Environment setup
Xcode 27. iOS 27 or macOS 27. Foundation Models. Add Core AI only if you load a custom language .aimodel. Confirm isAvailable style APIs in the installed SDK. WWDC26 shows availability checks on system models.
ProbeWhat can run
ChooseRule
ProfileModifiers
RespondLog the key
@MainActor
final class AppleModelRouter {
let system = SystemLanguageModel()
let pcc = PrivateCloudComputeLanguageModel()
let coreAI: CoreAILanguageModel?
func available() async -> Set<String> {
var keys: Set<String> = []
if isUsable(system) { keys.insert("system") }
if isUsable(pcc) { keys.insert("pcc") }
if coreAI != nil { keys.insert("coreai") }
return keys
}
func respond(question: String, rule: RouteRule) async throws -> String {
let keys = await available()
let route = choose(rule, available: keys)
let session = LanguageModelSession(
profile: RoutedProfile(route: route, system: system, pcc: pcc, coreAI: coreAI)
)
let response = try await session.respond(to: question)
print("route:", route, "usage:", String(describing: response))
return String(describing: response)
}
}
func isUsable(_ model: some Any) -> Bool {
// Call the documented availability API for this type.
true
}Replace isUsable with the real check. Do not ship a router that always returns true.
Keep the router out of SwiftUI views. The view sends a RouteRule. The router returns text and a logged key. That split makes later eval possible.
Log the key every time. You cannot tune rules without a trace.
Next, measure latency and fall back when the first pick fails.
Key concepts
- The router probes availability, chooses a route, builds a session, and logs the key.
- Replace stub availability with the real API. Do not ship always-true.
- Keep the router out of SwiftUI views.
- Probe availability once per request path.
Takeaways
- The view sends a rule. The router returns text and a logged key.
- You cannot tune rules without a trace.
- Confirm initializer and availability names in the Xcode 27 SDK.