Best Open-Source Grammar Checkers 2026: 6 Options Compared

Open-source grammar checkers are less polished than Grammarly but you control where your text goes. LanguageTool is the closest all-rounder, covering many languages through its server and apps, and its free plan limits text length and AI rewrites per day. Harper runs locally with no network call and aims at fast English checks inside editors. Vale, proselint, textlint and write-good are linters: they run in a terminal or a code editor against Markdown, documentation and other text files, flagging style rules rather than rewriting sentences. Choose LanguageTool if you write emails and essays in many places. Choose Harper if you want quick offline checks while writing in an editor. Choose Vale if you maintain documentation and want your team's style guide enforced automatically in CI. textlint handles natural-language text with plugins and is popular in Japanese-language projects. proselint and write-good are older, simpler tools; write-good is a naive linter and has seen few updates since early 2025. The honest trade-off is accuracy: open-source checkers catch rules and patterns, while the commercial products use larger models, so expect to miss some style suggestions. Licences below are from the repositories as checked on the date shown.

LanguageTool

LGPL-2.1 rule-based checker for many languages. Free plan limits each text field to 2,000 characters and 3 AI rephrasings a day; the open server has no such cap.

Price: Free (Free plan with 2,000 characters per text field and 3 AI sentence rephrasings a day); paid from $5.83 per month, billed annually ($24.90 billed monthly, Premium) (LanguageTool pricing, checked 2026-10-22).

Platforms and license: Web, Browser extension, macOS, Windows, Self-hosted. LGPL-2.1 (open source) (license source).

Project activity: 15.2k GitHub stars · last commit Oct 27, 2026

Best for: Writers in several languages.

Worth knowing: The open-source core can be run as your own server for 25+ languages.

Harper

Apache-2.0 grammar checker that runs locally and offline, with no paid tier listed. Fast enough to use live in an editor.

Price: Free (open-source grammar checker that runs locally, no paid tier listed) (Harper pricing, checked 2026-10-22).

Platforms and license: Browser extension, Editor plugins, Self-hosted. Apache-2.0 (open source) (license source).

Project activity: 17.2k GitHub stars · latest release v3.0.0 (Oct 27, 2026) · last commit Oct 27, 2026

Best for: Private offline checking.

Worth knowing: English only; plugins cover VS Code, Neovim, Helix, Emacs, Zed and Obsidian.

Vale

MIT markup-aware prose linter for Markdown and docs, driven by style rules you choose. Needs configuration before it is useful.

Price: Free (Free and open source prose linter) (Vale pricing, checked 2026-10-24).

Platforms and license: Command line. MIT (open source) (license source).

Project activity: 6.3k GitHub stars · latest release v3.25.1 (Oct 22, 2026) · last commit Oct 24, 2026

Best for: Documentation teams.

Worth knowing: GitHub: a markup-aware linter for prose. License taken from the repository metadata and LICENSE file; description from the project README. No paid plan was checked.

proselint

BSD-3-Clause prose linter for English that flags cliches and weak phrasing. Main development has moved to Codeberg, with GitHub as a mirror.

Price: Free (Free and open source prose linter) (proselint pricing, checked 2026-10-24).

Platforms and license: Command line (Python). BSD-3-Clause (open source) (license source).

Project activity: 4.6k GitHub stars · latest release v0.16.0 (Nov 14, 2025) · last commit Sep 4, 2026

Best for: Style checks on drafts.

Worth knowing: GitHub repository is a mirror; main development is on Codeberg. License taken from the repository metadata and LICENSE file; description from the project README. No paid plan was checked.

textlint

MIT pluggable linter for natural-language text, with rules you add as plugins. Runs on Node.js.

Price: Free (Free and open source pluggable linter for natural-language text) (textlint pricing, checked 2026-10-24).

Platforms and license: Command line (Node.js). MIT (open source) (license source).

Project activity: 3.2k GitHub stars · latest release v15.8.0 (Aug 1, 2026) · last commit Oct 27, 2026

Best for: Custom rules and non-English text.

Worth knowing: License taken from the repository metadata and LICENSE file; description from the project README. No paid plan was checked.

write-good

MIT naive linter for English prose from the npm world. Simple and quick, but last updated in March 2025, so expect no new rules.

Price: Free (Free and open source naive prose linter) (write-good pricing, checked 2026-10-24).

Platforms and license: Command line (Node.js). MIT (open source) (license source).

Project activity: 5.1k GitHub stars · No commits since Mar 10, 2025

Best for: Quick passive-voice checks.

Worth knowing: GitHub: naive linter for English prose; last commit March 2025. License taken from the repository metadata and LICENSE file; description from the project README. No paid plan was checked.

Which one to pick

• You want grammar help in email and web forms across languages: LanguageTool
• You write offline and do not want text leaving the computer: Harper
• You enforce a style guide across documentation in CI: Vale
• You need custom checks for non-English text: textlint