OpenAI Realtime API vs ElevenLabs Conversational
Under every AI voice agent platform (Vapi, Retell, Bland) there's a voice engine doing the actual TTS + STT + LLM work. The two leaders are OpenAI Realtime API and ElevenLabs Conversational AI. They optimize for different things.
OpenAI Realtime API
What it is: A unified speech-to-speech API. You speak, the model processes audio directly (no STT step), generates audio response (no TTS step). Uses GPT-4o voice.
Strengths:
- Lowest latency in the industry — typical 200-400ms first-token, full response in <1s
- Native turn-taking — handles interruptions naturally
- Best at understanding emotion in caller's voice (frustration, urgency)
- Tight integration with OpenAI tools — function calling, structured output
- Lower cost at high volume — $0.06-0.10/min usage
Weaknesses:
- Voice quality is good, not great — fewer voice options than ElevenLabs
- Limited custom voice cloning — pre-set voices mainly
- English-first — best in English, good in major European languages, weaker in low-resource
Verdict: Best when latency + cost matter most. Outbound sales, high-volume support.
ElevenLabs Conversational AI
What it is: Best-in-class TTS + custom voice cloning + integrated conversational layer. Uses Turbo v2.5 model.
Strengths:
- Best voice quality in the market — indistinguishable from human in head-to-head tests
- Best voice cloning — 1 minute of audio creates a usable clone
- Excellent multilingual — 30+ languages with native quality
- Custom voice library — thousands of preset voices to pick from
- Brand voice consistency — same voice across all your AI products
Weaknesses:
- Slightly higher latency than OpenAI Realtime (350-600ms first-token)
- More expensive at high volume — $0.10-0.18/min
- Separate STT/LLM/TTS pipeline — more components to fail
Verdict: Best when voice quality + brand consistency matter most. Premium customer-facing inbound, hospitality, brand-driven outbound.
Head-to-Head
| Dimension | OpenAI Realtime | ElevenLabs Conv |
|---|---|---|
| First-token latency | 200-400ms | 350-600ms |
| Full response | <1s | <1.5s |
| Voice quality | Good | Excellent |
| Voice cloning | Limited | Best in class |
| Languages | 20+ | 30+ |
| Cost / min | $0.06-0.10 | $0.10-0.18 |
| Best for | Latency-critical | Quality-critical |
How Vapi/Retell/Bland Use Them
The voice engine choice is mostly transparent on the platforms:
- Vapi — defaults to OpenAI Realtime, can opt for ElevenLabs voices
- Retell — both options, smart routing based on use case
- Bland — most flexible, you choose explicitly per agent
- Custom build — pick whichever fits, swap as needed
Many production deployments use a hybrid: OpenAI Realtime for routing/intent classification, ElevenLabs for the actual voice output (best quality + low latency).
When to Pick OpenAI Realtime
- High-volume outbound (cost-sensitive)
- Real-time conversation where 100ms matter (rapid turn-taking)
- English-heavy use cases
- Cost-critical SMB deployments
- Need for emotion detection in caller's voice
When to Pick ElevenLabs
- Premium brand voice (luxury, hospitality)
- Multilingual deployment with quality requirement
- Custom voice clone of brand spokesperson
- Customer-facing inbound where voice quality is the trust signal
- Lower volume but higher conversion focus