The AI Chat connects your Apple Health data to the AI model of your choice. Every number in an answer was computed on your iPhone before the model ever saw it — the AI reads the results, it never does the arithmetic.
Apple Health has been quietly building a longitudinal dataset about your body. Every night of sleep, every workout, every resting measurement adds a data point. The Health app can surface that data as a chart, but its charts cap at recent windows and keep each metric in its own silo.
The questions worth asking need the full timeline and multiple metrics at once: Did your resting heart rate trend down the year you started running, then creep back up? Does your HRV drop predictably every winter? Are your highest-effort training weeks actually followed by lower sleep quality?
Tau. bridges your data to the AI; it doesn't host a model of its own. You bring the key, so the cost and privacy terms are the provider's, not Tau's — and nothing you send ever passes through a Tau server, because there isn't one.
A normal AI chat only knows what you paste into it. Tau. opens every session with a map of your record: every metric you actually have, its unit, the first and last day it was recorded, the devices behind it, its all-time high and low, and the last seven days in full. From there the model asks Tau. for whatever a question needs — any other day, any window, any statistic, any correlation — and your iPhone computes it and hands back the result.
The same question, answered for two different readers.
Your API key lives in the iOS Keychain. The app has no server, no account and no analytics.
Reproductive health is a setting, and it is off until you turn it on. While it is off, nothing in that category reaches any model, even a paid one — periods, ovulation and pregnancy tests, contraception, cervical mucus, sexual activity, lactation. The metrics are kept out of the context entirely, and every tool refuses to return them: the context says plainly that the data is withheld by a setting rather than missing, so the model cannot mistake your silence for an empty record. Turn the setting on and the category becomes readable like any other, with a dedicated menstrual block (bleeding dates and cycle lengths) added so cycle questions can be answered. You can turn it back off at any time.
These aren't aspirational examples. Each one maps to data Tau. actually holds and statistics it actually computes on demand — your full daily history, the tested trends, the weekly and yearly cycles, the rank correlations between any two metrics.