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.
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.
Running one of these inside your own environment, with your own data and your own security rules, is the kind of work Reveneau does. Read how a forward deployed engagement works.