r/MachineLearning: iOS 27 Siri Uses WaveRNN and FastSpeech2 — Neural Architecture Discovered in iOS Simulator Espresso Model Files

A developer reverse-engineering iOS 27 Simulator files found that Apple's rebuilt Siri uses WaveRNN for neural vocoding and FastSpeech2 for text-to-speech synthesis, stored in Apple's Espresso on-device ML format. A compiled CoreML model for concert-style content ranking was also discovered alongside a simple logistic regression component. The finding reveals Apple chose established open-architecture TTS models rather than a novel proprietary speech synthesis stack for its Gemini-backed Siri rebuild, a choice that may constrain voice differentiation from competing AI assistants.