LanguageTool

What is LanguageTool?

LanguageTool is an open source proofreading tool that checks spelling, grammar, punctuation, and style in more than 30 languages. It uses a rule-based engine — thousands of error patterns written in XML and Java, plus n-gram data for confusing word pairs — and exposes an HTTP API you can run on your own server.

What is LanguageTool best for?

Writers, developers, and privacy-conscious teams who want grammar checking they control. It fits anyone integrating a proofreading API into their own app, running an in-house checker so text never leaves their infrastructure, or wanting a free browser and office checker without a Grammarly subscription.

What can LanguageTool do?

• Check spelling, grammar, punctuation, casing, and style in 30+ languages, with six English variants (US, UK, and more)
• Run as a self-hosted HTTP API server for integration into your own apps
• Proofread in the browser via extensions for Chrome, Firefox, Edge, Safari, and Opera
• Plug into LibreOffice, Microsoft Word, Google Docs, and email clients
• Flag confusable word pairs (their/there, its/it’s) using n-gram statistics
• Ship as a standalone desktop app plus GUI and command-line versions

Is LanguageTool free?

Yes — the core engine is free and open source under LGPL-2.1, and self-hosting has no license cost. The company also sells a hosted Premium plan that adds 20,000+ extra checks, AI paraphrasing, and a custom style guide, and raises the per-field limit from 2,000 to 150,000 characters. Premium’s advanced detection is not part of the open-source download.

Where does LanguageTool fall short?

• The open-source core catches typical errors but doesn’t judge full grammatical correctness — an unusual but valid-looking sentence can pass, and it’s weaker at contextual rewriting than Grammarly’s AI.
• The best-quality checks (Premium’s 20,000+ extra rules, AI paraphrasing, style guide) are cloud-only and paid; self-hosting gives you the base rule set, not the full Premium experience.
• N-gram data that powers confusable-word detection is a large separate download (multiple GB per language), and the server is a Java service that wants a healthy chunk of RAM.

What does LanguageTool replace?

LanguageTool is a self-hostable alternative to Grammarly, ProWritingAid, and Microsoft Editor. It does the same grammar-and-style checking, but the engine is open source and you can run it yourself so your text is never sent to a third-party service.

FAQ

Is LanguageTool open source? Yes. The core engine is licensed under LGPL-2.1-or-later and developed openly on GitHub. The hosted Premium tier and its advanced checks are a separate commercial product built on top.

Can I self-host LanguageTool for free? Yes. LanguageTool ships an HTTP API server you can run yourself — community Docker images exist — with no license fee. You only pay for the machine it runs on.

Is LanguageTool a good Grammarly alternative? For privacy and cost, yes — you keep your text on your own servers and pay nothing for the software. Grammarly’s AI still tends to give more polished, context-aware rewrites, so it depends on whether control or suggestion quality matters more to you.

What do I need to run LanguageTool? Java 17 or newer and a server with enough memory for the language models. For confusable-word detection you also download large n-gram datasets per language.