We have a watchOS app in TestFlight that is currently crashing on any device running watchOS 11 and up. I have an older Apple Watch SE that's capped at 10.6.1 that can run the app just fine, but the same exact install on a newer device fails. This started happening after I updated my MacBook to macOS Sequoia and Xcode to 16. In order to get the Watch app to run on the 10.6.1 device, I had to change the Swift Optimization Level from -O to -Osize. I already filed Feedback on this (FB15691116). I've tried so many different things and have come up with nothing to show for it. When I look in the IPS file, I see that the crash point in my code on the watchOS 11 device is at KinesiaUOnWatch`@objc SessionDelegator.session(_:activationDidCompleteWith:error:): 0x3a3f48 <+0>: sub sp, sp, #0x60 0x3a3f4c <+4>: stp x26, x25, [sp, #0x10] 0x3a3f50 <+8>: stp x24, x23, [sp, #0x20] 0x3a3f54 <+12>: stp x22, x21, [sp, #0x30] 0x3a3f58 <+16>: stp x20, x19, [sp, #0x40] 0x3a3f5c <+20>: stp x29, x30, [sp, #0x50] 0x3a3f60 <+24>: add x29, sp, #0x50 0x3a3f64 <+28>: mov x19, x4 0x3a3f68 <+32>: mov x23, x2 0x3a3f6c <+36>: mov x22, x0 0x3a3f70 <+40>: mov w0, #0x0 ; =0 0x3a3f74 <+44>: bl 0x89818 ; symbol stub for: type metadata accessor for Swift.MainActor 0x3a3f78 <+48>: mov x24, x0 0x3a3f7c <+52>: adrp x25, 161 0x3a3f80 <+56>: ldr w25, [x25, #0x654] 0x3a3f84 <+60>: mov x20, x0 0x3a3f88 <+64>: bl 0x8980c ; symbol stub for: static Swift.MainActor.shared.getter : Swift.MainActor 0x3a3f8c <+68>: mov x21, x0 0x3a3f90 <+72>: adrp x0, 166 0x3a3f94 <+76>: add x0, x0, #0xc40 ; lazy protocol witness table cache variable for type Swift.MainActor and conformance Swift.MainActor : Swift.Actor in Swift Incident Identifier: 08E2F667-7965-4C86-B85E-9E2F03EAA963 Beta Identifier: AD390666-55CB-43B3-9B14-BD3E70F456EB Hardware Model: Watch6,1 Process: KinesiaUOnWatch [389] Path: /private/var/containers/Bundle/Application/B571E983-F2B0-40EF-9F0D-8C471CAEB3FB/KinesiaUOnWatch.app/KinesiaUOnWatch Identifier: com.glneurotech.kinesiau.watchapp Version: 5.1.0 (510271) AppStoreTools: 16B39 AppVariant: 1:Watch6,1:11 Beta: YES Code Type: ARM64_32 (Native) Role: Foreground Parent Process: launchd [1] Coalition: com.glneurotech.kinesiau.watchapp [464] Date/Time: 2024-12-03 15:12:51.1876 -0500 Launch Time: 2024-12-03 15:12:50.0000 -0500 OS Version: Watch OS 11.1 (22R585) Release Type: User Report Version: 104 Exception Type: EXC_BREAKPOINT (SIGTRAP) Exception Codes: 0x0000000000000001, 0x000000002053dd4c Termination Reason: SIGNAL 5 Trace/BPT trap: 5 Terminating Process: exc handler [389] But, as I said, this exact app runs just fine on an older watchOS 10.6.1 device. Has anyone else encountered something like this, or have any ideas on stuff to try?