Most grammar checkers send your text to a server. You paste something in, it travels over the internet, gets processed by AI on external infrastructure, and comes back with suggestions.
For everyday writing, that's fine. For anything sensitive, it's worth knowing what you're handing over. I wrote about what specifically happens with Grammarly if you want the full breakdown.
This post covers the tools that don't do that. No cloud, no account, no copy of your text on someone else's machines.
What to actually look for
Not all "private" or "offline" tools are equally private or equally offline. A few things worth checking:
Where does processing happen? On-device means inference runs on your hardware. Some tools say "offline" but still phone home for activation, telemetry, or model updates.
Is an account required? An account creates a user profile. Even if text isn't stored, usage metadata often is.
What platform does it run on? Most private grammar tools are desktop or browser-based. iPhone options are thin.
Is it rule-based or AI? Rule-based tools (like Harper) are fast and deterministic but catch fewer subtle errors. AI-based tools catch more but require larger local models.
Comparison at a glance
| Tool | Platform | Offline? | Account? | Price |
|---|---|---|---|---|
| Harper | Browser ext, Neovim, VS Code | Yes (always) | No | Free |
| Refine | macOS only | Yes (always) | No | Paid (7-day trial) |
| LanguageTool | Browser, desktop, mobile | Self-hosted only | Optional | Free tier / paid |
| iOS spell check | iPhone (built-in) | Yes | No | Free |
| Proofed | iPhone | Yes (always) | No | Free / $4.99/mo |
Harper
Platform: Browser extension (Chrome, Firefox, Edge), Neovim, VS Code, Obsidian Offline: Always. No network calls. Account: Not required. Price: Free. Open source.
Harper is a rule-based grammar checker written in Rust. It runs entirely in your browser or editor and checks text as you type. No server, no account, no generative AI anywhere in the pipeline.
The "no generative AI" part is a deliberate design choice. Because it uses rules rather than a language model, it's fast, under 10ms per check, and completely deterministic. You know exactly what it's checking for.
What it catches well: incorrect articles (a/an), subject-verb agreement, common word confusions, punctuation issues. What it misses: nuanced grammar errors that require understanding context, style suggestions, rewrites.
For developers and writers who live in VS Code or Neovim, Harper is excellent. For anyone who wants a grammar check on iPhone, it doesn't have an app.
Good for: Developers, Obsidian/Neovim users, anyone who wants a browser extension that works offline. Not for: iPhone users, anyone who needs AI-quality error detection or rewriting.
Refine
Platform: macOS 14+ only (system-wide) Offline: Always. Local AI models. Account: Not required. Price: Paid, with a 7-day free trial.
Refine is a macOS menu bar app that works system-wide. Once installed, it can check grammar in any app on your Mac: Notes, Messages, email clients, web apps, whatever. It uses local AI models so everything stays on your machine.
The system-wide approach is genuinely useful. You don't have to paste text anywhere. You just write, and Refine watches.
The tradeoff is platform lock-in. macOS 14 or newer, no Windows, no iPhone. And it's paid. r/macapps reception has been positive for a Mac-focused offline Grammarly alternative.
If you write primarily on Mac and want a Grammarly replacement that runs locally, Refine is probably the most polished option right now.
Good for: Mac users who want system-wide grammar checking without cloud. Not for: iPhone, Windows, or anyone looking for a free tool.
LanguageTool
Platform: Browser extension, desktop app, web, mobile apps Offline: Self-hosted only (requires running a local Java server) Account: Optional for cloud, not needed for self-hosted Price: Free (open source core), paid for premium features
LanguageTool is open source and technically offline-capable, but getting there requires work. The cloud version sends text to their servers by default. For offline use, you run a local LanguageTool server (Docker or Java), then point your browser extension or app at localhost.
The self-hosted route gives you solid grammar checking with no data leaving your machine, and it supports a wide range of languages. Some editors like Zettlr have built-in support for pointing at a local LanguageTool instance.
The catch: this is a developer workflow. There's no one-click offline mode. If you're comfortable running a Docker container, it works well. If you're not, it's a headache.
The mobile apps send text to LanguageTool's cloud by default. There's no easy offline mobile path without significant technical setup.
Good for: Technical users who want self-hosted grammar checking with broad language support. Not for: Non-technical users, anyone who wants something that works out of the box.
iOS built-in spell check
Platform: iPhone (built-in) Offline: Yes, always. Account: No. Price: Free, comes with iPhone.
iOS has a spell checker that works in most text fields. It underlines misspellings and offers corrections on tap. It works offline and requires nothing.
It's not a grammar checker. It won't catch grammatical errors, won't flag sentence structure problems, won't offer rewrites. For typos and outright misspellings, it does the job. For anything more, it's not enough.
iOS 18 added Writing Tools, which includes grammar checking and rewriting powered by Apple Intelligence. It works offline once the models are downloaded, which is a real step forward. The limitation is that it's only available in supported apps (Notes, Mail, Messages, Pages) and only on Apple Intelligence-compatible devices. You can't run it on arbitrary text pasted from elsewhere.
Good for: Quick spell checks, basic autocorrect. Not for: Grammar analysis, checking text from external sources, any device running iOS 17 or older.
Proofed
Platform: iPhone Offline: Always. Apple Foundation Models (on-device AI). Account: Not required. Price: Free (5 checks/day), Pro at $4.99/month or $29/year.
Proofed is the app I'm building to fill the gap that everything above leaves: offline, AI-quality grammar checking specifically for iPhone.
It uses Apple's Foundation Models framework, which runs inference locally with no network calls. Grammar checking, spelling, rewrites, and tone adjustment all happen on your device. I wrote about how that works technically if you want the details.
The free tier gives you 5 grammar checks per day, which covers most casual use. Pro removes the limit and adds text rewriting and tone adjustment.
What it doesn't do: browser extension, desktop app, Windows, Android. It's iPhone-only. If you write primarily on a Mac or in a browser, Refine or Harper will serve you better.
Good for: iPhone users who want offline grammar checking without an account or cloud processing. Not for: Desktop-first workflows, non-Apple devices.
The verdict
There's no single tool that covers everything. Here's how I'd break it down:
On iPhone, offline: Proofed is the only purpose-built option. iOS Writing Tools works too but only in certain apps.
On Mac, offline: Refine for a polished paid experience, Harper if you want free and open source (browser extension only).
For developers: Harper is hard to beat. VS Code, Neovim, Obsidian support, completely free, fast.
For broad language support, self-hosted: LanguageTool if you're willing to run the server.
The privacy grammar checker space is small but growing. Cloud-first tools like Grammarly are strong on features but that comes with the tradeoff of your text leaving your device. Whether that matters depends on what you're writing.
FAQ
Is there a private alternative to Grammarly?
Yes. Harper is free and open source (browser/editor). Refine is a paid macOS app. Proofed is for iPhone. None of them match Grammarly's full feature set, but all of them keep your text on your device.
What is the best offline grammar checker for iPhone?
Proofed is built specifically for this. It uses Apple's on-device AI with no internet required. iOS's built-in Writing Tools also works offline but only in certain apps.
Is LanguageTool private?
The cloud version sends text to LanguageTool's servers. The self-hosted version (running locally via Docker or Java) keeps everything on your machine. Setup requires some technical comfort.
Does Harper work on iPhone?
No. Harper is available as a browser extension, and as a plugin for editors like Neovim, VS Code, and Obsidian. There's no iPhone app.
What grammar apps don't require an account?
Harper, Refine, and Proofed all work without creating an account. The free tier of LanguageTool requires an account for some features but the self-hosted version doesn't.
Proofed is on the App Store. No account. No internet. Offline grammar checking for iPhone.