KaiReader

Privacy Policy

Last updated: August 17, 2026

KaiReader is developed by Kaifuz Studio. It runs on your device and keeps what you read, save and say there. Sign-in is optional and we keep no profile of you; a few opt-in features do reach the network — an optional account if you sign in, sentence translation and the Kai writing partner, and the Penpal letter exchange (all below). No analytics SDK, no advertising. This page says exactly which features reach the network, what they send, and which of them require a Pro purchase.

What stays on your device

Your saved vocabulary and its review schedule, highlights, notes and imported pages, reading position, study-language settings, feed list and mute list, and the offline dictionaries are all stored only on your device. They are never uploaded to us. Uninstalling the app, or clearing its data, removes them.

The app also keeps a plain record of your own study history — a line each time you save a word, grade a review card, mark a passage or drop one, with the time it happened. It is what lets the app show you what you did rather than only what you have. It is written to a file on your device, capped so it cannot grow without limit, and it goes nowhere else.

Where articles come from

KaiReader fetches the RSS and news sources you add, directly from your device to those publishers. Nothing about which articles you open is reported to us — we do not operate a server that sees your reading. Those publishers can see the request the way any RSS reader or browser would: your IP address and the fact that a feed or page was fetched.

Sentence translation (文) — a Pro feature, and the only one that sends article text to us

Per-sentence translation and the vocabulary glossary are KaiReader Pro only. Without a Pro purchase the app never contacts this service at all — the free tier reads, looks words up and reviews vocabulary entirely on your device. There is no free trial of it that quietly sends your reading somewhere.

When a Pro reader turns 文 on, the paragraphs of that article and its language code are sent to a Kaifuz Studio service running on Cloudflare Workers, which forwards them to a third-party large-language-model provider (currently OpenAI) to produce the translation and glossary. The result is returned to your device and cached there.

What is sent is the article text you are reading — public published content, not anything private you wrote — together with an entitlement token proving a Pro purchase. Your name, email and saved vocabulary are not sent. We do not store the article text or the responses on our side; they are relayed and discarded. The provider processes it under its own terms and, on the API plans we use, does not use it to train models. If you never enable 文, this request never happens.

Word lookup — a separate path, and not Pro-only

Tapping a word uses the dictionaries bundled with the app and works fully offline. If the word is not in them, KaiReader asks Google Translate's public endpoint for a meaning — and this happens on the free tier too, so it is worth knowing about. It sends only that one word, straight from your device to Google, with no account, token or identifier attached; it does not go through our service, and we never see it. Google handles it under its own privacy policy.

On iOS you may also hand a word to Apple's built-in Translate and Look Up panels. Those are part of iOS and governed by Apple's privacy policy.

Kai — the AI writing partner

In the Penpal tab you can write short letters to Kai, an AI partner that replies in the language you are learning. Where your device has an on-device model (Apple Intelligence), or you have entered your own AI provider key, your messages are answered there and nothing is sent to us. Otherwise, if you are signed in, the message you write to Kai and your recent turns in that conversation are sent to our Cloudflare Worker, which forwards them to a third-party large-language-model provider (Cloudflare Workers AI or OpenAI) to produce the reply. We do not store the conversation on our side; it is relayed and discarded, and the provider processes it under its own terms and does not use it to train models. Your name, email and saved vocabulary are not sent with it.

Microphone and pronunciation practice

Pronunciation scoring asks for microphone and speech-recognition permission, and only records while you hold the practice control. KaiReader requests on-device recognition wherever the system supports it for your language, so the audio normally never leaves the device. Where the platform cannot recognise that language on-device, the system's own speech service handles it — on iOS that is Apple, on Android the recogniser installed on your phone. KaiReader does not upload or share your recordings, and scoring keeps no audio: it is discarded as soon as a transcript is produced, and only the score is shown.

Recording a take to play back against the model voice is a separate control, and that one does keep the audio — it has to, in order to play it back. One take at a time, written to the app's temporary storage on your device, replaced by the next recording and cleared when you close the word or the system reclaims the space. It is never uploaded, and nothing is sent anywhere to produce it.

Purchases

KaiReader Pro is sold through the App Store and Google Play. Payment is handled entirely by Apple or Google — we never see your card or billing details. To unlock the Pro server feature above, the app sends the store-signed purchase receipt to our Cloudflare Worker, which verifies the signature and returns a short-lived token. That token identifies the purchase, not you: for App Store purchases it is derived from Apple's anonymous original transaction id, which is not linked to your name or Apple Account.

Optional sign-in

KaiReader can optionally sign in with Google or Apple, so usage of the server features can be attributed to an account rather than to a shared app-wide key. Signing in is never required, and it does not unlock paid features — only a purchase does. If you sign in, we receive the account id and email address from the sign-in token in order to issue a session (with Sign in with Apple this may be Apple's private-relay email, if you choose to hide yours). We do not read your contacts, Drive, or any other data from Google or Apple, and signing out discards the session.

Abuse prevention

Our Cloudflare Worker counts requests per purchase and per IP address so a leaked token cannot run up an unbounded bill. These counters hold a request count against a purchase identifier or an IP address, expire within roughly two days, and are not used to build any profile.

Penpal — optional, and the one place your words leave the device by choice

Penpal is an opt-in language-exchange feature, off by default (the human letter exchange is currently disabled in the app; the Penpal tab hosts the Kai AI partner described above). If you never turn it on, nothing in this section applies and nothing about you is stored on our side. Turning it on requires signing in (Google or Apple, as above) and confirming you are at least 16.

When you use Penpal we store, on a Kaifuz Studio database running on Cloudflare (D1): the pseudonymous handle you pick, the languages you speak and are learning, your short bio, and the letters you send and receive. Other users see your handle, languages and bio — never your real name, your email, or your location. Your reading history and saved vocabulary are never part of Penpal.

To keep the space safe, every letter is checked by an automated content-safety system (a third-party large-language-model provider) before it is delivered; unsafe messages are held, not sent. You can block and report other users at any time. You can delete your Penpal profile whenever you like — that removes your profile and anonymises your past messages. Payment and Pro have nothing to do with Penpal; it is free and separate.

What KaiReader never does

No advertising, no third-party analytics or attribution SDKs, no cross-app or cross-site tracking, and no sale or sharing of personal information. We do not collect your contacts, photos, location or device identifiers.

Children's privacy

KaiReader is not directed to children under 13, and we do not knowingly collect personal information from children.

Your control

Leaving 文 off means the app never contacts our service. Staying inside the bundled dictionaries — they cover the common vocabulary — means no word is sent to Google either. You can decline the microphone permission and still use every other feature, sign out at any time, and export or delete your saved vocabulary from Settings. Removing the app removes everything it stored.

Changes to this policy

We update this page when the app's behaviour changes. The date above says when it was last revised.

Contact

For privacy questions, email privacy@kaifuz.com.