Privacy Policy

Tandem — read and listen, in sync. Last updated: April 26, 2026.

The short version: Tandem helps you switch between an ebook and audiobook without losing your place. Your books stay on your device. Tandem works without an account. If you choose to sign in, we store only the minimum needed to sync your reading position, highlights, and notes across devices. We don't track your reading habits, don't sell your data, and don't use analytics.

Data We Collect

Tandem does not require an account. You can use the app fully without signing in, and no personal information is collected in that case. Specifically:

No account required. Sign-in is entirely optional. Without an account, no name, email, or identifying information is ever requested.
No analytics or tracking. Tandem contains no analytics SDKs, crash reporters, or advertising frameworks.
No cloud storage of your books. Your EPUB and audiobook files are stored locally on your device only.

Optional Account & Sync Data

If you choose to create an account (via Supabase Auth), Tandem stores the following to enable cross-device sync:

Data	Where stored	Why
Email address	Supabase Auth	Authentication only
Display name	Supabase database	Chosen by you, shown in social features
Reading progress (chapter & paragraph position per book)	Supabase database	Resume where you left off on any device
Highlights and notes	Supabase database	Sync across your devices

All account data is secured with row-level security — only you can read or write your own records. Books, audio files, and reading content are never uploaded; account data is limited to sync metadata (positions, highlights, notes).

You can delete your account and all associated data at any time from within the app.

Data Stored Locally on Your Device

Data	Where stored	Why
Imported EPUB files	App Documents container	Library persists across launches
Imported audio files	App Documents container	Paired audiobooks persist
Reading progress	App Documents container	Resume where you left off
Sync anchors (paragraph → audio time)	App Documents container	Makes switching between reading and listening accurate
Reader settings	UserDefaults / iCloud preferences	Preferences persist and sync across your Apple devices

This data never leaves your device unless you explicitly export or share a file yourself using the iOS/macOS share sheet, or you sign in and choose to sync reading progress, highlights, and notes (see "Optional Account & Sync Data" above).

Network Services

Audio transcription for reading-listening sync

To line up your ebook and audiobook when you switch modes, Tandem sends short audio clips (typically 10–30 seconds) from your audiobook to OpenAI's Whisper transcription service. The service returns a text transcript which Tandem uses to locate the matching part of your ebook.

Only audio from files you have imported is ever sent.
Only short clips are sent, not entire audiobooks.
Transcription results are cached on your device so repeated syncs of the same passage do not require new network calls.
OpenAI's processing of this data is governed by their privacy policy. Per their API data usage policy, audio submitted through the API is not used to train their models.

Account sync (when signed in)

If you are signed in, Tandem communicates with Supabase to sync reading progress, highlights, and notes. Only the metadata listed above is transmitted — never your books or audio files. All connections use HTTPS.

No other network activity

Beyond the transcription service and optional account sync described above, Tandem makes no other network requests. There is no telemetry, no analytics, no remote configuration, and no advertising.

Permissions

Speech Recognition (on-device, iOS only)

When available, Tandem can use Apple's on-device SFSpeechRecognizer as an alternative to the OpenAI service. This runs entirely on your device. No audio or transcription data is sent to Apple or any other service when on-device recognition is used.

Microphone

Tandem never records from your microphone. The microphone usage description in Info.plist is required by iOS/macOS for apps that link SFSpeechRecognizer, but no live microphone audio is ever captured.

Third-Party Open Source Packages

Package	Purpose	Privacy impact
ZIPFoundation	Parsing EPUB files (ZIP archives)	None — runs on-device
swift-kindle	Kindle format support	None — runs on-device

Children's Privacy

Tandem can be used without an account and collects no personal information in that mode. The optional account feature requires an email address. The app is appropriate for all ages; users under 13 should not create an account without parental consent.

Changes to This Policy

If this privacy policy changes, the "Last updated" date above will reflect the change. Significant changes will also be noted in the app's release notes.

Contact

Questions about this privacy policy: privacy@tandembooks.co