Privacy Policy

Last updated: November 9, 2026

Fast Writing (the "App") is an English-learning app operated by Thunderstruck Works ("we"). This policy explains what information the App collects, uses, and shares. The Japanese version prevails if the two versions differ.

1. Accounts

The App does not require registration. It generates an anonymous identifier for each device (device UUID) to manage learning progress. You do not need to provide your name or email address. An optional nickname, your answers, and your conversations may contain personally identifiable information depending on what you enter.

2. Information we collect and process

• Learning data (written answers): For AI grading, the English you enter and the question text are sent to our servers and to our AI grading provider (Google Gemini API).
• Voice input: Voice input is converted to text by your device's operating-system speech recognition (provided by Apple or Google). The App does not send recordings to our servers. The resulting English text is sent for grading as your answer.
• AI chat conversations: When you use the Premium AI tutor, your messages and the responses are processed and stored.
• AI character conversations: Conversation logs, each character's memory, and affinity are stored on your device. To generate responses, recent conversation (up to 16 messages), memory, and English level are sent through our server to the Google Gemini API. Generated responses are stored on our server for 30 days so they can be recovered if the connection drops. For Premium character voices, audio files generated from the English response are stored on our server.
• AI response reports: If you report a response, we store the response, your optional reason, the device identifier, and the type and creation time of the item on our server, and we review it. Full conversations and conversation memory are not included. Slack receives only the report ID, type, and time received, not the report text, reason, or device identifier.
• Usage data: Grading counts, study time, streaks, ranking points, and similar data linked to the device identifier. To prevent abuse, we also process IP addresses for grading and usage counts per IP address.
• Public ranking name: If you set an optional nickname, it is processed and shown publicly as your ranking name.
• Push notification token: If you allow notifications, the token is stored to deliver reminders.
• Purchase information: When you subscribe, receipt information is processed through the App Store or Google Play and RevenueCat and stored to manage and audit billing status.
• Local data on your device: Progress and settings are stored mainly on your device (AsyncStorage).

3. Sharing with third parties and processors

• Google Gemini API (AI grading, chat, character conversations, voice generation): Answers, conversations, and similar text are sent for processing. Before you first use an AI feature, the App explains what will be sent and asks for your permission. You can withdraw permission under "Sending information to AI" in Settings.
• RevenueCat / App Store / Google Play (billing).
• Expo Push (push notification delivery).
• Langfuse (quality monitoring): In production, the text of answers and chats is masked, and only metrics such as token usage and processing time are sent. Character conversation text and memory are not sent.
• Sentry (error investigation, when enabled): Error information, the location in the program where it occurred, request IDs, and similar data are processed.

All of this processing is to provide the App. We do not sell data for advertising or track you. We require these recipients to provide protection equal to or greater than this policy. These providers may operate servers outside Japan, so information may be processed abroad.

4. Retention and deletion

• Running "Delete learning data" in Settings deletes chat history, character conversation responses and audio, public ranking nicknames and scores, and push tokens and delivery data on the server, and invalidates access using the old device secret.
• If Langfuse is used, deletion of traces linked to the device is recorded and processed until none remain with the external service. This may finish after deletion on the device and our servers because of in-flight transmissions and external outages.
• By default, AI chat text is kept for 90 days, and character conversation responses and audio for 30 days. IP and device usage counts for abuse prevention are deleted after about 30 days, and daily points, scores, and rankings after about 90 days. Usage counts needed to avoid re-issuing the day's quota are kept after deletion.
• AI response reports are kept for the same period as the original response and erased when you delete learning data. Retention of the content-free Slack notification follows our Slack settings.
• Billing receipts and the identifiers and entitlement status needed to keep purchases and refunds consistent are retained after deletion for auditing, refunds, and fraud prevention. The ledger used to detect duplicate billing events is pruned after 365 days.
• Past data in backups may remain during the backup retention period. When restoring a backup, we check that deleted data is not republished.
• Deletion erases progress, settings, and saved phrases on the device and switches to a new identifier and secret. Information in secure storage managed by the operating system may not be erased by uninstalling alone.

5. Your choices and rights

• You can withdraw permission to send data to AI at any time under "Sending information to AI" in Settings. Learning features other than AI remain available.
• You can stop notifications in your device settings or the App's notification settings.
• To request access, correction, or deletion of your data, contact us below. See Data Deletion for the steps.

6. Security

Communication is encrypted with HTTPS. A secret token for each device prevents others from manipulating your data.

7. Children's privacy

The App is for a general audience and does not knowingly collect personal information from children under 13.

8. Changes to this policy

We may revise this policy as needed. We will announce important changes in the App.

9. Operator and contact

Operator: Thunderstruck Works

Email: thunderstruckworks@gmail.com