Apple Watch Ultra 4 and Series 12 launch tomorrow, preceded by a code leak from X user pdfu detailing four key watchOS 27 changes. The Readiness app appears in the default Home Screen layout after the Workout app, positioned as a competitor to the Oura Ring and Whoop, with Mark Gurman confirming it will be a variant of the Vitals app. Strength training features are expanding inside the native Workout app, now capable of describing individual sets, storing repetition counts, weight, equipment, body side, and duration. A new optical-sensing system hints at significant internal sensor upgrades for the Apple Watch Ultra 4 and potentially Series 12. Additionally, the new S-class chip will offer 128GB of storage, a significant increase for latest models. While some features tie to new hardware, the Readiness app may be available for older watchOS 27 devices. Apple is set to announce these updates alongside the new hardware tomorrow.

Key entities mentioned: Apple, watchOS 27, Apple Watch Ultra 4, Series 12, Oura Ring, Whoop, Mark Gurman, pdfu

Secondary keywords: Apple Watch features, watchOS 27 leak, Apple Watch Ultra 4 specs, Readiness app functionality