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Harper

A free, open-source grammar checker that runs entirely on your device.

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About Harper

Harper is an English grammar checker written in Rust that does all its checking locally, so no text is sent to a server and no account is needed. Its creator, Elijah Potter, now develops it at Automattic, the company behind WordPress.com. Because it is small and fast, it shows up in more places than most checkers: browser extensions for Chrome, Edge and Firefox, a macOS desktop app that works system-wide, Obsidian and WordPress plugins, and a language server for code editors such as VS Code, Neovim, Helix, Emacs and Zed, where it checks comments and Markdown. Developers can also embed it via a JavaScript library or Rust crate. It is free and Apache-licensed. The trade-off is scope: Harper focuses on grammar, spelling and common style mistakes, has no AI rewriting, and only supports English. For privacy-conscious writers and developers that is often exactly the point.

Best for: Privacy-minded writers and developers who want grammar checks without the cloud.

Pros and cons

What works

  • Runs fully offline, with no account or data upload
  • Works in browsers, Obsidian, WordPress and code editors
  • Free and open source under Apache 2.0

Why people leave

  • English only
  • No AI rewriting or tone suggestions
  • Catches fewer issues than commercial checkers

Top Harper alternatives

See all 5
  1. 1LanguageTool logo
    LanguageTool

    Many more languages and deeper checks, with an optional self-hosted server.

    66.5
  2. 2Vale logo
    Vale

    Enforces custom style guides on docs, complementing grammar checks.

    62.6
  3. 3Grammarly logo
    Grammarly

    Far more suggestions plus AI rewriting, if cloud processing is acceptable.

    59.6
  4. 4Antidote logo
    Antidote

    Another locally installed checker, paid, with English and French.

    57.2
  5. 5ProWritingAid logo
    ProWritingAid

    Deeper style, readability and overused-word reports for long-form writers, which Harper's quick grammar checks don't attempt.

    55.3

Why people left Harper

In reviewers' own words, from reviews of the apps they moved to. These are their opinions, not ours.

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Harper: common questions

Is Harper free?

Yes. Harper is free, open-source software.

Is Harper open source?

Yes. The source code is on GitHub at Automattic/harper.

What platforms does Harper run on?

Harper is available for macOS, Windows, Linux and Browser extension.

What are the best alternatives to Harper?

The highest-ranked Harper alternatives right now are LanguageTool, Vale and Grammarly. See the full ranked list for pricing, platforms and verified facts.

See all 5 Harper alternatives