The most private way to check grammar is with a tool that runs on your own device: your phone’s built-in spell check, an open-source checker you run yourself, or an AI model that works offline on your phone. Popular services like Grammarly send your text to their servers to analyze it, which is fine for a birthday message and a problem for a confidential report, a medical note or a legal letter.
Here’s how the options compare, what an offline AI does well and badly as a proofreader, and prompts that stop it from rewriting your whole document.
Does Grammarly send your text to its servers? #
Yes. Grammarly’s suggestions are generated in the cloud, which is why it needs an internet connection to work. The same is true of most AI writing assistants and of cloud chatbots used as proofreaders: your text travels to the company, is processed there, and is handled under that company’s privacy policy.
For most everyday writing, that’s a reasonable trade. It becomes a problem when:
- your employer or client forbids sending documents to third-party services
- the text includes health, legal or financial details about you or someone else
- you’re writing something you simply don’t want stored anywhere else, like a personal letter or a journal entry
In those cases, you want a checker that never sends the text anywhere.
Private grammar checker options compared #
| Option | Where it runs | Catches | Works offline | Best for |
|---|---|---|---|---|
| Phone’s built-in spell check | On the device | Spelling, some autocorrect | Yes | Quick typing |
| Apple Writing Tools (Apple Intelligence) | Mostly on-device; some requests go to Apple’s Private Cloud Compute | Grammar, rewording | Partly | Recent iPhones that support it |
| LanguageTool (self-hosted) | Your own computer or server | Grammar, style, many languages | Yes, once set up | Technical users, desktop writing |
| Grammarly and similar | Company servers | Grammar, clarity, tone | No | Convenience, when privacy isn’t a concern |
| Offline AI app on your phone | On the device | Grammar, clarity, tone, rewrites | Yes | Private documents, anything sensitive |
A few notes:
- Built-in spell check is private but limited. It catches misspellings, not a sentence that says the opposite of what you meant.
- Apple Intelligence runs many requests on the device and sends larger ones to Apple’s Private Cloud Compute servers, which Apple says don’t store your data. It’s a good middle ground if your phone supports it. Our guide to phone AI privacy settings shows where to control it.
- LanguageTool has an open-source core you can run yourself. That’s the strongest private option for desktop writing, but setup takes some technical comfort.
- Keyboards are their own question. Some third-party keyboards send what you type to the cloud for predictions. We look at that in is your keyboard private?
Using an offline AI as a proofreader #
A language model that runs on your phone can do more than spell check. It can fix grammar, flag unclear sentences, adjust tone and explain why a sentence is wrong. The catch: it doesn’t underline mistakes as you type. You paste text in, and it gives you a corrected version back.
The other catch is that language models love to rewrite. Ask for “better grammar” and you may get a document that’s been reworded top to bottom. The prompts below keep it on a short leash.
Prompts that fix mistakes without rewriting #
Minimal corrections:
Proofread the text below. Fix only spelling, grammar and punctuation errors. Don’t change word choice, sentence order or tone. Return the corrected text, then a list of every change you made.
Show me, don’t fix it:
Don’t rewrite this. List each error you find, quote the sentence, and explain the problem in one line.
Specific English:
Proofread this in British English. Keep “-ise” spellings and single quotation marks.
Clarity pass:
Point out any sentence that’s ambiguous or hard to follow. Suggest a clearer version for each, but leave everything else alone.
Tone check:
Does this email sound rude, passive-aggressive or too casual for a client? Point to specific phrases.
The “list every change” instruction is what makes AI proofreading trustworthy. You can scan the list in ten seconds and reject anything you don’t agree with.
Use a low temperature #
Proofreading isn’t creative work. A lower temperature setting makes the model stick closely to your text and pick the most likely correction instead of an inventive one. If your app has presets, use the precise one. Our explainer on AI settings like temperature covers why this matters.
How to proofread privately on your phone #
Personal LLM runs open AI models such as Qwen 3.5 and Gemma 4 directly on iPhone and Android, with no account and no server. After the one-time model download, it works in airplane mode, and the text you paste never leaves the phone.
For proofreading:
- Download a model on Wi-Fi. Qwen 3.5 4B is the app’s recommended first choice; Qwen 3.5 9B gives better results on phones with 6 GB of RAM or more.
- Start a new chat and set a system prompt such as “You are a careful proofreader. Fix only errors and list every change.” You can make that the default for new chats in Settings.
- Choose the Precise preset.
- Paste a section of your text (a few paragraphs at a time works better than a whole report).
- Copy the corrected version back with one tap, or select just the sentence you need.
Phone-sized models catch most grammar and spelling mistakes and are good at clarity suggestions. They’re weaker on subtle style rules and can occasionally “fix” something that was correct, which is another reason to ask for a list of changes. For long documents, you can also attach a text or Markdown file and ask about specific sections.
What an AI proofreader misses #
- Facts. It won’t know the meeting was on Tuesday, not Thursday.
- Your house style. Unless you tell it (Oxford comma, “email” vs “e-mail”), it guesses.
- Names and technical terms. It may “correct” a product name or a drug name into a common word. Check proper nouns carefully.
- Long documents at once. Paste sections. A model can only keep a limited amount of text in view, and quality drops near that limit.
Frequently asked questions #
Is there a grammar checker that works offline? #
Yes. Your phone’s built-in spell check works offline, LanguageTool can be self-hosted, and an AI app that runs a language model on your phone can proofread with no connection at all. Grammarly and most AI writing tools need the internet.
Is Grammarly safe for confidential documents? #
Grammarly processes text on its servers, so check your employer’s or client’s rules before using it on confidential material. Many organizations restrict third-party writing tools. An on-device option avoids the question, because the text never leaves your device.
Can ChatGPT check grammar? #
Yes, and it does it well, but your text is sent to OpenAI and stored as part of your chat history, subject to your privacy settings. For sensitive writing, use a temporary chat, strip identifying details, or proofread with an offline model instead.
Why does AI change more than just my grammar? #
Language models are trained to produce polished text, so a vague request like “improve this” invites a rewrite. Tell it to fix only errors, keep your wording, and list each change, and use a low temperature setting.