Charm and Harper are both private, on-device writing tools for Mac, and that shared privacy is worth stating plainly up front. The difference is breadth and maturity. Harper is a free, open-source grammar and spell checker that is strongest inside code editors and the browser, with a system-wide Mac app that is still in beta. Charm is a mature native app that corrects spelling and grammar and predicts your next word across every Mac app today, for a one-time $9.99. Harper's price and its developer-focused integrations are real strengths; Charm's edge is proven system-wide coverage and word prediction.
What is Harper and where does it run?
Harper is an open-source grammar checker written in Rust, now maintained by Automattic. It is fast, lightweight, and fully on-device, with no account, no telemetry, and no cloud. Its most mature integrations are developer-facing: a language server that plugs into code editors like VS Code, Neovim, and Zed, plus browser extensions for Chrome and Firefox, an Obsidian plugin, and a WordPress plugin. Within those surfaces it checks and highlights as you type.
There is also Harper Desktop, a macOS app that aims to check text system-wide across apps like Mail, Slack, and Notes. As of mid-2026 it is in beta - its own developer describes it as not yet ready for general use - so the system-wide promise is real in intent but not yet a polished, proven capability.
Charm ships system-wide correction today. It uses the macOS Accessibility API together with a keyboard-event tap to work in native apps, browsers, code editors, and Terminal, as a mature product rather than a beta.
System-wide coverage: mature vs early
If you write mostly in a code editor or the browser, Harper is excellent and covers you well right now. If you want one tool that corrects everywhere - the Mail compose window, a Slack message, a Pages document, a note - Harper's answer is Harper Desktop, which is still early beta. Charm's system-wide coverage is the product itself, and it is shipping and stable.
Word prediction
Harper is a checker: it detects and corrects grammar and spelling. It does not predict your next word. Charm includes Oracle, which suggests your next word as you type so you can accept it with the Tab key. If you want prediction as well as correction, Charm has it and Harper does not.
| Feature | Charm | Harper |
|---|---|---|
| On-device and private | Yes | Yes |
| System-wide across every Mac app | Yes - shipping | Beta (Harper Desktop) |
| Strongest surfaces | Every app | Code editors and browser |
| Real-time spelling and grammar | Yes | Yes, within supported surfaces |
| Word prediction | Yes - Oracle | No |
| Languages | English-focused | English only |
| Open source | No | Yes - Apache-2.0 |
| Pricing | $9.99 once | Free |
Price and privacy
Harper is free and open-source, with no account and no limits, and it is fully on-device. That is a genuine strength, and the on-device privacy is something Charm shares. Both keep your writing on your Mac.
Charm is $9.99 once. For that one-time price you get mature system-wide coverage and word prediction, where Harper's system-wide app is still beta and it has no prediction. Both are English-focused, so language coverage is not a real point of difference here.
Where does Harper have the edge?
Harper is a superb choice for developers and for anyone who wants a free, private, open-source checker. It is extremely fast and light on memory, it is first-class inside code editors through its language server, and it checks code comments and Markdown well. It is backed by Automattic and actively developed. If your writing happens mostly in an editor or the browser and you want a free tool with no account, Harper is hard to beat - and if you want to inspect or contribute to the source, it is open.
Frequently asked questions
What is the difference between Charm and Harper?
Both are on-device, private writing tools. Harper is a free, open-source grammar and spell checker strongest in code editors and the browser, with a beta system-wide Mac app. Charm is a mature native app that corrects spelling and grammar and predicts your next word across every Mac app today, for a one-time $9.99.
Does Harper have a Mac app?
Yes, but Harper Desktop is in beta as of 2026 and its developer describes it as not yet ready for general use. Harper's mature surfaces are code editors and the browser. Charm's system-wide coverage across every Mac app is a shipping, stable product.
Is Harper free?
Yes. Harper is free and open-source under the Apache-2.0 licence, with no account and no usage limits. Charm is a one-time $9.99 purchase that adds mature system-wide coverage and word prediction.
Is Harper private?
Yes. Harper is fully on-device with no cloud, no telemetry, and no account, and it never trains on your writing. Charm shares this on-device privacy and extends correction across every Mac app.
Does Harper do word prediction?
No. Harper detects and corrects grammar and spelling; it does not predict your next word. Charm includes Oracle, a word-prediction feature you accept with the Tab key, alongside its spelling and grammar correction.