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Silence can mean “I’m finished,” but it can also mean “I’m thinking.” In OpenAI’s Realtime API, one silence-based option ends a turn after 500 milliseconds without detected speech by default. That is a configurable setting for one mode—not a universal rule for voice agents.
How a silence timer decides you are done
Silence-based endpointing does not have to understand whether a sentence is complete. It detects speech, then waits for a configured stretch of time without detected speech. If that interval passes, the system treats the turn as finished and can respond.
OpenAI calls this mode server_vad, or server-side voice activity detection. Its Realtime API documentation gives silence_duration_ms a default of 500 milliseconds. The timer is part of that API configuration; other products and configurations may use different behavior. OpenAI’s Realtime API VAD guide and API reference describe the setting.
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What changing 500 ms changes
A shorter silence interval can make the agent respond sooner after the speaker stops. OpenAI’s documentation puts it plainly: “With shorter values turns will be detected more quickly.” The trade-off is that a brief pause may be mistaken for the end of the turn, cutting off a hesitation or a pause before the speaker continues. A longer interval gives the speaker more room to continue, but can make the system wait longer when they really are finished.
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There is no universally correct interval established by the documentation. The right choice depends on the interaction: a rapid exchange may prioritize prompt responses, while a setting that tolerates pauses may better suit people who speak slowly or gather their thoughts between phrases. These are trade-offs, not a guarantee that any one value will perform best.
Silence-based and semantic detection compared
OpenAI’s Realtime API also offers semantic VAD. Rather than using only a fixed silence tail, it uses a model to estimate whether the utterance sounds complete. Its eagerness setting controls how readily it responds; a less eager configuration can allow more time when the utterance seems unfinished. Microsoft Foundry documentation likewise describes server and semantic VAD as voice-agent configuration choices.
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| Approach | What drives the decision | Timing trade-off | Can behavior be tuned? |
|---|---|---|---|
| Server VAD | Detected speech followed by a configured interval of silence. | A shorter interval can speed up responses but may treat a brief pause as turn completion. | Yes. OpenAI documents configurable silence timing for server VAD. |
| Semantic VAD | A model’s estimate of whether the user has finished speaking. | It may wait longer when the utterance seems unfinished; eagerness changes how soon it responds. | Yes. OpenAI documents eagerness settings; Microsoft Foundry also presents VAD configuration choices. |
The official documentation explains the approaches and their controls; it does not establish a controlled, universal performance ranking. Semantic detection is not automatically better for every conversation. See OpenAI’s VAD guide, its Realtime API reference, and Microsoft Foundry’s voice-agent documentation.
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OpenAI’s documented server-VAD configuration includes three controls with distinct jobs:
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thresholdsets the activation threshold used to detect voice activity.prefix_padding_msspecifies audio to include from before detected speech.silence_duration_mssets how long silence must last to detect that speech has stopped.
The guide’s example uses threshold: 0.5, prefix_padding_ms: 300, and silence_duration_ms: 500. These are values in a configuration example, not general standards. The 500-millisecond default applies to OpenAI’s documented server-VAD setting, not every voice agent.
For OpenAI semantic VAD, the API reference describes low, medium, and high eagerness with maximum timeouts of 8, 4, and 2 seconds, respectively; auto is equivalent to medium. These are API-specific configuration descriptions, not universal timing rules. Check the current Realtime API reference when implementing them, since API behavior and documentation can change.
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When the agent should stay quiet
Detecting an endpoint is not the same as deciding that every sound is a request. OpenAI’s voice prompting guide recommends a no-op or wait behavior when the input is silence, background noise, hold music, television audio, side conversation, or speech not directed to the assistant. That distinction helps prevent an agent from responding just because it heard audio or a pause. See OpenAI’s voice-agent prompting guide.
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