An app built with Xcode 27.1 for a macOS 15 deployment target called Logger.isEnabled(type:). In the SDK's swiftinterface the method sits under Logger's @available(macOS 11.0, ...) with no availability of its own, so it compiled without a warning. On macOS 15.6.1, libswiftos.dylib doesn't export it, and dyld stopped the app at launch: Symbol not found: _$s2os6LoggerV9isEnabled4typeSbSo0a5_log_E2_ta_tF Expected in: /usr/lib/swift/libswiftos.dylibThe macOS 15.5 SDK's libswiftos.tbd doesn't list the symbol and the 27 SDK does. I haven't checked macOS 26. Switching to OSLog.isEnabled(type:) fixed it. Which macOS version first exports this symbol? Write-up: https://yardlet.app/blog/logger-isenabled-macos-15.html