LanguageTool
Grammar, style and spelling checker for 30 plus languages. The LGPL Java core runs as a local HTTP server, so drafts never leave your machine. Cloud Premium adds closed ML rules.
Overview
Twenty-one years of accumulated grammar rules, written in Java and published under LGPL-2.1 or later, is an unusual thing to find in a category dominated by SaaS. The core repository holds the rule sets for more than 30 languages, the tokenisers and taggers behind them, and a standalone HTTP server with a plain JSON API. Point curl at it and you get back offences with offsets, rule ids and suggested replacements, which makes it easy to drop into a CI job that lints prose the same way you lint code.
The commercial side is a hosted service with browser extensions, desktop apps and integrations for Word, Google Docs and most editors. Paid Premium adds the pieces that are not in the repository: extra checks the company puts at 20,000 plus, Picky Mode, a shared style guide and the AI paraphrasing. In December 2025 the browser add-on itself moved behind the subscription, which annoyed a lot of long-time free users. The workaround still exists and is officially documented, the extension can be pointed at a server you run.
Sensible use for a multilingual technical writer is layered. Self-hosted LanguageTool for the mechanical pass, spelling, agreement, doubled words, spacing and your own terminology rules, running locally over drafts you would rather not paste into a web service. A general model afterwards for the things rules cannot see, argument order, ambiguity, whether the sentence actually means what you intended. It will not replace either an LLM or a careful editor, and it does not pretend to, which is part of why it is still worth having.
Key Features
- ✓ Rule engine covering 30 plus languages and dialects, including six English varieties
- ✓ Standalone HTTP server with a documented JSON API, requires Java 17 to 21
- ✓ Optional n-gram data for confusion pairs, roughly 3 GB for German and 15 GB for English
- ✓ Browser add-on, desktop apps and editor plugins that can be pointed at your own server URL
- ✓ Custom rules in XML or Java for house terminology and banned phrasing
- ✓ Personal dictionary, shared team style guide and Picky Mode on the paid tier
Where it holds
- • Self-hosting is a first-class path, not a token gesture, and keeps unpublished writing off someone else's servers
- • One engine across English, German, French, Spanish, Portuguese and Dutch, which suits people drafting in more than one
- • LGPL-2.1 or later core with genuine ongoing development, 6.8 landed 5 May 2026 and the repo sits near 14.9k stars
- • Rules are readable and extendable, so you can encode your own terminology instead of arguing with a black box
Where it breaks
- • The strongest rules and the ML rewriting live in the closed cloud tier, self-hosted output is noticeably weaker
- • The browser extension went Premium-only in December 2025 unless you connect it to your own server
- • Rules-based core has little context awareness, it will miss things a general LLM catches immediately
- • Coverage is weighted to European languages, Chinese support is minimal, so it does nothing for CN drafts
- • Full n-gram data for English is a 15 GB download and wants real RAM before it earns its place
My Take
Run the JAR, point an editor plugin at localhost:8081 and you have a checker that never phones home. For anything under NDA or embargo that is the entire argument, and it is a good one. The honest caveat is the split: most of the quality sits in closed cloud rules and the ML paraphraser, so self-hosted results land closer to a very good spellchecker than to Grammarly. Version 6.8 shipped 5 May 2026 and wants Java 17 to 21, and the full English n-gram set is a 15 GB download before confusion-pair detection starts pulling its weight.
Quick Info
- Pricing:
- freemium
- Openness:
- Open source
- Licence:
- LGPL-2.1-or-later
- Starting at:
- Free tier checks up to 2,000 characters per text field with basic grammar and spelling and three AI paraphrases a day. Premium raises the limit to 150,000 characters and adds 20,000 plus extra checks, Picky Mode, personal style guide and dictionary, and unlimited rephrasing. List prices move with promotions and include VAT, around $19.90 monthly, near $7 a month on annual and about $4.17 a month on the two-year plan. Team pricing is per seat and scales from 2 to 1000 plus users. Self-hosting the LGPL server costs nothing beyond the machine, realistically a $10 to $15 VPS, and break-even against Premium arrives at roughly two users.
- Added:
- Aug 2026
- Updated:
- Aug 2026
Use Cases
Judge it on your own work
The notes above say where LanguageTool holds and where it breaks. The fastest check is your own workload.
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