# Cartesia

> Real-time voice AI on state-space models — Sonic streaming TTS, Ink STT with native turn detection, and Line, a code-first voice-agent platform.

Cartesia builds voice AI on state-space models: Sonic streaming TTS — vendor-claimed sub-100ms model latency, 42 languages, emotion controls — Ink streaming STT with turn detection native to the model, and Line, a code-first platform for deploying voice agents with hosted infra, telephony, and evals. Freemium credits; commercial use starts at the low-cost Pro tier.

Website: https://www.cartesia.ai

Cartesia is the latency specialist of voice AI — founded by the creators of the state-space model architecture, and betting that **conversation-grade voice is a realtime systems problem**. Its stack covers both directions (Sonic out, Ink in) and, with Line, the agent platform that runs them.

## Highlights

- **Sonic TTS** — streaming-first synthesis with vendor-claimed sub-100ms model latency; Sonic 3.5 (May 2026) spans 42 languages with emotion and laughter controls.
- **Ink STT** — streaming transcription with **turn detection native to the model** (turn-start/turn-end events, no external VAD), plus careful handling of phone numbers, emails, and IDs; Ink-2 launched May 2026 (English-first).
- **Line** — the voice-agent platform: SDK/CLI with one-command deploys, hosted infra, provisioned phone numbers, recordings/transcripts, latency dashboards, and built-in evals.
- **Voice cloning** — instant (Pro) and professional tiers.
- **SSM pedigree** — the architecture bet (efficient streaming inference) is the product's whole thesis.

## In an AI-assisted workflow

Sign up, take an API key, and stream over WebSocket — or let Line own the loop. In a [voice-agent pipeline](/guides/voice/build-a-voice-agent), Cartesia typically slots in as the TTS (and now STT) where time-to-first-audio defines how human the agent feels; native turn detection removes one of the pipeline's trickiest components.

> [!NOTE]
> Plan mechanics worth knowing: the free tier is **non-commercial** (commercial use starts at Pro), credits meter TTS ~6× faster than STT, and the older T2A API was deprecated in March 2026 — build against the current endpoints.

## Good to know

$64M Series A led by Kleiner Perkins (March 2025); a larger late-2025 raise is third-party-reported but not vendor-confirmed, so we don't state it. Hosted/proprietary (the GitHub org carries SDKs). Against the field — [ElevenLabs](/tools/elevenlabs)' breadth, [Deepgram](/tools/deepgram)'s enterprise STT, [Vapi](/tools/vapi) as the assemble-don't-build alternative to Line — see [Best TTS APIs](/guides/voice/best-tts-apis-2026) and [Best STT APIs](/guides/voice/best-stt-apis-2026).

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_Source: https://agentscamp.com/tools/cartesia — Tool on AgentsCamp._