Cartesia

What is Cartesia?

Cartesia is an AI voice platform built around State Space Models (SSMs), a research lineage its founding team helped pioneer at the Stanford AI Lab before founding the company in September 2023. It ships three products. Sonic-3.6 is the current real-time text-to-speech model, with laughing and emotional control across 40+ languages. Ink-2 is the streaming speech-to-text model.

Line is the voice-agent development platform that sits on top of both. The company has raised $191 million across three rounds. A $27M seed (December 2024, led by Index Ventures), a $64M Series A (March 2025, led by Kleiner Perkins), and a $100M Series B in October 2025, led by Kleiner Perkins with Index Ventures and Lightspeed returning and NVIDIA joining as a new strategic investor.

The Series B was announced alongside Sonic-3, a 42-language TTS model Cartesia said was preferred over competitors by 62% of blind human testers, and a successor Sonic-3.6 now sits on the pricing page. Cartesia's customers page lists ServiceNow, Together AI, Replicant, Thoughtly, Lorikeet, Fundamento, Bolna, Maven AGI, Forethought, Simple AI, 2X Solutions, Cresta, Rasa and others.

Several rival voice-agent platforms in this index, including Vapi, Retell AI and LiveKit Agents, are themselves named as Cartesia customers and license Sonic and Ink as components of their own stacks.

What does Cartesia do?

Line, Cartesia's voice-agent layer, offers two build paths: a no-code Agent Builder for prototyping without writing code, and an open-source (Apache 2.0, 103 GitHub stars as of this listing's verification) Python SDK for full programmatic control.

The SDK connects to more than 100 LLM providers via LiteLLM — including OpenAI, Anthropic and Google models by name, plus open-weight models — rather than locking a buyer into Cartesia's own reasoning layer, and supports multi-agent handoffs (a main agent can transfer a call to a specialized agent mid-conversation via an agent_as_handoff() primitive), real-time interruption handling, tool calling against external APIs and databases, and built-in web search.

Deployment is Cartesia's own managed runtime — the docs describe it as deploying "in seconds" with auto-scaling — and every call is recorded with saved audio, transcripts and latency/system metrics for debugging.

A distinct, narrower self-hosting path exists for the underlying Sonic and Ink models only (deployable into a customer's own GCP, AWS or on-premise environment on an Enterprise contract) — that option does not extend to Line's voice-agent runtime itself, which stays on Cartesia's managed infrastructure regardless of tier.

Cartesia also publishes its own MCP server (cartesia-mcp) so tools like Cursor or Claude Desktop can call its TTS/STT/voice-management APIs directly.

How Cartesia works

1. 1Start on Sonic (TTS) and Ink (STT) directly via API, or use Line for full voice-agent orchestration — via the no-code Agent Builder for prototyping, or the open-source Python SDK for production control.
2. 2In the SDK, connect any of 100+ LLM providers through LiteLLM (OpenAI, Anthropic, Google or open-weight models), define tools the agent can call, and configure multi-agent handoffs for specialized routing.
3. 3Deploy through Line's managed runtime — Cartesia hosts the compute, auto-scaling and audio pipeline; every call is recorded with transcripts and latency/system metrics available for review.
4. 4Billing draws from two separate pools per tier: a monthly credit allotment for Sonic/Ink usage, and a prepaid balance for Line voice-agent minutes at a flat $0.06/min ($0.014/min telephony) that doesn't change by tier.

Who is Cartesia for?

• ✓Engineering teams that want an open-source voice-agent SDK (not a closed black box) with genuine LLM choice across 100+ providers, and are comfortable with Cartesia hosting the runtime rather than running it themselves.
• ✓Teams prioritizing speech-model quality and latency first — Sonic and Ink are the core product several rival platforms in this category license as a component — who want the option to build the agent layer on top of the same models directly.
• ✓Startups and solo builders who want a genuine $0/month starting point (20K TTS/STT credits plus a small prepaid agent balance) before committing spend, scaling into paid tiers that stay flat-rate on voice-agent minutes ($0.06/min) regardless of plan.

Not forRegulated enterprises that specifically need the voice-agent runtime itself (not just the speech models) fully self-hosted — Deepgram Voice Agent API offers that; Cartesia's Line layer does not.

Why use Cartesia?

1. A genuine $0/month starting tier (20K credits, $1 prepaid agent balance) rather than only a time-limited trial, with flat $0.06/min voice-agent pricing that doesn't change across paid tiers.
2. Line's SDK is open source (Apache 2.0) and connects to 100+ LLM providers via LiteLLM, so a buyer isn't locked into Cartesia's own reasoning layer the way several closed-source competitors in this category are.
3. Speech-model quality several rival platforms in this very index (Vapi, Retell AI, LiveKit Agents) rely on directly, per Cartesia's own published customer list. ServiceNow, Together AI, Replicant and Forethought are also named customers on the same page.
4. A well-capitalized company ($191M raised, Series B led by Kleiner Perkins with NVIDIA as a new strategic investor, October 2025) with SOC 2 Type II compliance on its public API.