What Is Harper, the Privacy-First Offline Grammar Checker?

Harper is an open-source grammar and spell checker that runs entirely on your own device. Every check happens locally—no cloud round-trips, no telemetry, and no LLM in the loop—so your writing never leaves your machine. It suits developers and writers who want fast corrections without sending drafts to a third-party service, and it covers US, UK, Canadian, Australian, and Indian English. If you need collaborative cloud editing or AI-generated rewrites, Harper is not built for that.

What "offline" and "no AI" actually mean

The site states plainly: "Every check happens locally. No cloud round-trips, no telemetry, no LLM in the loop." In practice, that means:

• Your text stays on your device. There is no upload step, so there is no server-side copy of your draft and no model trained on your writing.
• Suggestions come from local rules and dictionaries, not a language model. The tool describes itself as "the only grammar checker that doesn't use AI," so you should expect deterministic, rule-based suggestions rather than generative rewrites.
• Speed follows from locality. The site advertises suggestions in under 10ms, which is plausible when no network request is involved.

One caveat: "offline" here describes where checking happens, not necessarily that you can install and run it with no internet at all. The site does not spell out installation or download requirements, so treat full air-gapped use as something to verify yourself rather than assume.

What it checks and who it's for

Harper targets everyday grammar and spelling issues across five English variants: US, UK, Canadian, Australian, and Indian English. That variant coverage matters if you write for mixed audiences or follow a specific style, since spelling conventions differ (for example, "organize" vs. "organise").

The site positions Harper for "students, journalists, and devs," and its keywords include markdown checking—useful if you write README files, docs, or notes in Markdown. The page does not detail which Markdown constructs are handled or how code blocks are treated, so test that against your own files before relying on it.

How to try it and judge fit

1. Go to writewithharper.com and use the "Try Harper" option on the page.
2. Paste a sample of your real writing—ideally one with deliberate errors and one in your preferred English variant.
3. Check whether the flagged issues match what you'd expect and whether suggestions appear quickly.
4. If you write Markdown, run a document with headings, lists, and code blocks to see how it behaves.

You'll know it fits if you want private, fast, rule-based checks inside your existing writing workflow. Look elsewhere if you need cloud sync, team review features, or AI-style rewriting—none of those are part of what Harper describes.

Quick comparison

Pricing and any upgrade terms are not specified on the page, so check the site directly if cost matters to your decision.