iOS 27 dev guide: hook apps into Siri via App Intents, call models locally via Foundation Models

iOS 27 gives developers two separate lanes. App Intents lets you register your app as a tool Siri can invoke—but only if your core features fit one of Apple's 12 predefined domain templates; otherwise you're stuck with fixed trigger phrases. Foundation Models lets your app call on-device, Private Cloud Compute, or third-party models for tasks like summarization and receipt scanning. The on-device model is free and unlimited but capped at a 4096-token context window; Apple recommends no more than 5 tools per request. The two systems don't mix: you can't swap Siri's reasoning engine, apps can't read each other's sandboxed data, and Apple keeps the global orchestration role. The real time sinks are debugging Siri's silent failures and the small on-device model occasionally hallucinating values—the post recommends Apple's evaluation framework for systematic regression testing.