LanguageTool
LanguageTool is open-source proofreading software under LGPL-2.1 for English, Spanish, French, German, Portuguese, Polish, Dutch and more than 20 other languages. Its README says it finds many errors a simple spell checker cannot detect, using rules contributors can extend. You can run your own HTTP server with a documented API, call it as a Java library, or use its LibreOffice and OpenOffice extension, all on your own machine.
What does LanguageTool do?
LanguageTool is "open-source proofreading software for English, Spanish, French, German, Portuguese, Polish, Dutch, and more than 20 other languages", and "it finds many errors that a simple spell checker cannot detect." Its checks are rules, which contributors can add, rather than a language model, so results are explainable and repeatable.
You can run it several ways: as your own HTTP server with an API documented in Swagger, through the public server over HTTP, as a Java library with published documentation, from the command line, in a graphical interface, or as an extension for LibreOffice and OpenOffice. Its install script offers the graphical, command-line and server modes. Many other tools on this page, including LTeX+, use it as their checking engine.
Key facts
- Licence: LGPL-2.1 or later for the core in this repository, which means a changed copy you distribute must share those changes under the same licence.
- Languages: English, Spanish, French, German, Portuguese, Polish, Dutch and more than 20 others; the repository description says 25 or more.
- Rule-based: contributors can add error-detection rules; no language model is involved.
- Run it as your own HTTP server with a Swagger-documented API, as a Java library, from the command line, in a graphical interface, or as a LibreOffice and OpenOffice extension.
- Build from source needs Java 17 and Apache Maven; a full clone downloads more than 500 MiB and needs more than 1,500 MiB on disk, by the README.
- Docker files exist only as community contributions, of which the README lists three.
- A public server is reachable over HTTP; the README states no paid tier for this repository.
- No GitHub releases; the newest tag when read was v6.8. 15,046 stars on 2026-09-17.
What does it replace, and where does it fall short?
LanguageTool replaces Grammarly for writing in languages other than English, and for anyone who wants the checker on their own server rather than a service that reads every sentence. Its API makes it the engine other tools build on. It is the multilingual pick on the open-source alternatives to Grammarly page.
Where it falls short: its rules catch grammar and style, not the rewriting and tone changes Grammarly sells, and it has no browser extension in this repository. Running it well means Java and a fair amount of disk. For AI rewriting across any app, Writing Tools; for a fast English-only checker inside editors, Harper.
How does LanguageTool run?
Run the install script, which offers a graphical interface, a command-line tool or a server, or build from source with Java 17 and Maven. As a server it exposes an HTTP API documented with Swagger, so editors and applications can send text to it. The LibreOffice and OpenOffice extension plugs into those programs. A public server exists for testing; the README describes no paid tier here.
Who is LanguageTool for?
Someone writing in several languages, or a team that wants one proofreading server its tools can call, with the text never leaving its network. Someone who wants AI rewriting rather than error checking should look at Writing Tools.
What limits does the README state?
From the README: a complete clone downloads more than 500 MiB and needs more than 1,500 MiB on disk; development builds are bleeding edge and may contain regressions; Apple M1 and M2 Macs need build steps that run under Rosetta.
Questions people ask
Is LanguageTool open source?
The core in this repository is, under LGPL-2.1 or later, which is OSI-approved. LGPL means that if you distribute a changed version of the library, those changes must be shared under the same licence. The README describes no paid tier for this repository.
How many languages does LanguageTool check?
English, Spanish, French, German, Portuguese, Polish and Dutch are named, plus more than 20 others, which the repository description sums up as 25 or more. That breadth is its main advantage over the English-only tools on this page.
Can I run LanguageTool on my own server?
Yes. It runs as an HTTP server with an API documented in Swagger, so your editor or application can send text to your own machine instead of a cloud service. It also runs as a Java library, a command-line tool, a graphical app or a LibreOffice extension.
How does LanguageTool compare with Grammarly?
LanguageTool finds grammar, spelling and style errors with explainable rules, in more than 25 languages, and can run entirely on your hardware. Grammarly adds AI rewriting and generation as a hosted service. The open-source alternatives to Grammarly page compares the five options.
Does LanguageTool use AI?
Not in this repository's description. Its checks are rules that contributors write and extend, which makes each correction explainable. For AI rewriting, Writing Tools uses a language model of your choice, locally or in the cloud.
Sources
- LanguageTool README and COPYING.txt (LGPL-2.1): github.com/languagetool-org/languagetool, read 2026-09-17.
Compared with the others
On the open-source alternatives to Grammarly page, LanguageTool is the pick for grammar in more than 25 languages. LGPL-2.1 proofreading with explainable rules, run as your own HTTP server with an API, from the command line, or inside LibreOffice.
Also on that page: Harper for fast english checking in your editor, Writing Tools for ai rewriting anywhere on screen, Vale for a team's own style guide, enforced.
More writing and grammar
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 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.
Added September 17, 2026. Every claim above comes from the project's README, LICENSE or model card, read on September 17, 2026, or from the GitHub API on the date shown in the panel. Found an error? Write to reveneau@licheo.com and it is fixed in the next weekly pass. Repository: github.com/languagetool-org/languagetool.
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.