OSI-licensed only · self-hostable
Grammarly checks spelling, grammar and style as you write and suggests rewrites. Open-source tools cover the checking part well, and most can run on your own machine or server so your text is not sent to a third party. None of them replicates Grammarly's generative rewriting.
Every tool below is under an OSI-approved licence (LGPL-2.1, Apache-2.0, MIT, BSD-3-Clause, MPL-2.0), checked against its repository. Some are grammar checkers, others are prose linters that enforce a style guide.
Licences checked against each repository on November 7, 2026.
| Tool | Best for | Licence | Runs as |
|---|---|---|---|
| LanguageTool | Multilingual grammar checking | LGPL-2.1 | Server |
| Harper | Fast, private English grammar checking | Apache-2.0 | Desktop app |
| Vale | Enforcing a style guide in docs and CI | MIT | Desktop app |
| proselint | Writing-advice linting | BSD-3-Clause | Desktop app |
| textlint | Pluggable prose linting | MIT | Desktop app |
| LTeX+ | Grammar checking in LaTeX and Markdown | MPL-2.0 | Desktop app |
Best for: Multilingual grammar checking
Open-source proofreading software for English, Spanish, French, German and more than 20 other languages, finding many errors a simple spell checker cannot detect.
Best for: Fast, private English grammar checking
An offline, privacy-first English grammar checker that the README describes as completely private, quick, with a small memory footprint, and loadable via WebAssembly.
Best for: Enforcing a style guide in docs and CI
A command-line linter for prose that turns writing guidelines into checks in editors and CI, running entirely offline.
Best for: Writing-advice linting
A linter for English prose that scans text and flags issues, drawing on advice from well-known writers and editors.
Best for: Pluggable prose linting
The pluggable linting tool for natural language, an ESLint-like linter for prose.
Best for: Grammar checking in LaTeX and Markdown
A language server, built on LanguageTool, that provides grammar and spelling errors in markup documents such as LaTeX and Markdown.
One email a week with the open-source projects, launches and releases added to TechLogHub. Unsubscribe in one click.
One email a week: new and trending developer tools, fresh comparisons, and what shipped. Unsubscribe in one click.
| If you need… | Start with |
|---|---|
| Full multilingual proofreading | LanguageTool |
| Private English checker with tiny footprint | Harper |
| Enforce a house style in docs | Vale |
| Checking prose in a repo or CI | Vale, proselint or textlint |
| Grammar checks in LaTeX or Markdown | LTeX+ |
Quick answers to common questions
LanguageTool is the best known: open-source proofreading for English and more than 20 other languages. Harper is a newer offline English checker.
Harper describes itself as offline and completely private, and Vale runs entirely offline. LanguageTool can be self-hosted.
No. They flag errors and style issues from rules. None of the projects here advertises generative rewriting.
Vale and textlint turn writing guidelines into automated checks, and Vale works in CI.
LanguageTool supports more than 20 languages beyond English, Spanish, French and German. Harper and proselint are English tools.
List it on TechLogHub and we'll review it for this page.
