Harper

Overview

Harper is a free, open-source grammar checker for people who want writing checks to stay on their device. It is best suited to English-language students, journalists, and developers who want to use one checker across browsers, editors, and desktop apps. Its local processing and zero-cost access make it an appealing focused tool, but writers who need another language will need a different option.

Key features

Harper flags capitalization, spelling, grammar errors, and awkward phrasing, with style suggestions to help refine a draft. The maker says suggestions arrive in under 10 ms; the more important distinction for many users is that checks run locally, without cloud round-trips, telemetry, or an LLM in the loop. That is a strong fit for writers who prefer not to send their text to an online service.

Harper supports English in British, American, Canadian, Australian, and Indian dialects. Choosing a regional variety helps keep suggestions aligned with a writer’s English, but the single-language scope rules it out for multilingual work. Harper Desktop adds practical controls: users can toggle rules, add dictionary entries, choose a dialect, and enable checking for individual apps.

Its reach is broader than a single desktop editor. Integrations include browser and editor extensions, a language server, a JavaScript library, a Rust crate, and native apps. Named options 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, making it relevant to text-based workflows as well as browser writing.

There are deployment caveats. The macOS desktop app is beta and requires macOS 14 or later on Apple Silicon or Intel. The Windows desktop app is also beta; users add apps to its integrations page themselves, and the maker said the binary was unsigned at the time of its August 28, 2026 post, so Windows may show a warning. Harper also will not run in Gmail unless Gmail’s own grammar checker is disabled.

Pricing

Harper costs nothing. The Free plan is free and includes grammar and spelling checking, local processing, and an Apache-2.0 open-source license. The Harper plan is also free, billed Free forever, with no account or credit card required and the same open-source licensing. There is no free trial because the product is free; there is no paid tier or seat or usage quota to weigh against the free access described here.

That simplicity suits individuals who want core checking without a subscription or sign-up. The tradeoff is not a cheaper plan with fewer allowances, but Harper’s narrow English focus and its reliance on the integrations that fit each user’s workflow.

Platforms

Harper is listed for Android, iOS, Linux, macOS, web, and Windows, as well as API and extension access. Browser and desktop apps are available, with the macOS and Windows desktop apps in beta. Users who need Gmail checking should account for the requirement to disable Gmail’s built-in checker.

Who it's for

Choose Harper if you write in English, value local processing, and want a free checker that can fit browser, editor, or desktop workflows. Its rule toggles, dictionary entries, and dialect selection are useful for writers who want to tune desktop checking. Look elsewhere if your work regularly switches languages or if you need Harper to run in Gmail alongside Gmail’s own grammar checker.

Pros and cons

• Pro: Local checking without cloud round-trips, telemetry, or an LLM in the loop suits privacy-conscious writers.
• Pro: Free access, no account or credit card requirement, and an Apache-2.0 license make it straightforward to adopt.
• Pro: Browser, editor, API, and native-app integrations give English writers several ways to bring checking into their workflow.
• Con: Support is limited to English and its listed dialects, so multilingual writers need another checker.
• Con: The desktop apps are beta, and Windows users may encounter a warning because the binary was unsigned at the time of the maker’s post.
• Con: Gmail checking requires disabling Gmail’s built-in grammar checker, an inconvenient tradeoff for users who rely on it.

Alternatives

For a free checker with explicit submission and credit limits, consider GrammarCheck.ai: its Free Plan includes 5,000 credits and 650 words per submission. LanguageTool is another option, with a Basic limit of 2,000 characters per text field and a Premium Monthly limit of 150,000 characters per text field; its plan prices are not given here.

Pick QuillBot if you want paraphrasing alongside basic grammar: its free plan allows paraphrasing up to 100 words in two modes. Outwrite is an alternative with a free plan covering spelling and grammar, a thesaurus, Google Docs, Microsoft Word, browser extensions, and a web app; it also offers a free trial.

Trinka is another freemium alternative for readers comparing grammar checkers. Wordvice AI may suit writers who want a defined free monthly allowance: Basic includes 5,000 words per month, 500 words per submission, two proofreading modes, and 15,000 AI Detector characters per month. Linguix offers free grammar and spelling checks, basic AI suggestions, limited paraphrasing, typing shortcuts, and a browser extension. For language editing and rewriting with limited free quotas, consider Writefull.

Browse Grammar checkers for more options, or AI Grammar Checkers if that is the category you want to compare.

Verdict

Harper is a strong choice for English writers who want a free, open-source checker that processes text locally and can slot into browser, editor, and desktop workflows. The main reason to choose it is that combination of privacy-conscious checking and broad integration access at no cost. Look elsewhere if you need more than English, or if your workflow depends on Gmail’s grammar checker staying enabled.