OpenAI Realtime API vs Vapi vs Retell AI vs Cartesia
A side-by-side look at scores, pricing and features — with RECATOOLS' ASEAN-aware verdict for each.
OpenAI Realtime API
Speech-to-speech voice-agent API, billed per audio token
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Vapi
Voice AI infrastructure for developers
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Retell AI
Voice AI for phone-call automation
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Cartesia
Ultra-low-latency voice AI on Mamba SSMs
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| RECATOOLS Score | 7.8 / 10 | 7.3 / 10 | 7.2 / 10 | 7.6 / 10 |
| Capability | ||||
| Value for money | ||||
| Ease of use | ||||
| ASEAN readiness | ||||
| API quality | ||||
| Pricing | Paid | Paid | Paid | Paid |
| Free tier | — | — | — | — |
| Paid from | — | — | — | — |
| Has API | ✓ | ✓ | ✓ | ✓ |
| Open source | ✗ | ✗ | ✗ | ✗ |
| Free to use | ✗ | ✗ | ✗ | ✗ |
| Users | — | — | — | — |
| Founded | 2024 | 2023 | 2023 | 2023 |
| Maker | — | — | — | — |
| Verdict | OpenAI's Realtime API does voice-to-voice natively instead of chaining speech-to-text, a text model and text-to-speech together, which is what killed the responsiveness of most 2023-era voice bots. gpt-realtime-2.1 score... |
Vapi is a developer-focused platform for building real-time voice AI agents, orchestrating speech-to-text, an LLM and text-to-speech into low-latency phone and web calls. Its appeal is flexibility: you bring your own mod... |
Retell AI is a developer-first platform for building production AI voice agents that handle real phone calls, with low-latency conversation, interruption handling, telephony (Twilio/SIP) integration and post-call analyti... |
Cartesia's Sonic models are a leading choice for real-time text-to-speech, prized for very low latency and natural prosody, making them well-suited to voice agents and interactive applications. The API is developer-frien... |
| Full review → | Full review → | Full review → | Full review → |
Comparisons cover up to 4 tools. Scores are RECATOOLS editorial assessments; verify current pricing on each vendor's site.