Open-source writing and grammar
Checkers and rewriters for prose: rule-based engines that explain every correction, style linters that enforce a team's own guide, and AI assistants that rewrite selected text anywhere on screen. All of them can run without sending your writing to a service.
An Apache-2.0 English grammar checker from Automattic that lints a document in milliseconds, uses a fraction of LanguageTool's memory by its own measure, runs privately in your editor or in the browser through WebAssembly.
An LGPL-2.1 proofreading engine for English, Spanish, French, German, Portuguese, Polish, Dutch and more than 20 other languages, which you can run as your own HTTP server with an API, or inside LibreOffice.
An MIT command-line prose linter that turns a team's writing guidelines into checks that run in an editor, in CI and beside code, offline, understanding Markdown, AsciiDoc and code comments rather than plain text.
A GPL-3.0 system-wide writing assistant for Windows, Linux and macOS that rewrites, proofreads or summarises selected text anywhere with one hotkey, using a local model through Ollama or a cloud model with your own key.
An MPL-2.0 language server that checks grammar and spelling in LaTeX, Markdown, Typst, AsciiDoc and 12 other markup formats with LanguageTool, in more than 20 languages, offline, with Java and LanguageTool bundled.
A free, Apache-2.0 Grammarly alternative that runs as a browser extension, a Microsoft Word add-in and a desktop tray app, checking grammar with your own free Google Gemini API key.
An Apache-2.0 browser extension that checks grammar and spelling with a model you choose, running against OpenRouter, Groq or a local Ollama server, with keys stored in the extension.
An MIT Grammarly-style tool for macOS that corrects grammar, rewrites tone, and translates selected text in any app, backed by OpenRouter, Groq, Cerebras, Gemini, Mistral, Ollama, LM Studio or your ChatGPT or Claude subscription.
An MIT desktop writing assistant for Mac and Windows that checks spelling, grammar and style across almost every app, with AI rewrites and tone shaping provided by Apple Intelligence on Mac and Phi Silica on Windows Copilot+ PCs, entirely on the device.
An MIT Grammarly-style Chrome extension that routes grammar fixes, rewrites and translations to Chrome's built-in Gemini Nano on your computer, or to any provider you choose with your own key, with no backend, no account and no telemetry.
Questions people ask
What is the best open-source alternative to Grammarly?
LanguageTool (LGPL-2.1) for grammar in more than 25 languages, self-hosted. Harper (Apache-2.0) for fast, private English checking inside editors. Writing Tools (GPL-3.0) for AI rewriting of selected text in any app. Vale (MIT) for enforcing a team's style guide. LTeX+ (MPL-2.0) for LaTeX and Markdown.
Can these check my writing without sending it anywhere?
Yes, all five. Harper runs in your editor or the browser, Vale is a local binary that its README says uploads nothing, LTeX+ bundles LanguageTool and Java to check offline, LanguageTool can run as your own server, and Writing Tools can use a local model through Ollama.
Which ones use AI, and which use rules?
Writing Tools is an AI rewriter: results come from whichever model you point it at. LanguageTool, Harper, Vale and LTeX+ are rule-based, so every correction is explainable and repeatable. Rules catch errors consistently; a model rewrites and changes tone.
Which support languages other than English?
LanguageTool covers more than 25 languages and LTeX+ more than 20, both using the same rule engine. Writing Tools works with any language its model supports. Harper is English only for now, and Vale enforces whatever rules you write, in whatever language you write them.
Paying for an AI tool you would rather own?
Reveneau sets up the replacement on your own servers, connects it to your product, and builds the software around it.