AssemblyAI argues Universal-3.6 Pro Realtime should remain the default for voice agents, with Sync HTTP used for second passes and recovery.
AssemblyAI says most voice agents should keep using streaming speech-to-text, while its Sync HTTP API should be reserved for narrower jobs such as second-pass verification, recovery after a dropped WebSocket, or custom low-volume systems that already handle turn detection. The company said in a blog post published on 2026-10-30 that Universal-3.6 Pro Realtime remains the default starting point because it returns partial transcripts during a caller's turn and manages turn-ending logic that Sync leaves to the client.
According to AssemblyAI, a streaming session can return partial transcripts while a caller is still speaking, detect when a turn is complete, and hold the turn open around entities such as phone numbers and order IDs so they are not split. The company said the stream also provides a SpeechStarted signal that can be used to stop text-to-speech output when a caller begins talking. By contrast, Sync uses a single HTTP request per finished clip and returns one transcript after the clip ends, with no socket, partials, or built-in turn detection.
The post frames that difference as an implementation choice rather than a universal performance verdict. For frameworks such as LiveKit or Pipecat, AssemblyAI said the streaming path matches common voice-agent architectures, while using Sync as the primary STT would require custom orchestration around voice-activity detection and endpointing.
AssemblyAI described three cases where Sync can still be useful. First, it can act as a second pass on low-confidence turns or high-stakes values such as card numbers, account IDs, or spelled-out names.
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