Local Proofreader

English | 日本語

A privacy-first Japanese proofreading assistant for Chrome. It checks the Japanese you type in web input fields — entirely on your device using Chrome's built-in Gemini Nano. Nothing is ever sent to the cloud.

What it does

As you type, it underlines likely mistakes — typos, grammar, punctuation, and awkward phrasing — shows a suggestion and the reason when you hover, and lets you fix just that spot with one click. It never rewrites your text on its own.

Features

• 🔒 100% on-device — your text never leaves your computer (no cloud, no sign-in)
• 〰️ Wavy underlines on likely mistakes, in real time as you type
• 💬 Hover or click an underline → suggestion + the reason → Apply fixes only that span
• ✋ Never auto-corrects — you decide what to change
• 🧩 Works in plain text boxes (textarea / input) and rich editors like Slack and Gmail
• ⚙️ Choose what to detect (typo / grammar / punctuation / awkward) from the popup. "Awkward phrasing" is off by default to avoid over-correction.

Requirements

• Chrome 149+ (Japanese works out of the box), or Chrome 148 with a flag (see Setup)
• Desktop only: macOS 13+, Windows 10/11, or Linux (no Android/iOS)
• About 22 GB free disk for the model, and a GPU with >4 GB VRAM or 16 GB RAM + 4 CPU cores
• The Gemini Nano model downloads once (a few GB) on first use

Setup

1. (Chrome 148 only) Enable Japanese
   • chrome://flags/#optimization-guide-on-device-model → Enabled
   • chrome://flags/#prompt-api-for-gemini-nano → Enabled multilingual
   • Restart Chrome
2. Build and load the extension

   pnpm install
   pnpm build

   Then open chrome://extensions, turn on Developer mode, click Load unpacked, and select the .output/chrome-mv3 folder.
3. Confirm it's ready — open the extension popup. When it shows "✓ 校正が利用可能" you're set. If the model isn't downloaded yet, click the download button (one-time, a few GB).

How to use

1. Click into any text box (a comment field, a Slack/Gmail message, …) and type Japanese.
2. A red wavy underline appears under likely mistakes.
3. Hover or click the underline → a tooltip shows the suggestion and why.
4. Click 適用 (Apply) to replace just that span. In some editors one-click apply isn't available yet — the tooltip says so, and you can fix it manually.
5. Open the popup to turn detection types on/off.

Privacy

Everything runs locally through Chrome's built-in Gemini Nano. Your text is never sent to any server and no account is required. The only network activity is the one-time model download, handled by Chrome itself.

Supported & not supported

• Supported: plain textarea / input[type=text], and many contenteditable editors (Slack, Gmail, …).
• Apply may be unavailable in some rich editors — detection and underlines still work; fix manually.
• Not handled: password / search / email fields (skipped on purpose), code editors, and Google Docs (canvas-rendered).

For developers

See CLAUDE.md for setup, commands, and architecture notes.

License

MIT © 2026 kokoichi206