LanguageTool
Open-source grammar, style, and spell checker for 30+ languages.
About
LanguageTool is a free and open-source grammar, style, and spell checker licensed under LGPL, developed by Daniel Naber and Marcin Miłkowski. It checks writing in more than 30 languages through thousands of pattern-based and AI-powered rules, catching grammar errors, punctuation mistakes, redundancies, and style issues that simple spell checkers miss. It runs as a browser extension, desktop app, web editor, and a self-hostable Java server with a public REST API.
The open-core model means the core engine is free while LanguageTool Premium (around 5 dollars per month) adds improved error detection, longer text limits, and an AI rephrasing assistant. Because it can be self-hosted, it is popular with privacy-conscious users and European enterprises with strict data governance. Developers can also integrate it into content pipelines as a proofreading step after LLM drafts.
What it can do
- Grammar, spelling, and style checks in 30+ languages
- Browser extension, desktop app, and web editor
- Self-hostable server with REST API
- AI-powered rephrasing assistant (Premium)
- Client libraries for Python, Node.js, and more
Setup
Use the web editor, install the browser extension, or self-host the Java server.
Prerequisites
Nothing for the free tier; a Java runtime for self-hosting.
Use cases
Works with
Trust score — 8/10
Scored 8/10 (verified tier, metrics-v1). Evidence: Verified via vendor site/docs/GitHub and reputable coverage, wave 92; Trending score 80/100 from researcher assessment.
Sources
Every fact above was checked against these sources. Where a claim comes from the provider and couldn't be independently verified, the text says so.