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 agents on gpt-realtime-2.1, billed per audio to...
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Vapi
Developer infrastructure for phone and web voice agents
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Retell AI
Build AI agents that answer and place real phone calls
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Cartesia
Low-latency speech models built on state space models, for voice agent...
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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 | Usage_based | Usage_based | Usage_based | Freemium |
| Free tier | — | $5 trial credit on sign-up; no lasting free plan | — | Free: 20K credits a month (about 27 minutes of Sonic speech) plus $1 a month of prepaid voice-agent usage |
| Paid from | From $0.60 per 1M text input tokens (gpt-realtime-2.1-mini); audio from $10 per 1M input | $0.05/min Vapi hosting fee plus provider costs at list price, no package required | $0.07-0.31 per minute all-in for voice agents, pay as you go | $5/month (Pro: 100K credits, commercial licence, instant voice cloning) |
| Has API | ✓ | ✓ | ✓ | ✓ |
| Open source | ✗ | ✗ | ✗ | ✗ |
| Free to use | ✗ | ✗ | ✗ | ✓ |
| Users | — | — | — | — |
| Founded | 2024 | 2024 | 2024 | 2024 |
| Maker | OpenAI | — | — | — |
| Verdict | The Realtime API removes the speech-to-text and text-to-speech hand-offs that made earlier voice bots slow, and since the August 2025 GA it handles phone calls over SIP, MCP tools and image input. gpt-realtime-2.1 adds c... |
Vapi is one of the most flexible building blocks for voice agents: pick the transcriber, model, voice and carrier, wire in tool calls and webhooks, hand calls between agents in squads, and simulate calls before they go l... |
Retell is one of the more complete toolkits for phone agents: low-latency conversation, human transfer, booking, batch calling, testing and QA are all on the platform, with a choice of models and your own telephony if yo... |
Cartesia is built for real-time voice: Sonic-3.6 starts speaking in under 90 milliseconds across 44 languages, Ink-2 transcribes with built-in turn detection, and Managed Agents ties them into phone agents. Its leaderboa... |
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Comparisons cover up to 4 tools. Scores are RECATOOLS editorial assessments; verify current pricing on each vendor's site.