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Harper

writewithharper.com

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Spec score 8.9 #19 of 88 in Grammar checkers

What it is

Harper is a free, open-source grammar checker that flags capitalization, spelling, awkward phrasing, and grammar errors. The maker says checks run locally, with no cloud round-trips, telemetry, or LLM in the loop, and that suggestions appear in under 10 ms. Harper is available through browser and editor extensions, a language server, a JavaScript library, a Rust crate, and native apps. Listed integrations include Chrome, Brave, Edge, Firefox, VS Code, Cursor, Obsidian, WordPress, Neovim, Zed, Helix, Emacs, and Sublime Text. Its language support covers English and British, American, Canadian, Australian, and Indian English dialects. Harper Desktop lets users toggle rules, add dictionary entries, select a dialect, and turn checking on or off per app. The desktop app is labeled beta for macOS 14 or later, and the Windows desktop app is also described as a beta; its binary was not signed at the time of the listed post. Harper is free, with no account or credit card required.

Spec sheet

What it does
Harper is an English grammar checker that detects spelling, capitalization, phrasing and grammar problems locally on the device.Read 14 Oct 2026 · writewithharper.com
Purpose
Harper is a free, open-source grammar checker that flags capitalization, spelling, awkward phrasing, and grammar errors.Read 7 Oct 2026 · writewithharper.com
Free plan
YesRead 20 Sep 2026 · writewithharper.com
Integrations
The maker lists browser and editor extensions, a language server, a JavaScript library, a Rust crate, and native apps.Read 7 Oct 2026 · writewithharper.com
Privacy
Checks run locally; the maker says Harper has no cloud round-trips, telemetry, or LLM in the loop.Read 7 Oct 2026 · writewithharper.com
Support
The maker directs users to Discord or GitHub discussions for questions.Read 7 Oct 2026 · writewithharper.com
License
The source code is released under the Apache License 2.0.Read 14 Oct 2026 · github.com
API and SDKs
Developers can integrate harper.js as a JavaScript library, harper-core as a Rust crate, or harper-ls through the Language Server Protocol.Read 14 Oct 2026 · writewithharper.com
Apps
Listed integrations include Chrome, Brave, Edge, Firefox, VS Code, Cursor, Obsidian, WordPress, Neovim, Zed, Helix, Emacs, and Sublime Text.Read 7 Oct 2026 · writewithharper.com
Browser extension
YesRead 20 Sep 2026 · writewithharper.com
Chrome Web Store privacy
The Chrome Web Store listing says the developer does not collect or use user data.Read 14 Oct 2026 · chromewebstore.google.com
Customization
Harper Desktop lets users toggle rules, add dictionary entries, choose an English dialect, and enable checking per app.Read 7 Oct 2026 · writewithharper.com
Desktop app
YesRead 20 Sep 2026 · writewithharper.com
Desktop availability
The Harper Desktop page lists macOS 14 or later on Apple Silicon and Intel and labels the app beta.Read 7 Oct 2026 · writewithharper.com
Documentation depth
Documentation includes integration guides, harper.js API material, Weir rule language documentation and contributor build/testing guides.Read 14 Oct 2026 · writewithharper.com
Feature: code comments
Harper can lint comments and documentation in many programming and markup languages through harper-ls.Read 14 Oct 2026 · writewithharper.com
Feature: custom rules
The Weir language lets users and organizations define custom linguistic rules and conventions.Read 14 Oct 2026 · writewithharper.com
Feature: dictionaries
The language-server integrations support custom dictionaries and code actions for adding words.Read 14 Oct 2026 · writewithharper.com
Feature: grammar suggestions
Harper underlines detected errors and offers suggested corrections while users write.Read 14 Oct 2026 · writewithharper.com
GitHub project scale
The public repository showed about 16.4 thousand stars, 720 forks and more than 800 open issues when accessed.Read 14 Oct 2026 · github.com
Gmail limitation
Harper will not run in Gmail unless Gmail’s built-in grammar checker is disabled.Read 7 Oct 2026 · writewithharper.com
Integration: browsers
Official browser integrations are available for Chrome, Brave, Microsoft Edge and Firefox.Read 14 Oct 2026 · writewithharper.com
Integration: editors
Official editor integrations include VS Code, Cursor, Neovim, Zed, Helix, Emacs and Sublime Text.Read 14 Oct 2026 · writewithharper.com
Integration: writing apps
Harper provides plugins or integrations for Obsidian, WordPress and a macOS desktop application.Read 14 Oct 2026 · writewithharper.com
JavaScript API
harper.js exposes LocalLinter and WorkerLinter implementations through a Promise-based Linter interface.Read 14 Oct 2026 · writewithharper.com
Language coverage
Supported English dialects are British, American, Canadian, Australian and Indian English.Read 14 Oct 2026 · writewithharper.com
Languages
Harper supports English and British, American, Canadian, Australian, and Indian dialects of English.Read 7 Oct 2026 · writewithharper.com
Languages supported
1Read 20 Sep 2026 · writewithharper.com
Limit: browser compatibility
A Reddit user reported being unable to install Harper on Waterfox.Read 14 Oct 2026 · reddit.com
Limit: language selection
A Reddit user reported receiving suggestions for valid non-US spellings in code-related text and was unsure about selecting multiple languages.Read 14 Oct 2026 · reddit.com
Limit: Obsidian popup layout
A Reddit user reported that suggestion popups can overlap linked previews and become awkwardly narrow at the edge of a window.Read 14 Oct 2026 · reddit.com
Limit: vocabulary coverage
The same Reddit discussion reported that obscure or archaic words were sometimes unknown to Harper.Read 14 Oct 2026 · reddit.com
Maker company
Automattic Inc. is a privately held company headquartered in San Francisco and founded in August 2005.Read 14 Oct 2026 · automattic.com
Maker size
LinkedIn lists Automattic in the 1,001–5,000 employee size range and says it has more than 1,730 employees across 92 countries.Read 14 Oct 2026 · linkedin.com
Ownership news
Automattic announced its acquisition of Harper and the addition of founder Elijah Potter to its team on November 21, 2024.Read 14 Oct 2026 · automattic.com
Performance
The maker says Harper usually returns suggestions in under 10 milliseconds.Read 14 Oct 2026 · writewithharper.com
Praise: developer workflow
A Reddit commenter described Harper as a fast language-server grammar checker that runs in the background on code comments.Read 14 Oct 2026 · reddit.com
Praise: Obsidian users
A Reddit user reported that Harper was much better than Obsidian's default spell checker and easy to customize with added words.Read 14 Oct 2026 · reddit.com
Processing model
Harper performs checks on-device without cloud round trips, telemetry or large-language-model processing.Read 14 Oct 2026 · writewithharper.com
Rating on Chrome Web Store
4.2 out of 5 from 128 ratingsRead 14 Oct 2026 · chromewebstore.google.com
Rating on Firefox Add-ons
4.1 out of 5 from 42 ratingsRead 14 Oct 2026 · addons.mozilla.org
Recent release
GitHub lists Harper version 2.12.0 released October 1, 2026, including new redundant-word detection and Chrome extension changes.Read 14 Oct 2026 · github.com
Self-hosting
The project is designed for local execution and supplies command-line, language-server, WebAssembly and native integration code rather than a hosted checking service.Read 14 Oct 2026 · github.com
Speed
Harper says it provides suggestions in under 10 ms.Read 7 Oct 2026 · writewithharper.com
Style suggestions
YesRead 20 Sep 2026 · writewithharper.com
Support channels
Users are directed to Discord and GitHub discussions for questions and support.Read 14 Oct 2026 · writewithharper.com
System requirements
The maker says Harper needs no GPU or special additional memory; the desktop application currently supports macOS 14 or later.Read 14 Oct 2026 · writewithharper.com
Text formats
The maker says harper.js and integrations built on it support plaintext and/or Markdown.Read 7 Oct 2026 · writewithharper.com
Who it serves
The homepage describes Harper as used by students, journalists, and developers.Read 7 Oct 2026 · writewithharper.com
Windows
The maker’s August 28, 2026 post says the Windows desktop app is available as a beta download, with users adding apps to the integrations page themselves.Read 7 Oct 2026 · elijahpotter.dev
Windows limitation
The maker said the Windows binary was not yet signed at the time of that post, so users would likely see a warning popup.Read 7 Oct 2026 · elijahpotter.dev

Plans and prices

Free Free all features · local processing · no credit card Read 14 Oct 2026 · writewithharper.com
Harper Free Free forever No account or credit card required · Apache-2.0 open source Read 7 Oct 2026 · writewithharper.com

Every Harper plan and price →

Strong points and limits

Strong points

  • Free and open-source under Apache-2.0.
  • Checks run locally, according to the maker.
  • Supports several English dialects.
  • Desktop app allows rule and dictionary customization.
  • Integrates with browsers and editors.

Limits

  • Supports English only.
  • macOS and Windows desktop apps are labeled beta.
  • Windows binary may trigger a warning popup.
  • Does not run in Gmail unless its checker is disabled.

Who it suits

Harper suits students, journalists, and developers who want local grammar and spelling checks in a browser, editor, or desktop app. Its dialect options and per-app desktop controls may suit writers who want to adjust how checks apply.

Harper in depth

Read the full write-up

Harper is a free, open-source grammar checker that keeps analysis on your device while covering browsers, editors and desktop apps. It is best suited to English-language students, journalists and developers who value privacy and configurable writing rules. The trade-off is a single-language scope and beta desktop software, so multilingual writers and users needing a polished Gmail workflow should look elsewhere.

Overview

Harper flags capitalization, spelling, grammar mistakes and awkward phrasing, with additional style suggestions. Its local-processing design means text is checked without cloud round-trips, telemetry or an LLM in the loop. That is a substantial advantage for private drafts, source code comments and other material you do not want sent to a remote service.

The maker says suggestions appear in under 10 ms, which should suit writers who want immediate feedback while composing. Harper is released under the Apache-2.0 open-source license, and no account or credit card is required. Support is handled through Discord or GitHub discussions rather than a conventional commercial support channel.

Key features

English support includes British, American, Canadian, Australian and Indian dialects. Choosing among those varieties helps prevent regional spellings from being treated as mistakes, but Harper does not cover other languages, making it unsuitable for multilingual editing.

Harper Desktop allows rule toggles, custom dictionary entries, dialect selection and per-app enablement. Those controls are useful when a writer needs to preserve specialist terms or apply different checking standards in different programs. Integrations span browser and editor extensions, a language server, a JavaScript library, a Rust crate and native apps. Named environments include Chrome, Brave, Edge, Firefox, VS Code, Cursor, Obsidian, WordPress, Neovim, Zed, Helix, Emacs and Sublime Text. Harper.js and integrations built on it support plaintext and/or Markdown, fitting both ordinary prose and text-based development workflows.

Gmail is a notable exception: Harper will not run there while Gmail's built-in grammar checker is enabled. Anyone who writes primarily in Gmail must change that setting before Harper can help.

Pricing

Harper has no paid tier. Both the Free and Harper plans cost free; the latter is billed free forever, with no account or credit card required. The plans provide grammar and spelling checking, local processing and the Apache-2.0 open-source license. There is no free trial because the product itself is free, and no seat, usage quota or renewal charge is stated. It is an uncomplicated choice for individuals and teams that need core checking without a subscription, though it also means there is no paid service tier to add broader language coverage.

Platforms

Harper runs on Android, iOS, Linux, macOS, Windows and the web, with browser extensions and an API. Desktop software deserves caution: macOS support is for version 14 or later on both Apple Silicon and Intel, and the app is marked beta. A Windows desktop beta is available, but the maker said its binary was not signed at the time of the August 28, 2026 announcement, so Windows may show a warning popup. Users also add Windows applications to the integrations page themselves, which makes setup less turnkey.

Who it's for

Choose Harper if you are a student, journalist or developer writing in English and want fast, device-local checks across several tools. It is particularly persuasive for privacy-sensitive work, open-source users and people who need dialect controls or custom dictionaries. It is a poor match for writers working in languages other than English, anyone requiring a non-beta desktop experience, or Gmail users unwilling to disable Gmail's checker.

Pros and cons

Pros

Local and private: Checks avoid cloud transfers, telemetry and LLM processing.

Broad workflow coverage: Browser, editor, desktop, API, language-server, JavaScript and Rust options reach both general and developer tools.

Useful English variation: Five English dialect choices reduce regional spelling friction.

Configurable desktop rules: Rule switches, dictionary entries and per-app controls accommodate specialist writing.

No-cost access: Free-forever use requires no account or card.

Cons

English only: Multilingual writers need another checker.

Desktop beta status: macOS and Windows users should expect a less mature experience.

Windows warning: The unsigned beta binary may trigger an operating-system popup.

Gmail constraint: Gmail's own grammar checker must be disabled.

Community support: Questions go through Discord or GitHub discussions.

Alternatives

For a broader comparison, see Grammar checkers and AI Grammar Checkers. Pick QuillBot instead if you want paraphrasing alongside basic grammar, with Android, iOS, desktop, web and browser access; its free plan limits paraphrases to 100 words. LanguageTool is the better direction when character-based limits and paid Premium tiers suit your editing workflow. Choose Outwrite for a freemium tool combining spelling, grammar, thesaurus, Google Docs, Microsoft Word and browser support. Microsoft Editor fits users already paying for Microsoft 365, including a 1 TB subscription benefit. Sapling is aimed at users who need API, self-hosted or web deployment and AI-detector quotas. GrammarCheck.ai offers credit-based plans, including a free 5,000-credit allowance. ORFO is the alternative for Russian, English and Ukrainian checking. Trinka is another freemium option for writers who need its broader product scope.

Verdict

Harper is the right choice for privacy-conscious English writers who want genuinely free, open-source checking across browsers, editors and desktop tools. Its local processing, dialect selection and rule customization are stronger reasons to choose it than the price alone. Look elsewhere if you need languages beyond English, a mature signed desktop release or uninterrupted Gmail checking.

Verdict

Harper provides free local writing checks across browser, editor, and desktop options, with English dialect selection and customization on desktop. Note the beta status of desktop apps and the Gmail limitation before choosing it.

Where Specifiction places it

Alternatives to HarperAll alternatives →