Harper

Harper is a privacy-first grammar checker that runs entirely on your own computer. There's no cloud, no data collection, and no account required. Every check happens locally, so your writing never leaves your device. It catches the usual suspects: misspellings, improper capitalization, awkward phrasing, and grammar issues. But unlike most online checkers, Harper does it all without phoning home.

Harper is built in Rust, which makes it fast — the site claims suggestions appear in under 10 milliseconds. That's noticeably zippier than Grammarly and similar tools that wait for a server round-trip. You can use it as a browser extension for Chrome or Firefox, a VS Code plugin, an Obsidian plugin, a Neovim LSP, a WordPress block editor plugin, a native desktop app for macOS, or even as a JavaScript library, Rust crate, or language server if you want to build your own integration. The source code is open and available under an Apache-2.0 license on GitHub.

Harper is ideal for anyone who writes a lot and cares about privacy — journalists, developers, technical writers, and students. If you're tired of grammar checkers that scan your text to train LLMs or sell your data, this is a solid alternative. It's also great for developers who want a grammar checker that integrates into their editor or CI pipeline. The tool only supports English right now, but it's straightforward to set up and genuinely fast.