CinderPrivacy Policy
Last updated October 16, 2026
The short version. Almost everything you log in Cinder stays on your phone. If you sign in to add friends or chat with Ash-ly, our server keeps only what those features need. If you turn on sync, a private copy of your log is kept so your other devices can use it. We don't show ads, we don't sell data, and Apple Health data never leaves your phone.
What stays on your device
Your fasts, goals, weights, water, food log, activity, workouts and training settings, programs, lifts and runs, sleep check-ins, keto settings and ketone readings, body details for estimates (sex, birth year, height), how-you-feel check-ins, notes and photos on your board, meal plans and settings are stored only in your browser or the Cinder app on your phone. We never receive them, unless you turn on Sync across devices (below). Clearing your browser data, deleting the app or using Erase this device removes them.
What reaches our server (only if you sign in)
Signing in is optional. It's needed for Friends, Ash-ly (the AI helper), the meal chef and photo estimates. Our server runs on Supabase. If you sign in we store:
- Your email address, to send you sign-in codes.
- The first name you choose and your 5-character invite code.
- What you choose to share with friends: when your current fast started and its goal, your last fast's length and end time, and how many fasts you did this week. You can switch sharing off at any time. We never share weight, water, food, notes or photos.
- Who you've added as friends, and encouragement notes you send (the text and who it's for; we keep your newest 60).
- The invite code you redeemed, if any, with the time.
- Daily counts of how many questions you asked Ash-ly and photo estimates you used, so we can keep the free allowance fair. Not the questions or photos themselves.
- Only if you turn on Sync across devices (Health → History & settings): a private copy of your fasts, goals, weights, water, food log, manually entered activity, workouts, lifts, runs, sleep check-ins, ketone readings, check-ins, meal plans and settings, so your other signed-in devices can use them. Only you can read it — not friends, not other users. Apple Health data and board photos are never included. Turning sync off stops further copies; deleting your account deletes it.
Only people you've added as friends can see what you share. The database rules enforce this on the server.
The shared food list
When you scan a barcode, pick a food found online, or add a food by hand, Cinder adds that food to a shared list so the next person can find it. What is shared: the food's name, brand, serving, barcode and nutrition numbers, plus a random number made on your phone so one phone counts once. What is not: your name, email, account, or when or how much you ate. You don't need an account for this. Anyone using Cinder can find these foods. Foods from Cinder's built-in lists and photo estimates are never sent. You can turn this off in Health → History & settings → Shared food list.
Help Ash-ly learn (optional)
This is off unless you turn it on in Health → History & settings → Help Ash-ly learn. When it's on and Ash-ly can't answer a question from its notebook, Cinder sends that question so we can teach Ash-ly the answer. What is sent: the question as you typed it (up to 160 characters), the app version, and the same random number made on your phone that the shared food list uses, so one phone can't flood the list. What is not: your name, email, account, or anything from your log. Questions with a link, an email address or a long number in them are never sent, but please keep personal details out of what you ask. Only we can read these questions, and they're deleted after 90 days.
Services that process data for us
| Service | What for | What it receives |
|---|---|---|
| Supabase | Accounts, friends, usage counts, the shared food list, and questions you choose to share with Ash-ly | The server data listed above |
| Resend | Sending sign-in codes | Your email address and the code |
| Anthropic | Answering questions you ask Ash-ly, the meal chef, and estimating food from photos | Your question and the recent conversation, a short summary of your day so Ash-ly can answer with real numbers (fasting hours, water, calories and protein so far, your units), your chef request, or the photo. We don't store these. Under Anthropic's commercial terms, this data isn't used to train its models. |
| Wikipedia (Wikimedia Foundation) | Looking up a general topic Ash-ly doesn't have in its notebook, when the live AI isn't available | Only the words of your question, sent from your device. No account details, log data or identifiers. Wikimedia's own privacy policy applies to the request. |
| Open Food Facts | Barcode lookups and online food search | The barcode or search words |
| USDA FoodData Central (api.data.gov) | Finding more foods and brands when Cinder's own lists come up short, and barcodes nobody else has | The search words or barcode, sent from your device |
| Cloudflare | Hosting the web app, and Web Analytics on the website | Standard web request information such as IP address. Web Analytics counts page views and how fast pages load; it uses no cookies and doesn't follow you to other sites. |
| jsDelivr | Delivering open-source code the app loads | Standard web request information such as IP address |
Asking first. Nothing goes to Anthropic until you say so: the first time a live answer, the meal chef or a photo estimate would send something, Cinder shows exactly what would be sent and asks. You can turn this off again any time in History & settings → Live AI answers.
Videos
The Board tab has a few optional videos hosted on YouTube. Nothing is loaded from YouTube until you tap play. When you do, the video plays in YouTube’s privacy-enhanced mode (youtube-nocookie.com), and YouTube (Google) receives standard request information such as your IP address and may store data on your device as described in Google’s privacy policy. “Open on YouTube” takes you to YouTube itself. Cinder doesn’t send YouTube anything about you or your fasting.
Apple Health and Health Connect
In the iPhone and Android apps, Cinder can read steps, workouts and active energy so it can estimate what you burn. That data is stored only on your phone. It is never sent to our server, never shared with friends, never used for advertising or marketing, and never sold.
Ash-ly on your iPhone (Apple Intelligence)
On iPhones with Apple Intelligence turned on, the Cinder app can write some of Ash-ly’s answers with Apple’s on-device model. Your question, the closest notes from Ash-ly’s notebook and a short summary of your day are processed entirely on your iPhone. They are not sent to us, to Apple or to anyone else. You can turn this off in Settings, under Ash-ly on this iPhone.
No ads, no tracking, no selling
Cinder has no advertising and no tracking across other apps or websites. The website (cinderfast.com, not the iPhone app) uses Cloudflare Web Analytics to count visits and page speed: it sets no cookies, keeps no profile of you, and never sees your fasting, food or health data. (The optional YouTube videos are the one place a third party serves content, and only when you tap play.) We don't sell or rent personal data.
Your choices and rights
- Export: Health → History & settings → Back up (everything, as a file) or Export CSV (fasts, weights and food log).
- Delete your account: Health → History & settings → Delete account, or Friends → Delete account. This permanently removes your server data.
- Erase this device: removes everything Cinder stores on your phone.
- You can also email support@cinderfast.com to access, correct or delete your data, or with any privacy question. Depending on where you live, you may have further rights (for example under the GDPR or California law), and we'll honor them.
Retention
Server data, including any synced copy of your log, is kept until you delete your account. Encouragement notes are trimmed to your newest 60. Daily usage counts are kept with your account and deleted with it. Questions shared through Help Ash-ly learn are deleted after 90 days. Foods on the shared food list stay on the list, since they aren't linked to anyone.
Children
Cinder is for adults 18 and over. We don't knowingly collect information from anyone under 18. If you believe a child has signed in, email us and we'll delete the account.
Security
Data travels over encrypted connections. Server access is limited by row-level security rules so each person can read only their own data and what friends chose to share with them.
Changes
If we change this policy in a way that matters, we'll update the date above and tell you in the app.