Kaku quietly catches grammar, particle, and keigo mistakes in your desktop apps — and explains them in plain English so you actually learn.
Available for macOS and Windows. No account required to try.
こんにちは、田中さん。
来週の打ち合わせの件で連絡しました。
確認お願い
Not a browser extension. A desktop assistant that surfaces in any input field — chat, mail, docs, notes, browsers.
And more — anywhere your operating system exposes a standard text input.
Pick the register that fits the moment. Kaku rewrites your selected text in casual, polite, or business-formal Japanese — never replacing the original until you confirm.
Sending a quick chat
明日の会議何時？
明日の会議は何時ですか？
明日のお打ち合わせは何時のご予定でしょうか？
Replying to a manager's email
資料 添付した
資料を添付しました
資料を添付いたしました
Highlighted spans show the changes Kaku made.
Watch a correction play, then send your own Japanese to Kaku AI for a real tone conversion. One free try — no sign-up.
Kaku flags grammar, particle, and keigo issues at the cursor. Press play to see a correction tooltip appear on a real example.
来週の打ち合わせの件で連絡しました。
The corrected sentence above is pre-filled below. Edit it, pick a register, then send it to Kaku AI.
✦ Only this text is sent to Kaku AI for the demo. We don’t store it.
Set up once and Kaku runs in the background. The next time you finish a Japanese sentence, it's there.
Grant accessibility permission once. Kaku only attaches to input fields, never to your screen contents or the apps in the background.
Yuki
明日の予定どうする？
明日の打ち合わせ
The local rule-based engine flags grammar, particle, vocabulary, and keigo issues. Nothing is sent to the cloud at this stage.
山田部長 · 14:02
返信お待ちしております
来週の打ち合わせの件で連絡しました。
Apply or dismiss every suggestion explicitly. Kaku never auto-replaces — and never breaks your IME or candidate window.
山田部長 · 14:02
返信お待ちしております
来週の打ち合わせの件でご連絡いたしました。
Kaku runs in the background of any desktop app and only surfaces when a completed sentence needs attention.
Kaku highlights and underlines grammar, particle, and vocabulary issues at the cursor. Apply from the tooltip — Kaku never auto-replaces your text.
To 田中部長 · tanaka@example.co.jp
Subject 来週の打ち合わせの件
田中部長
来週の打ち合わせの件で連絡しました。
ご連絡いたしました
連絡しました
Use 謙譲語 with a senior colleague
Hide explanation
Every non-trivial correction comes with a short English explanation, tuned to your selected level: beginner, intermediate, or advanced.
Select any sentence and rewrite it in 普通体, 丁寧語, or ビジネス丁寧語. See a side-by-side diff before you apply.
Kaku's hardest rule: nothing leaves your device unless you ask for it. Here's what that means in practice.
Where almost everything happens.
Only the selected passage you choose to rewrite.
Read the full Privacy Policy below for the precise data flow.
Start free with 5 AI rewrite credits every day. Upgrade to Pro for an extra 2,000-credit monthly pool.
Free
$0
5 AI rewrite credits / day
Pro
$9 / mo
5 free credits / day + 2,000 / month pool
Short, honest answers to the questions we hear most.
Yes. Kaku's correction engine runs entirely on your device — your text is not transmitted while you write. The Tone Converter is the only feature that sends text to the cloud, and only the passage you explicitly select. Metadata is retained for 30 days for service operation; the source text is not stored unless you submit a 👍/👎 feedback rating.
Any desktop application that exposes a standard text input field through your operating system — chat (Slack, Discord, LINE, Teams, Messages), email (Outlook, Apple Mail, Gmail in the browser), documents (Word, Google Docs, Pages), and notes (Notion, Apple Notes, Obsidian). It doesn't replace your browser-based writing tools — it sits alongside them, in the native apps you already use.
Yes. Kaku is designed not to fight your IME. It only triggers after you commit a sentence — your candidate window, per-key suggestions, and conversion behaviour are untouched. It works with macOS Japanese input, Google Japanese Input, and ATOK on Windows.
Japanese writing only, for now. Explanations are in English (and tuned to your selected level). Adding a Japanese-language explanation option for native speakers writing keigo is on the roadmap.
Two things. First, Kaku is a native desktop assistant — it works in every app, not just your browser. Second, every correction carries a plain-English explanation so you actually learn the rule, not just accept a fix. Tone conversion across three Japanese registers (普通体・丁寧語・ビジネス丁寧語) is also unique to Kaku.
No. The on-device correction engine is lightweight (median latency under 200 ms). Cloud calls only happen when you explicitly open the Tone Converter, so background CPU and network usage are negligible.
Yes. Cancel from Settings → Subscription. Cancellation stops future renewals; your current billing period continues until its end, then your account reverts to the Free plan.
Your account, settings, sessions, and any feedback ratings you've submitted are deleted within 30 days. Subscription audit records may be retained as required by accounting laws — see the Privacy Policy for details.
Didn't find what you're looking for? Email us at hello@kaku.example.
Kaku is a lightweight desktop app — not a browser extension. Sign in with Google after install to enable cloud features.
Installers are not yet available. Want to be notified when builds ship? Drop us a line.
