Harper

Harper is a free, open-source English grammar checker built by Automattic and released under the Apache-2.0 license. It runs entirely on-device — no data ever leaves your computer — and is written in Rust for speed, delivering suggestions in under 10ms. The project positions itself as a private alternative to cloud-based grammar tools, and the source code is publicly available on GitHub with over 10,000 stars as of mid-2026.

What It Is

Harper is a local-first grammar checker that catches improper capitalization, misspelled words, awkward phrasing, and broken grammar without sending your writing to any server. It is distributed as a language server (harper-ls), a JavaScript/WebAssembly library (harper.js), a Rust crate (harper-core), browser extensions for Chrome and Firefox, editor extensions for VS Code, Neovim, Helix, Emacs, Zed, and Sublime Text, an Obsidian plugin, a WordPress block-editor plugin, and a native macOS desktop app. Users can pick the integration that matches their workflow or build their own using the published libraries.

Architecture and Performance

Harper is written in Rust and compiled to WebAssembly where needed, which lets it run in browsers and JavaScript environments without a server. The project states it takes less than 1/50th of LanguageTool's memory footprint and lints documents in milliseconds rather than seconds. Because all processing is local, there is no network round-trip latency and no dependency on GPU hardware — the FAQ explicitly states no GPU is required.

Privacy Model

Every grammar check happens locally on the user's device. Harper has no telemetry, no cloud pipeline, and no LLM in the loop. The project's FAQ states it is "the only widespread and comprehensive grammar checker that is truly private," and that writing data is never used to train models. No account or credit card is required to start using it.

Integration Breadth

Harper covers a wide range of writing environments through a single underlying engine:

• Editors: VS Code, Neovim, Helix, Emacs, Zed, Sublime Text (via harper-ls language server)
• Note-taking: Obsidian plugin with inline vault checks
• Browsers: Chrome Web Store extension, Firefox AMO add-on
• CMS: WordPress block-editor plugin
• Desktop: Harper Desktop for macOS, checking every text field system-wide
• Developers: harper.js for web apps and TypeScript/JavaScript codebases; harper-core Rust crate for Rust programs

Language Support

Harper currently supports English and its dialects: British, American, Canadian, Australian, and Indian English. The project notes that support for other languages is on the horizon but that English support will be made "truly amazing" before diversification. The core is described as extensible to support other languages, and community contributions are welcomed.

Update: v2.4.0

The latest release is v2.4.0, published on June 4, 2026, with the repository last pushed on June 10, 2026. The homepage footer lists the current stable version as v1.0, while the GitHub release tag reflects v2.4.0, indicating active and rapid development. The project has an open issue count of 603 and 356 forks, signaling a growing contributor community. Automattic, the company behind WordPress.com, sponsors the project as an official open-source initiative.