Harper

Overview

Harper is a grammar and spelling checker for writers who want local checks across browsers, editors and desktop apps. Its strongest case is privacy-conscious English writing, especially for people who value open-source software and a choice of integrations. Writers who need support beyond English should choose another tool.

Key features

Harper flags capitalization, spelling, grammar errors and awkward phrasing, and also offers style suggestions. The maker says suggestions arrive in under 10 ms. Checks run locally, with no cloud round-trips, telemetry or LLM in the loop, making Harper a considered option for text users would rather not send to a service.

It supports English in British, American, Canadian, Australian and Indian dialects. That is useful for choosing a regional variety, but the single-language scope rules it out for multilingual work. In Harper Desktop, users can toggle rules, add dictionary entries, select a dialect and enable checks per app.

Integrations span browser and editor extensions, a language server, a JavaScript library, a Rust crate and native apps. Named options include Chrome, Brave, Edge and Firefox, along with VS Code, Cursor, Obsidian, WordPress, Neovim, Zed, Helix, Emacs and Sublime Text. Harper.js and integrations built on it support plaintext and/or Markdown, which makes the tool relevant to both prose and text-based workflows.

There are practical caveats. Harper will not run in Gmail unless Gmail’s built-in grammar checker is disabled. Harper Desktop is labeled beta; its macOS version requires macOS 14 or later and runs on Apple Silicon and Intel. The Windows desktop app is also beta, and the maker’s August 28, 2026 post says its binary was not yet signed, so users would likely see a warning popup. Windows users must add apps to the integrations page themselves.

Pricing

Harper’s two free entries describe the same basic proposition: the Free plan costs nothing and includes grammar and spelling checking, local processing and an Apache-2.0 open-source license. The Harper plan is also free, with Free forever billing, no account or credit card required and the same license. There is no paid tier or trial to weigh against the free option, and no per-seat or usage quota is given. That simplicity suits anyone seeking a no-cost checker, but it does not change the English-only scope or beta status of the desktop apps.

Platforms

Harper’s platform range includes Android, iOS, Linux, macOS, Windows, web, API and browser extension. It has both a browser extension and desktop app, alongside editor integrations and developer-facing libraries. That breadth gives it a plausible place in varied writing setups, though the Gmail constraint and the beta desktop caveats may matter to users who rely on those environments.

Who it's for

Harper is best suited to students, journalists and developers who write in English and want local grammar checks across browsers, editors or desktop apps. Its dialect selection and desktop rule controls add useful flexibility for English writing. It is a poor match for writers who need multiple languages, or Gmail users unwilling to disable the built-in checker. Questions go to Discord or GitHub discussions.

Pros and cons

• Pros: Local processing, with no cloud round-trips, telemetry or LLM in the loop, is a strong fit for privacy-conscious writing.
• Pros: Free forever, open-source checking with no account or credit card requirement keeps the price barrier low.
• Pros: Browser, editor, API and native-app integrations offer options for different writing workflows.
• Cons: English is the only supported language, so multilingual writers need another checker.
• Cons: Gmail requires users to turn off its built-in grammar checker before Harper will run there.
• Cons: The desktop apps are beta; the Windows app may trigger a warning because its binary was unsigned at the time of the maker’s post.

Alternatives

Choose QuillBot instead if you want grammar checks alongside paraphrasing: its free plan includes basic grammar, two paraphrase modes and paraphrasing up to 100 words, with limited humanizing and AI Detector features. GrammarCheck.ai may suit readers who prefer a freemium service with a stated 5,000-credit allowance and 650 words per submission on its free plan; its Lite plan is $4 a month.

LanguageTool is another option for readers who want a freemium checker with extension, iOS, macOS and Windows platforms; its Basic plan allows 2,000 characters per text field. Linguix is worth considering for browser-based writing where basic AI suggestions, limited paraphrasing and typing shortcuts are useful alongside grammar and spelling checks.

Pick Outwrite if you want a free checker that also lists a thesaurus, Google Docs and Microsoft Word support. Sapling may be a better fit for users seeking AI-detector quotas and snippets: its free plan includes 2,000 AI detector characters per check, five detector files per batch and 20 snippets.

Trinka is another freemium alternative. Wordvice AI may suit readers who want a defined free proofreading allowance: its Basic plan includes 5,000 words per month, 500 words per submission, two proofreading modes and 15,000 AI Detector characters per month.

For a broader shortlist, browse Grammar checkers or AI Grammar Checkers.

Verdict

Choose Harper if you want a free, open-source English checker that keeps processing local and fits into browser, editor or desktop workflows. Its privacy-oriented approach and broad integrations are the main reasons to choose it; its one-language limit, Gmail requirement and beta desktop apps are the reasons to look elsewhere.