OpenAI's Realtime API was one of the first products to make building voice agents feel accessible. Stream audio in, get audio back—simple idea, big impact. But as developers move from prototype to production, a different set of requirements kicks in: speech accuracy on real-world entities, cost predictability, and a developer experience that doesn't fight you. AssemblyAI's Voice Agent API launched in April 2026 as a direct alternative—same simplicity, fundamentally different architecture. Here's an honest comparison of the two. Feature AssemblyAI Voice Agent API OpenAI Realtime API Pricing $4.50/hr flat ~$18/hr (per-token) ASR model Universal-3 Pro Streaming (#1 WER) GPT-4o multimodal Word accuracy 94.07% (6.3% mean WER) 93.13% Missed entity rate (emails, phones, names) 16.7% 23.3% End-to-end latency ~1 second (~150ms P50 STT) ~1 second Languages EN, ES, FR, DE, IT, PT 99+ (lower accuracy) Turn detection Speech-aware VAD (semantic + neural) Basic VAD Mid-session updates Prompt + voice + tools + turn…