Cartesia Sonic vs OpenAI Realtime API
Cartesia Sonic vs OpenAI Realtime API: side-by-side comparison of two voice agent platforms systems — architecture, taxonomy, license, pricing, MCP/A2A support, and direct edges.
Cost & capability
| Cartesia Sonic | OpenAI Realtime API | |
|---|---|---|
| Capability band | competent | frontier |
| Capability composite | 58 | 78 |
| Cost tier | mid | premium |
| $/Mtok input | — | 40 |
| $/Mtok output | — | 80 |
| Use cases | Latency Sensitive, Scoped Agentic | Latency Sensitive, Scoped Agentic |
Where they differ (15)
Rows where both sides have data and the values disagree — the shortlist of dimensions that actually distinguish these two systems.
| Cartesia Sonic | OpenAI Realtime API | |
|---|---|---|
| Capability band | competent | frontier |
| Capability composite | 58 | 78 |
| Cost tier | mid | premium |
| Type | Real-time TTS + voice-agent platform (Cartesia) | Speech-to-speech LLM API (OpenAI Realtime) |
| Created | 2024 (founded) | 2024-10 |
| GitHub | github.com/cartesia-ai (SDKs) | openai/openai-realtime-agents — ~6.9k stars (MIT reference app on Agents SDK), last commit n/a; API itself closed source |
| Pricing | Free (20K credits/mo); Pro $5/mo; Startup $49/mo; Scale $299/mo; Enterprise custom; managed voice agents $0.06/min | Audio token-based: $40/$80 per 1M input/output audio tokens (Oct-2024 launch) |
| Funding | $64M Series A Q1-2025 (Index + Lightspeed) | OpenAI parent — $40B round Mar-2025 ($300B val) |
| Backend storage | Stateless API; caller manages state | Stateless session API; caller manages durability |
| Deployment | Both (self-hosted + managed cloud) — managed API; on-prem (incl. air-gapped), customer VPC and OEM under enterprise contract | API (Cloud); no self-host |
| API surface | REST + WebSocket APIs + Python/JS SDKs | WebSocket + WebRTC + REST |
| Multi-tenancy | Multi-tenant Cloud | API multi-tenant |
| MCP | yes — official Cartesia MCP server (cartesia-ai/cartesia-mcp) | searched not found |
| Optimised for | Lowest-latency TTS substrate for voice agents | Native speech-to-speech with GPT-4o; lowest-latency LLM voice tier |
| Anti-fit | TTS-led platform — not turnkey for full call orchestration (use with Vapi/Pipecat/LiveKit) | Stateless platform — caller builds orchestration, memory, telephony adapter |
At a glance
| Cartesia Sonic | OpenAI Realtime API | |
|---|---|---|
| Section | Voice agent platforms | Voice agent platforms |
| Tier | T1 | T1 |
| Type | Real-time TTS + voice-agent platform (Cartesia) | Speech-to-speech LLM API (OpenAI Realtime) |
| Created | 2024 (founded) | 2024-10 |
| Latest release | Sonic-3.6 (current model; release date not stated) | — |
| GitHub | github.com/cartesia-ai (SDKs) | openai/openai-realtime-agents — ~6.9k stars (MIT reference app on Agents SDK), last commit n/a; API itself closed source |
| Pricing | Free (20K credits/mo); Pro $5/mo; Startup $49/mo; Scale $299/mo; Enterprise custom; managed voice agents $0.06/min | Audio token-based: $40/$80 per 1M input/output audio tokens (Oct-2024 launch) |
| Funding | $64M Series A Q1-2025 (Index + Lightspeed) | OpenAI parent — $40B round Mar-2025 ($300B val) |
| Backend storage | Stateless API; caller manages state | Stateless session API; caller manages durability |
| Deployment | Both (self-hosted + managed cloud) — managed API; on-prem (incl. air-gapped), customer VPC and OEM under enterprise contract | API (Cloud); no self-host |
| API surface | REST + WebSocket APIs + Python/JS SDKs | WebSocket + WebRTC + REST |
| Multi-tenancy | Multi-tenant Cloud | API multi-tenant |
| MCP | yes — official Cartesia MCP server (cartesia-ai/cartesia-mcp) | searched not found |
| A2A | searched not found | searched not found |
| OpenTelemetry | searched not found | searched not found |
| Optimised for | Lowest-latency TTS substrate for voice agents | Native speech-to-speech with GPT-4o; lowest-latency LLM voice tier |
| Anti-fit | TTS-led platform — not turnkey for full call orchestration (use with Vapi/Pipecat/LiveKit) | Stateless platform — caller builds orchestration, memory, telephony adapter |
Pros & cons
Cartesia Sonic
Pros: Lowest-latency TTS in market; Mamba-author founders; $64M Series A; SOC 2.
Cons: TTS-only at platform layer — needs orchestrator; SSM voice quality polarising vs Eleven.
OpenAI Realtime API
Pros: Native speech-to-speech (skips STT/TTS); OpenAI brand + price/perf; WebRTC support.
Cons: Audio token pricing is expensive at scale; stateless (caller builds memory/orchestration); OpenAI-only model choice.