LanguageTool is an AI-powered writing assistant that checks texts for grammar, spelling, punctuation, and style errors across more than 30 languages and dialects. It goes beyond basic spell checking by detecting advanced issues such as incorrect word order, wrong tense usage, overused phrases, wordiness, and inconsistent formatting. The tool supports six varieties of English alone, as well as German, Spanish, French, Dutch, Portuguese, and many more, making it suitable for international teams and multilingual professionals.

The service is available as a browser add-on for Chrome, Firefox, Edge, Safari, and Opera, and integrates directly into Google Docs, Microsoft Word, Apple Pages, LibreOffice, Apple Mail, Gmail, Thunderbird, and Outlook. Dedicated desktop apps for macOS and Windows provide in-app checking across most native applications. An online editor is also available for distraction-free writing with live suggestions.

Premium users gain access to more than 20,000 additional checks for major languages, text fields of up to 150,000 characters, unlimited AI-powered paraphrasing, and a personal Style Guide for maintaining consistent wording. Team plans add shared Style Guide management, a Team Dictionary for industry-specific terminology, and user management for groups of up to 20 members. Larger organizations can contact sales for custom pricing and on-premise deployment options.

LanguageTool also exposes a Proofreading API for developers who want to integrate language checking into their own applications. The core technology is open source and available on GitHub, allowing community review and contributions. Since its launch in 2003, the platform has grown to serve millions of users and has accumulated over 2 million active browser add-on installations.