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Silence can mean “I’m finished,” but it can also mean “I’m thinking.” A voice agent using silence-based endpointing treats a pause as a possible end of your turn: it detects speech, then waits for a configured interval without detected speech before responding. OpenAI’s Realtime API documents 500 milliseconds as the default silence interval for its server_vad mode—not a universal rule for voice agents.
How a silence timer decides your turn is over
Voice activity detection (VAD) identifies when audio contains speech. In server VAD, the system uses detected speech and the silence that follows to decide when a speaker has stopped. The silence_duration_ms setting controls how long that quiet interval must last before the turn is treated as complete. OpenAI documents a default of 500 ms for this setting in its Realtime API. OpenAI’s Realtime API VAD documentation
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The timer is a practical shortcut, not proof that a thought is complete. A pause of half a second might be the end of a request, or just a breath before the next phrase. The agent responds based on its configured endpointing behavior, not on certainty about the speaker’s intention.
Why 500 ms is adjustable
A shorter silence interval can make an agent respond sooner, but it also gives a brief pause less time to resolve. OpenAI’s Realtime API documentation says, “With shorter values turns will be detected more quickly,” and warns that shorter values may cause the system to respond during a short pause. A longer interval gives a speaker more room to continue, at the cost of waiting longer after they really have finished. OpenAI’s Realtime API VAD documentation
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That is a responsiveness-versus-interruption tradeoff, not a contest with one best number. The right setting depends on the application’s conversational pace and how disruptive a premature response would be. The cited documentation describes configuration and behavior; it does not establish a universally optimal interval or a controlled performance ranking.
Silence-based and semantic VAD compared
| Approach | What drives the decision | Timing behavior | Effect of a short pause |
|---|---|---|---|
| Server VAD | Detected speech followed by a configured interval without detected speech | Responds after the configured silence interval is reached | A pause long enough to meet the interval can trigger an end-of-turn decision |
| Semantic VAD | A model estimates whether the utterance sounds complete | Response timing varies with the estimated completion and eagerness setting | May wait longer if the utterance seems unfinished |
OpenAI’s Realtime API offers both approaches, and Microsoft Foundry documentation also describes server and semantic VAD as voice-agent configuration choices. Semantic detection changes the signal used to estimate turn completion; the documentation does not show that it is always more accurate or preferable. OpenAI Realtime API VAD documentation; Microsoft Foundry voice-agent documentation
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Semantic VAD eagerness in OpenAI’s API
OpenAI documents low, medium, and high eagerness for semantic VAD, with maximum timeouts of 8, 4, and 2 seconds, respectively; the API reference says auto is equivalent to medium. These are API-specific configuration descriptions, not general timing standards. Consult the current API reference when configuring an integration because these details may change. OpenAI Realtime API reference
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In OpenAI’s server-VAD configuration, threshold is the activation threshold, prefix_padding_ms controls audio included before detected speech, and silence_duration_ms sets the silence interval used to detect speech stopping. The documentation’s example uses threshold: 0.5, prefix_padding_ms: 300, and silence_duration_ms: 500. Those are example configuration values; only the 500 ms value is identified as the documented default for the silence interval, and none is a universal standard. OpenAI’s Realtime API VAD documentation
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When silence should not trigger a reply
Endpointing is not the same as deciding whether audio is intended for the assistant. Silence, background noise, hold music, television audio, side conversation, or speech not directed at the assistant should not automatically prompt a response. OpenAI’s voice prompting guidance recommends a no-op or wait behavior in those situations. OpenAI’s voice prompting guide
That distinction matters: a system may detect sound or a pause without having a meaningful user turn to answer. Turn timing and the decision to engage are related parts of voice interaction, but they solve different problems.
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