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.
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, 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' breadth, Deepgram's enterprise STT, Vapi as the assemble-don't-build alternative to Line — see Best TTS APIs and Best STT APIs.
Frequently asked questions
- What makes Cartesia different from ElevenLabs?
- Architecture and posture. Cartesia's models are state-space (SSM/Mamba lineage — its founders created the architecture), built for streaming-first, low-latency realtime voice; ElevenLabs leads on voice variety, expressiveness, and its broader audio product surface. For interactive agents where every millisecond of latency is conversational quality, Cartesia is the specialist; for produced audio and voice breadth, ElevenLabs.
- What is Cartesia Line?
- Their voice-agent platform (GA August 2025): code-first SDK and CLI with one-command deploys, hosted infrastructure, phone numbers and SIP (beta), call recording and transcripts, latency metrics, and LLM-as-judge evals — defaulting to Sonic + Ink, so the whole loop runs on Cartesia's stack at per-minute pricing.
- Is there a free tier?
- Yes — monthly free credits covering meaningful testing, but it's non-commercial: a commercial-use license starts at the inexpensive Pro plan, which also unlocks instant voice cloning. Credits meter both TTS and STT (TTS burns them several times faster).
Related
- Best Text-to-Speech APIs in 2026The TTS APIs worth building on — ElevenLabs for quality and breadth, Cartesia Sonic for realtime latency — and how to choose for agents vs produced audio.
- Best Speech-to-Text APIs in 2026The STT field, honestly ranked — Deepgram and AssemblyAI's hosted duel, Whisper as the open baseline, Cartesia Ink for latency — and how to pick by workload.
- Realtime Voice Agents: Build on LiveKit, Buy Vapi, or Pipeline with PipecatThe three ways to ship a realtime voice agent in 2026 — open infrastructure, managed platform, or OSS pipeline framework — and how speech-to-speech models change it.
- ElevenLabsA voice-AI platform for high-quality text-to-speech, voice cloning, dubbing, and real-time conversational agents, via API.
- DeepgramA voice-AI platform with fast, accurate speech-to-text (Nova) and low-latency text-to-speech (Aura), plus a bundled Voice Agent API.
- How to Build a Voice Agent: The STT → LLM → TTS PipelineHow to build a real-time voice agent: the STT → LLM → TTS pipeline, the latency budget that makes or breaks it, and how to wire each stage.
- AssemblyaiSpeech AI platform: Universal STT models (promptable Universal-3 Pro), a flat-rate Voice Agent API, and speech understanding — summarization, sentiment, PII redaction.
- LivekitOpen-source realtime infrastructure — a WebRTC server plus the LiveKit Agents framework for production voice AI, with turn detection, telephony, and cloud.
- VapiThe API-first voice-agent platform — assemble phone and web agents from any STT/LLM/TTS mix, with telephony, squads, and tool calling handled for you.