# OpenAI Realtime API vs Cartesia Sonic 2: which realtime voice stack wins in 2026?

**Source:** https://promtable.com/compare/openai-realtime-vs-cartesia

> OpenAI Realtime API is the integrated voice-mode stack inside OpenAI. Cartesia Sonic 2 is the specialised low-latency TTS for production voice agents. Pick OpenAI for OpenAI-native, Cartesia for fastest end-to-end voice.

---
OpenAI Realtime API is the integrated voice-mode stack inside OpenAI. Cartesia Sonic 2 is the specialised low-latency TTS for production voice agents. Pick OpenAI for OpenAI-native, Cartesia for fastest end-to-end voice.

## At a glance

| Dimension | OpenAI Realtime API | Cartesia Sonic 2 |
|---|---|---|
| Form factor | Full STT + LLM + TTS pipeline | TTS only — bring your own STT + LLM |
| End-to-end latency | ~500-800 ms | **Sub-150 ms TTS first byte** ✓ |
| Voice naturalness | Strong with GPT-realtime voices | **Top tier** ✓ |
| Multilingual coverage | **Good** ✓ | Growing |
| Integration complexity | **Single API — easiest path** ✓ | Compose STT + LLM + TTS yourself |
| Voice cloning | Limited (preset voices) | **Available + controllable** ✓ |
| Best for | OpenAI-native realtime voice | Latency-critical production voice agents |

## Verdict

OpenAI Realtime API is the right pick for OpenAI-native stacks that want the simplest path to a realtime voice assistant — single API, voices included. Cartesia Sonic 2 is the right pick for production voice agents where sub-150ms latency is the hard requirement and you compose your own STT + LLM + TTS pipeline. Many production agents in 2026 use Deepgram or AssemblyAI for STT, Claude / GPT for LLM, and Cartesia for TTS.

## When to pick which

- **OpenAI Realtime API** — OpenAI-native stacks, simplest realtime voice path.
- **Cartesia Sonic 2** — Lowest-latency production voice agents, cloning needs, composable pipeline.

## FAQ

### OpenAI Realtime or Cartesia in 2026?

OpenAI for simplest single-API path; Cartesia for lowest latency and composable pipelines.

### Cheapest realtime voice stack?

OpenAI Realtime API tends to be cheaper at low scale; composable Cartesia stack can be cheaper at high scale.

### Best for voice cloning?

Cartesia or ElevenLabs — OpenAI Realtime is limited to preset voices.

## Related

- [/compare/elevenlabs-vs-cartesia](https://promtable.com/compare/elevenlabs-vs-cartesia)
- [/glossary/voice](https://promtable.com/glossary/voice)
- [/guides/ai-voice-production-2026](https://promtable.com/guides/ai-voice-production-2026)

*Last updated: 2026-06-01*
---

Original page: https://promtable.com/compare/openai-realtime-vs-cartesia
Maintained by Promtable (https://promtable.com). Content: CC BY 4.0. Cite as "Promtable — https://promtable.com/compare/openai-realtime-vs-cartesia".
Contact: info@vibecodingturkey.com.