Okay, I finally fixed the issue. Installing Rosetta allows Xcode to use the iOS 18.5 simulator runtime. Here's the command: softwareupdate --install-rosetta I didn't need Rosetta installed before I downloaded the XCode 26 beta the first time, so I don't know why this changed.

Update: When I run xcrun simctl -v, I get more details about why the runtime cannot be opened: iOS 18.5 (18.5 - 22F77) - com.apple.CoreSimulator.SimRuntime.iOS-18-5 (unavailable, liblaunch_sim.dylib could not be opened) [/Library/Developer/CoreSimulator/Volumes/iOS_22F77/Library/Developer/CoreSimulator/Profiles/Runtimes/iOS 18.5.simruntime] - Error Domain=com.apple.CoreSimulator.SimError Code=401 "The iOS 18.5 simulator runtime is not available." UserInfo={NSLocalizedDescription=The iOS 18.5 simulator runtime is not available., NSUnderlyingError=0x6000024e34b0 {Error Domain=NSPOSIXErrorDomain Code=86 "Bad CPU type in executable" UserInfo={NSLocalizedFailureReason=A Rosetta update is required}}, NSLocalizedRecoverySuggestion=Try reinstalling Xcode or the simulator runtime., NSLocalizedFailureReason=liblaunch_sim.dylib could not be opened} The failure reason is interesting, I'm on a M4 Max, so I don't see why I could need Rosetta.

If only someone from Apple would actually read this forum post....

I see both of those volumes in my Disk Utility, it appears that they are mounted. Here's what I see under runtimes when I run xcrun simctl list: iOS 18.5 (18.5 - 22F77) - com.apple.CoreSimulator.SimRuntime.iOS-18-5 (unavailable, liblaunch_sim.dylib could not be opened)

When I run xcrun simctl list in terminal, iOS 18.5 is listed in the runtimes. So the runtime has been downloaded, but Xcode can't find it. I installed it through Xcode, so I don't understand why Xcode can't find it.