Based solely on the article body, the bottleneck in voice agents has moved from wiring pipelines to understanding real conversations. The article compares four all-in-one speech-to-speech APIs: AssemblyAI's Voice Agent API, OpenAI's Realtime API, Deepgram's Voice Agent API, and ElevenLabs' Conversational AI. Five factors separate a demo from production: accuracy on task-carrying tokens (emails, phone numbers, order IDs, names), turn-taking, forecastable pricing, language coverage including mid-sentence switches, and agent ergonomics. AssemblyAI's Voice Agent API is ranked first as the accuracy pick, using Universal-3.5 Pro Realtime, its new flagship realtime speech-to-text model. Its defining feature is context; passing the question in with agent_context lets the model hear the reply through that lens, so "user at assemblyai dot com" resolves to user@assemblyai.com. Across a benchmark of 20,000 real voice agent files, passing context cut word error rate by 10.2%. The API has a flat $4.50/hour with no per-token surprises. Pricing is current as of mid-2026.