Voice Agents (2025)
Build conversational AI that speaks in real-time. From OpenAI's Realtime API to specialized platforms like Vapi and Retell AI. Low latency (< 500ms) conversation is now possible.
The State of Voice AI (2025)
Traditional Voice AI (STT → LLM → TTS) was slow (2-4s latency). In 2025, end-to-end multimodal models and optimized infrastructure have brought latency down to 300-800ms, enabling truly interruptible, natural conversations.
Key Capabilities
- Low Latency: Sub-second response times.
- Interruptibility: Stop speaking when the user talks.
- Emotion: Detect and express emotions.
- Function Calling: Trigger actions during calls.
Leading Players
- OpenAI Realtime API: Native GPT-4o voice (~230ms).
- Vapi: Developer platform for voice agents.
- Retell AI: Focus on telephony & scheduling.
- Bland AI: Enterprise phone agents.
OpenAI Realtime API
A WebSocket-based API that streams audio input and output directly. No separate transcription or text-to-speech steps needed.
import asyncio
import websockets
import json
import base64
API_URL = "wss://api.openai.com/v1/realtime?model=gpt-4o-realtime-preview-2024-10-01"
HEADERS = {"Authorization": "Bearer YOUR_API_KEY", "OpenAI-Beta": "realtime=v1"}
async def voice_chat():
async with websockets.connect(API_URL, extra_headers=HEADERS) as ws:
# 1. Initialize Session
await ws.send(json.dumps({
"type": "session.update",
"session": {
"modalities": ["text", "audio"],
"voice": "alloy",
"turn_detection": {"type": "server_vad"}
}
}))
# 2. Receive Events
async for message in ws:
event = json.loads(message)
if event["type"] == "response.audio.delta":
# Play audio chunk
audio_bytes = base64.b64decode(event["delta"])
play_audio(audio_bytes)
elif event["type"] == "input_audio_buffer.speech_started":
# Handle interruption
stop_playback()
# Use asyncio.run(voice_chat())Platform Field Guide
Choosing between building with the raw API vs. using a managed platform.
| Platform | Best For | Models | Latency (Approx) |
|---|---|---|---|
| OpenAI Realtime | Developers building custom web/app agents. | GPT-4o | ~230ms |
| Vapi | Orchestration layer. Mix & match models. | GPT-4o, Claude, Groq | ~500-800ms |
| Retell AI | Telephony (Phone Calls). | Custom LLMs | ~800ms |
| Bland AI | Enterprise scale phone automation. | Proprietary | < 400ms (Turbo) |
Common Use Cases
Customer Support
24/7 phone support capable of handling complex queries.
Inbound Booking
Scheduling appointments for clinics, salons, and services.
Real-time Translator
Voice-to-voice translation with preserved intonation.