Hi Brendan, Thanks a lot for the quick and substantive reply — this is more than I expected to learn. Good to have the x18 approach confirmed: that's exactly how I ended up solving it — my loader is linked with -Wl,-platform_version,macos,11.0,12.3 after tracing the behavior to machine_task_process_signature() in the published xnu sources. The new per-thread "x18 custom ABI" mode in macOS 26.5 with a public com.apple.security.* entitlement is great news — a durable public mechanism was exactly the piece I considered missing. I'll study <os/arch/arm64.h> and the xnu tests; looking forward to your test app and the Wine-side integration. Given that, I'll hold off pushing my current changes for now and keep running on my own tree — it already unblocks the parts I need for my project. If the situation changes — the Apple work becomes public or the x18 API lands in Wine — I'd appreciate a ping; on my side I'll be following your activity as well. Thanks again, Alexey