does the README state?
From the README: "Harper currently only supports English, but the core is extensible to support other languages", and "We consider long lint times bugs."
Questions people ask
Is Harper open source?
Yes. Harper is Apache-2.0 with no added conditions, maintained by Automattic. The README describes no paid or hosted tier; you run it in your editor, in your app or in the browser.
Does Harper work offline?
Yes. It runs where you write: as a language server on your machine, as a library in your application, or compiled to WebAssembly in the browser. The README describes it as completely private, with nothing uploaded for checking.
Which editors support Harper?
VS Code, Neovim, Helix, Emacs and Zed through the harper-ls language server, plus an Obsidian plugin. For web applications, harper.js on npm embeds the same checker, and a WebAssembly build runs it in a browser page.
Does Harper support languages other than English?
Not yet. The README says Harper currently supports English only, and that its core is extensible to support other languages in future. For more than 25 languages today, LanguageTool is the option on this page.
How does Harper compare with Grammarly?
Harper checks English grammar and style in your editor in milliseconds, free and private, with no account. Grammarly adds AI rewriting and generation across apps and websites as a hosted service. The open-source alternatives to Grammarly page compares the five options by need.
Sources
- Harper README and Apache-2.0 LICENSE: github.com/Automattic/harper, read 2026-09-17.
Compared with the others
On the open-source alternatives to Grammarly page, Harper is the pick for fast english checking in your editor. Apache-2.0 from Automattic, linting in milliseconds inside VS Code, Neovim, Helix, Emacs, Zed and Obsidian, or in the browser through WebAssembly.
Also on that page: LanguageTool for grammar in more than 25 languages, Writing Tools for ai rewriting anywhere on screen, Vale for a team's own style guide, enforced.
More writing and grammar
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Added September 17, 2026. Every claim above comes from the project's README, LICENSE or model card, read on September 17, 2026, or from the GitHub API on the date shown in the panel. Found an error? Write to reveneau@licheo.com and it is fixed in the next weekly pass. Repository: github.com/Automattic/harper.
Running one of these inside your own environment, with your own data and your own security rules, is the kind of work Reveneau does. Read how a forward deployed engagement works.