Harper Automattic/harper

Fast, offline English grammar and spell checker from Automattic, written in Rust, with browser extensions, editor plugins and a language server.

Creator kit + Compare GitHub ↗ Install guide (official) ↗

Harper at a glance

Checked 15 Oct 2026

Harper is an offline grammar and spell checker for English only (US, UK, Canadian, Australian and Indian English), written in Rust and maintained at Automattic (v2.12.0, October 2026). It ships as Chrome and Firefox extensions, an Obsidian plugin, a language server for VS Code, Neovim, Helix, Emacs and Zed, a CLI and a WebAssembly library. Apache-2.0. Every check runs on your device with no LLM involved, so it does no AI rewriting, tone changes or plagiarism checks, and it does not support other languages yet.

Cite this pageRepoQuarry. “Harper.” Checked 15 Oct 2026. https://repoquarry.dev/projects/harper

How much of Grammarly does it replace?

2 of 5 jobs. English grammar and spelling in the browser and editors, fully offline; no AI rewrite, tone or plagiarism check.

Grammar and spelling checks Partial replacement

Style and clarity suggestions Not covered

AI rewriting and tone changes Not covered

Works in the browser and other apps Partial replacement

Plagiarism check Not covered

For style and clarity suggestions, AI rewriting and tone changes, plagiarism check, see other Grammarly alternatives. You may also need to build an interface, an API server and hosting yourself.

Facts, with sources

Every answer links to where we found it. We read the documentation; we did not install it.

Report a correction

QuestionAnswerSourceChecked

Runs on my machine? Yes, every check runs on your device Every check happens locally. No cloud round-trips, no telemetry, no LLM in the loop. writewithharper.com ↗ 1d ago

Code licence Apache-2.0 the source code is freely available under the Apache-2.0 license. writewithharper.com ↗ 1d ago

Model weights licence No separate model weights Every check happens locally. No cloud round-trips, no telemetry, no LLM in the loop. writewithharper.com ↗ 1d ago

Commercial use Yes, Apache-2.0 github.com · LICENSE ↗ 1d ago

Paid services needed? None required You don't need a credit card to start using Harper writewithharper.com ↗ 1d ago

Hardware Not documented; the README claims under 1/50th of LanguageTool's memory footprint take less than 1/50th of LanguageTool's memory footprint github.com · harper#readme ↗ 1d ago

Operating systems Windows, macOS and Linux binaries (x64 and ARM), plus Chrome and Firefox extensions harper-ls-x86_64-pc-windows-msvc.zip github.com · v2.12.0 ↗ 1d ago

Setup effort Easy; browser extension, editor extension or prebuilt binary github.com · harper#links ↗ 1d ago