Glossary · AI & Agents
OpenAI Realtime API
Overview
What the OpenAI Realtime API is
The OpenAI Realtime API is an interface for building real-time, spoken conversations with an OpenAI model. Instead of chaining separate speech-to-text, language, and text-to-speech services, audio flows directly into a single multimodal model that can respond with both audio and text.
It reached general availability in 2025 with the gpt-realtime speech-to-speech model, which added stronger instruction following, more reliable function calling, and image input alongside audio. It is one approach to voice AI, distinct from pipeline platforms that orchestrate discrete STT, LLM, and TTS components.
How the OpenAI Realtime API works
A client opens a persistent connection over WebRTC (recommended for browsers and devices) or WebSocket, then streams microphone audio to the model and receives generated speech back with low latency. Because the model hears the audio directly, it can pick up tone and interruptions rather than working from a flat transcript.
It supports server-side voice activity detection for natural turn-taking, function calling to trigger tools and look up data mid-conversation, and MCP server integration. You define instructions, voice, and available tools when configuring the session.
Where it fits in a voice-AI stack
The Realtime API is a speech-to-speech engine: it collapses transcription, reasoning, and speech synthesis into one model, which can lower latency and simplify architecture versus a multi-vendor pipeline. It still needs a telephony layer, such as Twilio, Telnyx, or SignalWire, to connect to actual phone numbers.
Orchestration frameworks like LiveKit and Pipecat, and platforms like Vapi and Retell AI, can use it as their model layer. Choose the speech-to-speech approach for responsiveness and simplicity; choose a composable pipeline when you need to mix best-of-breed STT, LLM, and TTS vendors or swap them independently.
Where we use it
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How our team puts OpenAI Realtime API to work in real projects.
FAQ
OpenAI Realtime API — common questions
What is the OpenAI Realtime API used for?
The OpenAI Realtime API is used to build low-latency voice agents and live audio applications. It powers phone and in-app assistants that listen, reason, and speak in real time, including support, scheduling, and interactive voice experiences.
How is speech-to-speech different from a traditional voice pipeline?
Speech-to-speech feeds audio directly into one multimodal model that returns speech, while a traditional pipeline chains separate speech-to-text, language, and text-to-speech services. The single-model approach can cut latency and preserve tone, but offers less control over each component.
Does the OpenAI Realtime API handle phone calls?
Not on its own. The Realtime API provides the conversational model over WebRTC or WebSocket, but you still need a telephony provider such as Twilio, Telnyx, or SignalWire to connect it to real phone numbers.
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