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What is Harper?
Harper is an open-source grammar and spell checker that runs entirely on your own device. It targets developers and writers who want proofreading without sending their text to a cloud service. According to its site, every check happens locally, with no telemetry and no language model in the loop, and its suggestions appear in under 10 milliseconds. It covers US, UK, Canadian, Australian and Indian English, and the site positions it for use across the places you write, including Markdown.
Who it suits
- Privacy-sensitive writers: journalists, students and anyone handling confidential drafts who don't want text processed remotely.
- Developers: people writing Markdown, docs or code comments who want fast, local checks rather than a browser-based service.
- Teams needing multiple English variants: the regional English coverage matters if you publish for several markets.
Trade-offs to weigh
- No AI rewriting: the site says it is "the only grammar checker that doesn't use AI." That favors predictability and privacy, but you won't get generative suggestions or tone rewrites that AI-based tools offer.
- Local-only means local setup: running on your device is the selling point, but you should confirm it integrates with your specific editor or workflow before committing.
- Open source: useful if you want to inspect or self-host; less relevant if you just want a finished consumer app.
Next step: if privacy is your priority, test Harper on a real Markdown document in your editor and check that the regional English variant you need is selectable. If you mainly want AI-assisted rewriting, compare it against a mainstream cloud checker to see which gap matters more.
How does Harper keep my writing private compared to cloud-based grammar checkers?
Harper keeps writing private by doing every grammar and spell check locally on your device. The page states plainly that "every check happens locally," with no cloud round-trips, no telemetry, and no LLM in the loop, so your text never leaves your machine and cannot be used to train models. Cloud-based checkers, by contrast, typically send your text to a remote server for analysis, which introduces a transfer step and a third party that may retain or process the content.
What the difference means in practice
- Data path: Harper's checks run on your hardware; cloud tools require an upload and a response before you see suggestions.
- Speed: Harper advertises suggestions in under 10ms, a benefit of skipping the network round-trip.
- Coverage: The page lists US, UK, Canadian, Australian, and Indian English, useful if you write for mixed audiences.
- Open source: The code can be inspected, which lets privacy-conscious teams verify the local-only claim rather than trust a policy statement.
A concrete scenario
A developer drafting internal design docs or a journalist working on an unpublished draft may not be allowed to paste sensitive text into a third-party service. With Harper, the same document can be checked without that exposure. A team that must document its tooling can also point to the open-source code as evidence.
How to decide
Choose Harper when confidentiality or offline work is the priority and you want fast, rule-based corrections. Choose a cloud checker when you need deeper stylistic rewriting or AI-generated suggestions and are comfortable sending text to a provider. If you want to compare, Grammarly is a well-known cloud option. As a next step, test Harper on a document containing names, code snippets, and jargon to confirm its suggestions fit your writing before relying on it for sensitive drafts.
Can I use Harper to check Markdown or code comments while I work in my editor?
Yes. Harper is designed to check writing where you actually work, including Markdown files and code comments in your editor, with checks running locally rather than through a cloud service.
What that means in practice
- Markdown: Because Harper handles prose inside files, it suits README files, documentation, blog drafts and notes written in Markdown. It checks spelling and grammar while you type, so corrections happen before you commit or publish.
- Code comments: Developers can catch typos and awkward phrasing in comments and docstrings without switching to a separate browser tab. This matters most in shared codebases, where a confusing comment costs reviewers time.
- Privacy and speed: The page states that every check happens on your device, with no telemetry and no LLM involved, and that suggestions appear in under 10ms. For teams working with confidential source code or unpublished drafts, that removes the usual concern about sending text to an external server.
Trade-offs to weigh
Harper is positioned as a rule-based checker rather than an AI rewriter, so expect it to flag clear errors and inconsistencies rather than restructure whole paragraphs. It also covers US, UK, Canadian, Australian and Indian English, which helps if your documentation uses a house style other than US English. If you need heavy stylistic rewriting or tone adjustment, a different tool may fit better; for accuracy and privacy during editing, Harper is the more direct choice.
A useful next step
Install the editor integration for the editor you use, open a Markdown file with a few deliberate typos and a code file with a misspelled comment, and confirm both are flagged. Then set your preferred English variant so you are not fighting false positives on spelling. For a broader comparison of editor-based checking, Grammarly and LanguageTool are the common alternatives, though their privacy models differ from Harper's local-only approach.
Does Harper support UK, Canadian, Australian, or Indian English, or only US English?
Harper covers US, UK, Canadian, Australian, and Indian English, according to the site, so it is not limited to US spelling and usage.
That matters most when you write for a specific audience. A UK-based editor can keep "-ise" spellings and British conventions, while someone preparing documentation for Canadian or Australian readers can select the matching variety instead of manually overriding US defaults. Indian English is also listed, which helps teams producing material for that market.
How to check it suits your work
- Open Harper and test a short sample in your target variety, ideally one containing known regional differences such as "-our"/"-or" or "-ise"/"-ize" endings.
- Confirm the checker flags the forms you consider wrong and leaves your preferred forms alone.
- If you work across several varieties, decide whether you need to switch settings per project or can standardise on one.
The practical trade-off is that broader dialect support is useful for international teams, but regional rules can still differ in nuance. Test with your own text rather than assuming every local convention is covered.
How do I set up Harper for my writing workflow if I want it to run offline?
Harper is designed to run entirely on your device, so "setup" mostly means installing a local build and pointing your editor or writing app at it rather than signing into a cloud account. The page's core claim is that every check happens locally—no cloud round-trips, no telemetry, no LLM in the loop—which is exactly the property you want for offline use. Expect to give up cloud syncing and any AI-style rewriting features; in exchange you get checks that work with no connection and writing that never leaves your machine.
A practical offline setup path
- Pick your entry point. Harper targets "every place you write," so decide whether you need it in a code editor (for Markdown, docs, comments) or a standalone writing surface. Install the build or extension for that environment from the official project.
- Confirm it's truly local. Once installed, disable your network briefly and check that suggestions still appear. If they do, you're running offline as intended.
- Set your English variant. The tool covers US, UK, Canadian, Australian and Indian English; choose the one matching your audience so spelling suggestions don't fight your house style.
- Tune for your files. For developer writing, scope checks to Markdown and prose files so code identifiers aren't flagged as misspellings.
- Keep it updated manually. Offline tools don't auto-sync rules from a server, so plan periodic updates from the source.
Who this suits, and the trade-offs
| Priority | Harper's fit |
|---|---|
| Confidential or client-sensitive drafts | Strong: nothing leaves the device |
| Fast feedback while typing | Strong: sub-10ms suggestions per the page |
| Multi-region English | Strong: five variants covered |
| Cloud sync across devices | Weak: local-only by design |
| AI rewriting or tone suggestions | Not offered: no LLM in the loop |
A concrete case: a freelance technical writer handling unpublished client specs can install Harper in their Markdown editor, set UK English, and draft on a train with no Wi-Fi—suggestions still appear, and the client's text never touches a server. A team that relies on shared cloud documents and AI rewriting would find the offline model a limitation rather than a feature.
As a next step, start with Harper in your primary editor at Harper, run the airplane-mode test above, and only then expand to a second writing app once you've confirmed the offline behavior matches your workflow.
Is Harper really faster than other grammar checkers for long documents?
Yes, plausibly—for long documents, Harper's local processing is where the speed advantage is most visible. The homepage claims suggestions in under 10ms and says every check happens locally with no cloud round-trips, so latency doesn't grow with document length the way a service that uploads text and waits for a response can. For a 50-page report, that difference is felt as you type rather than as one long wait.
Two caveats keep this from being a blanket "fastest" claim:
- What "faster" measures. Harper's figures describe suggestion latency on-device. A cloud checker may be slower per keystroke but offer deeper stylistic rewriting that Harper, as a non-AI, rule-based tool, doesn't attempt. Speed and depth are different axes.
- Your hardware. Local checking runs on your machine, so performance depends on your device and editor integration. On an older laptop with a very large file open, expect some variance.
A practical test: open your longest real document—a thesis chapter, a 10,000-word Markdown README, a legal contract—and type in it for five minutes with each tool. Note when suggestions appear and whether you ever wait. That tells you more than any benchmark.
Whose observation counts: the sub-10ms and "completely private" claims are Harper's own, per the site—not an independent measurement. Treat them as the vendor's stated performance and verify on your own files.
For comparison, established cloud-based checkers include Grammarly and LanguageTool, the latter offering a self-hosted option worth considering if offline operation is your priority but you want broader language coverage than one tool provides.
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