Harper
Offline English grammar and spell checker that runs locally in browsers, editors and a desktop app.
Harper checks English grammar in milliseconds on your own device, with no cloud and no LLM.
Harper is an English grammar checker written in Rust that runs entirely on your device. It suits writers, students and developers who want Grammarly-style checks without sending text to a server.
What it is
Harper lints English text for spelling, capitalization, awkward phrasing and grammar, and shows suggestions as you type. It ships as a language server, a JavaScript library (harper.js), a Rust crate, browser extensions, editor plugins and a desktop app.
Who it's for
It suits people who care about privacy and speed, including developers who want grammar checks in comments and Markdown in their editor. It does not suit anyone who needs languages other than English or AI rewriting of whole paragraphs.
Key features
Local-only checking
Every check happens on your device with no cloud round-trip, no telemetry and no LLM in the loop, per writewithharper.com.
Language server
harper-ls plugs into VS Code, Neovim, Helix, Emacs and Zed to lint prose and code comments.
Many integrations
Chrome, Edge and Firefox extensions, an Obsidian plugin, a WordPress block-editor plugin and a macOS desktop app.
WebAssembly build
Small enough to load via WebAssembly, so the checker runs in a web page.
harper.js library
A JavaScript library and Rust crate let you embed the grammar engine in your own app.
Fast and light
The README claims lint times in milliseconds and under 1/50th of LanguageTool's memory footprint.
Up and running in 3 steps
Our honest verdict
What's great
- Your writing never leaves your computer, which is the project's main reason to exist.
- Suggestions arrive in under 10ms according to the project site, so checking keeps up with typing.
- Works in the places you already write: browser, Obsidian, WordPress, VS Code, Neovim and more.
- No account is needed, and the engine can be embedded through harper.js or a Rust crate.
Where it falls short
- English only; the README says other languages are welcome contributions but not supported yet.
- It is a rule-based checker, so it will not rewrite paragraphs or adjust tone the way LLM-based tools do.
- The desktop app is described as macOS, so Windows and Linux users depend on extensions, editor plugins or the language server.
- Google Docs support is still being fixed: the v3.1.0 release notes describe bug fixes for sentences spanning pages and for tables.
Pricing & license
Harper is free under the Apache-2.0 license, and writewithharper.com shows no paid plan. The only cost is your own device.
Open remote AI-training roles
880 open remote AI-training roles, updated November 15, 2026; most pay $20–$175 an hour. Zekai lists them free; the application goes to the employer’s platform.
Alternatives
These reviewed AI tools do the same job instantly — from our scored directory.
Use it, or have us set it up
FAQ
Does Harper send my text anywhere?
According to writewithharper.com, every check happens locally with no cloud round-trips and no telemetry.
Which languages does it check?
English only. The README says the core is extensible but other languages need contributions.
How does it compare with LanguageTool?
The README says LanguageTool needs gigabytes of RAM and a ~16GB n-gram dataset, while Harper uses far less memory and lints in milliseconds.
