Cartesia Managed Agents (Line)
Managed voice agents combining Cartesia Ink-2 speech recognition, an LLM you choose and Sonic-3.6 TTS, with phone numbers, SIP and evals. Suits teams that like Sonic's low-latency voices and want a managed agent runtime.
Overview
Best for: Phone and web agents that want Sonic voices with a managed runtime and simple per-minute billing.
At a glance
$0.06/min base excludes LLM tokens; Cartesia phone numbers add $0.014/min. Subscription plans include monthly credits. LLM chosen from a per-account list.
pcm_16000, pcm_24000, pcm_44100 (16-bit) or mulaw_8000, headerless mono
Streamed audio_output events (speaking-pace delivery option)
Configurable per agent; list not read.
Cartesia Sonic voice library.
Not stated on the pages read.
Twilio regional routing documented; Cartesia hosting regions not stated.
Not stated on the pages read.
Features
- system, webhook and client tools
- dynamic variables
- agent versioning
- turn-taking and interruptions
- outbound and batch calling
- call metrics/evaluations
Pricing
| What | Price | Unit |
|---|---|---|
| Agent calling | $0.06 | per minute |
| Cartesia-provided phone number telephony | +$0.014 | per minute |
| LLM | per model token prices | per 1M tokens |
$0.06 base plus $0.014 telephony on Cartesia numbers plus LLM tokens (small models add little; own estimate).
Not token-billed (LLM portion is).
Free tier: Subscription plans include monthly credits; free LLM promo ended 2026-10-01.
Source: docs.cartesia.ai
Setup
- Create an agent in the Cartesia Playground or via API (instructions, voice, LLM, tools).
- Server: open the agent WebSocket with X-API-Key and cartesia_version; browsers: mint an agent access token.
- Send session_create with input_format, wait for session_ready, stream audio_input, play audio_output, clear on audio_output_clear.
Endpoint
wss://api.cartesia.ai/v1/agents/websocket/{agent_id}?cartesia_version=2026-08-14Authentication
X-API-Key header (server) or access_token query parameter (browser/mobile)
Quick start javascript
// npm i ws (create the agent in the Cartesia Playground or via API first)
import WebSocket from "ws";
const ws = new WebSocket(
`wss://api.cartesia.ai/v1/agents/websocket/${process.env.CARTESIA_AGENT_ID}?cartesia_version=2026-08-14`,
{ headers: { "X-API-Key": process.env.CARTESIA_API_KEY } } // browsers: ?access_token=... instead
);
let ready = false;
ws.on("open", () => {
ws.send(JSON.stringify({
type: "session_create",
audio: { input_format: "pcm_16000", output_delivery: "speaking_pace" },
}));
});
ws.on("message", (raw) => {
const ev = JSON.parse(raw.toString());
if (ev.type === "session_ready") { ready = true; console.log("call", ev.call_id); }
if (ev.type === "audio_output") play(Buffer.from(ev.audio, "base64"));
if (ev.type === "audio_output_clear") stopPlayback(); // user interrupted
});
// headerless mono 16-bit PCM, 20-100 ms chunks
export function sendPcm(buf) {
if (ready) ws.send(JSON.stringify({ type: "audio_input", audio: buf.toString("base64") }));
}
function play(pcm) {} function stopPlayback() {}Written from the current docs. Check the vendor's SDK version before you ship.
Warnings
Free LLM period just ended
LLM usage was free until October 1, 2026; from now on LLM tokens are billed on top of $0.06/min. Budgets built during the promo are too low.
Versioned API header
Connections require cartesia_version (2026-08-14 in current docs); older version strings may behave differently.
Never ship the API key to clients
Browsers cannot set WebSocket headers; use short-lived agent access tokens.
Cascaded pipeline
STT, LLM and TTS are separate stages; tone and emotion in the caller's voice are not passed to the LLM.
Telephony surcharge
Cartesia numbers add $0.014/min; Twilio import or SIP is billed by your carrier.
Plus 14 warnings that apply to all voice-to-voice APIs. See category warnings.
Limits
- Concurrency not documented on the pricing page
- max_output_tokens per response 1-4096
Models and products
| Name | Status |
|---|---|
| Ink-2 (STT) + chosen LLM + Sonic-3.6 (TTS) | GA |
Docs and sources
Docs
Sources used
- docs.cartesia.ai/pricing.md
- docs.cartesia.ai/agents/introduction.md
- docs.cartesia.ai/line/integrations/websocket-api.md
- docs.cartesia.ai/agents/models.md
Concurrency limits, languages, latency, compliance.