Harper

Harper is a free, open source grammar and spell checker that runs entirely on your device. It is written in Rust and compiled to WebAssembly so it can run inside the browser. It was started by Elijah Potter, who now works at Automattic, and the code is released under the Apache-2.0 licence.

Its site says every check happens locally, with no cloud round trips, no telemetry and no AI model in the loop. It supports English and its main dialects, and it offers no tone rewriting, paraphrasing or AI drafting, so it is a proofreader, not a writing assistant. It runs in Chrome, Firefox and Edge, and the project also ships editor plugins, an Obsidian plugin and a macOS desktop app.

Those privacy points are the developers' claims, and we did not audit the extension's network traffic. Its manifest allows connections only to itself and to writewithharper.com, and we could not find out what the latter is for. It is much smaller than the commercial options, with about 30,000 Chrome users, so ratings rest on few reviews. If you write in English and want the basics without an account, it is a good fit. It replaces the basics of Grammarly, not the extras.