The "NanoRegistry pairing compatibility index" is a MobileAsset
apparently used to specify compatibility between
iPhone and Apple Watch hardware and software versions.
It was added on iOS 18.2 beta 3.
The asset type identifier is com.apple.MobileAsset.NanoRegistryPairingCompatibilityIndex
.
As of 11 December 2024
it still doesn't exist on either Mesu or Pallas, so we haven't seen what an asset actually looks like yet, but it will presumably appear on Pallas in the future.Software
The asset is downloaded and parsed by nanoregistryd on iOS. It's not used by watchOS, only by the iOS side.
Asset contents
We have yet to see any actual asset, and a lot of the logic in nanoregistryd is incomplete, so this mostly speculation.
Based on code analysis by lunotech, the AssetData will contain a file called NanoRegistryPairingCompatibilityIndex.plist, with the following structure:
{
"iPhone": {
"WatchX,Y": <min>
...
},
"Watch": {
"iPhoneX,Y": <max>
...
}
}
Where <min> and <max> are integers with CompatibilityVersions. The iPhone only pairs with an Apple Watch with product type WatchX,Y if the watch's compatibility version is <min> or higher. The Apple Watch only pairs with an iPhone with product type iPhoneX,Y if the iPhone's compatibility version is <max> or lower.
This data is only consulted if the watch's CompatibilityVersion is already higher than what iOS has hardcoded as the maximum supported version.