Cartesia alternative: Sonex Labs vs Cartesia for multilingual voice agents

Why do teams compare Cartesia and Sonex Labs?

Cartesia built its name on speed. Its Sonic page promises first audio in under 90 milliseconds, and it now also offers Ink, a streaming speech-to-text model, and a managed agents product with its own phone numbers. That puts it alongside full-stack voice platforms such as Sonex Labs, not only other text-to-speech engines.

Teams usually weigh the two for one of three reasons. The first is that a language they need isn't on Cartesia's list. The second is that their speech to text has to handle languages beyond the five Ink supports, because an agent that can speak to a caller also has to understand them. The third is that per-character credit pricing gets hard to forecast as volume grows. This article compares Cartesia with Sonex Labs on those points and on latency and procurement. Every Cartesia fact links to Cartesia's own pages, checked on 24 October 2026.

Five things should drive the decision. The first is whether both directions, speech in and speech out, support every language your callers use, including dialects and mixed-language speech. The second is how audio holds up on 8 kHz phone lines. The third is the full response time of a turn, measured from where your servers run. The fourth is total monthly cost, with plan fees, overage and telephony included. The fifth is what your procurement team needs documented before you sign.

What are Cartesia and Sonex Labs?

Sonex Labs is a voice platform for calls in many languages. Pāṇini, its text-to-speech model, covers 250+ languages in 34 writing systems and starts streaming audio in under 200 ms (time to first byte) (text-to-speech docs). Valmiki, its speech-to-text model, covers 100+ languages with automatic language detection, billed by the second (speech-to-text docs). Sonex voice agents combine both with telephony and a choice of LLM, and answer in about 300 ms end to end. Prices are per character, per second of audio and per agent minute, with no credit conversion (pricing), and every language page has a live playground.

Cartesia is a voice AI company whose Sonic text-to-speech models are designed for low-latency streaming. The current model is Sonic 3.6, with 44 languages and a dated snapshot for production pinning (TTS models). Ink-2, released on 17 September 2026, is its streaming speech-to-text model (STT models). Its agents product connects to Twilio, to SIP trunks and to numbers bought from Cartesia (Twilio guide, SIP guide). Everything is billed in credits under monthly plans from free to $299, plus custom enterprise plans (pricing).

Cartesia and Sonex Labs at a glance

Which covers the languages your callers speak, in both directions?

On the speech-to-text side, Valmiki covers 100+ languages (speech-to-text docs). Ink-2 lists five: English, French, Hindi, Japanese and Spanish (STT models). An agent built entirely on Cartesia can therefore speak to a Tamil, Bengali or Telugu caller but can't transcribe what that caller says without a second speech-to-text provider. On Sonex, recognition and synthesis come from one platform, on one bill, and Valmiki detects the caller's language automatically.

On the text-to-speech side, Pāṇini covers 250+ languages (languages), including 36+ Indian languages and dialects. Beyond the major Indian languages, it includes Bhojpuri, Marwari, Haryanvi, Garhwali, Maithili, Chhattisgarhi, Mewari, Saraiki, Bundeli, Braj, Angika and Bhili, and the price per character is the same for every one. Pāṇini reads mixed-language text such as Hinglish naturally in one voice, and one voice or clone can speak across languages.

Sonic 3.6 speaks 44 languages, including Hindi, Bengali, Tamil, Telugu, Gujarati, Kannada, Malayalam, Marathi, Punjabi, Odia and Urdu (TTS models), so it covers the large Indian languages. Its code-switching guide describes Hindi and English in one generation and doesn't describe other language pairs (advanced capabilities). On quality evidence, Sonic 3.6 was fourth on the Artificial Analysis arena on 24 October 2026, with an Elo of 1292; we don't publish a comparable score. For languages both support, open the language's page under text to speech, try the live playground, and run a blind test with native speakers.

How quickly does a caller hear a reply?

Sonex quotes under 200 ms to first byte for Pāṇini and about 300 ms end to end for a full voice agent turn, from when the caller stops speaking to when the agent starts. Cartesia claims first audio in under 90 ms for Sonic (Sonic), a lower synthesis figure, though it doesn't publish the test conditions behind it.

On a phone call, synthesis is only one part of the delay. The caller also waits for speech to text to finish, for the LLM to produce its first tokens, and for the telephony network to carry the audio. Cartesia's own buffering guide covers the trade-off between buffering for smoother prosody and sending audio sooner. So measure the whole turn, from the moment the caller stops speaking to the moment they hear a reply, on your own stack and in the languages you will run.

How do they sound on a real phone line?

Pāṇini is trained for 8 kHz phone audio. Cartesia's output format guide lists 8,000 Hz output with μ-law and A-law encoding for telephony, and its API supports wideband rates up to 48 kHz. A voice shaped on wideband audio is downsampled on the way to the handset, so what you hear on headphones is not what a caller hears. Neither company has published a side-by-side phone-line test, so put both voices on a real call and listen on an ordinary handset.

What does procurement get?

Sonex agents work on Sonex numbers or your own telephony, with no minute charge for bring-your-own telephony, and you can bring your own LLM key at zero markup. For security documents and contract terms, ask us for what your review needs.

Cartesia documents more of this up front. Its pricing page states that it is SOC 2 Type II certified, and its enterprise plans list DPAs, BAAs, SSO and security questionnaires, with on-prem or VPC deployment through its sales team. Enterprise customers can get dedicated deployments in regions including the United States, the European Union, the United Kingdom, India and Australia. It also documents Twilio and SIP trunking integrations (Twilio guide, SIP guide). Ask both vendors for their certifications and deployment terms in writing during procurement.

What does each cost at 1M and 10M characters, and per agent minute?

Sonex charges $0.03125 per 1,000 characters pay as you go, or $0.028125 on a $5-a-month plan, for every language (pricing). Cartesia sells credits in plans, with one text-to-speech character costing one credit. The plans are Free (20,000 credits), Pro at $5 (100,000 credits), Startup at $49 (1.25 million) and Scale at $299 (8 million). Overage costs $65, $45 or $38 per million credits on those paid plans (pricing). All prices were checked on 24 October 2026.

The assumptions are as follows. Prices are in US dollars, before tax. For each vendor we used the cheapest plan that covers the volume, with overage enabled where it applies. Free allowances are ignored. Cartesia's credits are one pool shared between text to speech and speech to text, so a plan used for both will run out sooner than shown.

At these two volumes, Sonex's text to speech costs about 36% less at 1M characters and about 24% less at 10M, and there is no plan to size or credit balance to watch. The gap narrows at volumes that fit a Cartesia plan exactly, for example 8M characters on Scale, which works out to about $37 per million.

For speech to text, Valmiki is $0.00525 per minute of audio, billed by the second, in every one of its 100+ languages (pricing). Cartesia's speech to text uses three credits per second of audio, and Cartesia gives $0.39 an hour on the Scale plan as its example, about $0.0065 a minute (pricing). At 10,000 minutes a month, that is $52.50 against about $65.

For voice agents, Sonex lists $0.055 per minute, including speech to text, text to speech and telephony on Sonex numbers. With your own telephony, no minute charge applies to it. For the LLM, you either bring your own key at zero markup or use Sonex's Vyas model at $0.30 per million input tokens and $0.60 per million output tokens. Cartesia lists $0.06 per minute of call time, plus $0.014 per minute on a Cartesia-provided number. LLM usage is free for a limited time on agents created in its interface, and each plan includes prepaid agent dollars equal to its monthly fee (pricing).

The table below adds an illustrative Vyas cost to Sonex. It assumes an agent sends 20,000 input tokens and receives 1,000 output tokens per minute of conversation, which comes to about $0.0066 a minute at pay-as-you-go rates. Your own prompt size and turn rate will move this number, so swap in yours. The Cartesia column assumes its current free-LLM offer, which Cartesia describes as temporary.

When is Sonex Labs the better choice?

Choose Sonex Labs when your callers speak languages that Ink-2 can't transcribe or Sonic doesn't speak. That covers Tamil, Bengali or Telugu speech to text, and Bhojpuri, Marwari, Garhwali, Maithili or Bundeli text to speech, along with the rest of a 250+ language list. Collections teams and call centres running campaigns across several states or countries can keep every queue on one platform, with the same per-character price for each language.

It also suits teams that would rather pay per character and per second than size credit plans, and teams that bring their own telephony or LLM key and don't want a charge on either. Sonex agents also transfer live calls to a human agent, support MCP and tool calling, connect to HubSpot, Salesforce, n8n, Zapier and Make, record and transcribe calls, and run outbound campaigns with calling windows, time zones and retry limits. If you're also weighing a more expressive engine, see Sonex Labs vs ElevenLabs.

When is Cartesia the better choice?

Choose Cartesia when synthesis latency is the deciding factor and your languages are within Sonic's 44, as with English-first interruptible assistants. It also fits when procurement needs SOC 2 Type II, BAAs and dedicated regional deployments documented up front, when you want a published leaderboard position, or when your speech to text stays within Ink-2's five languages.

How should you run a bake-off?

Take 30 to 50 lines from real calls, with names, account numbers, dates and the language mixing your customers actually use. Shortlist Sonex voices on each language's playground page, then run the lines through Pāṇini and Sonic 3.6 in the format you will send to the phone network, play them over a live phone call, and have native speakers score the recordings blind. For speech to text, transcribe the same 50 real call recordings with Valmiki and Ink-2, in every language your callers use, and count errors on the entities that matter. Measure time to first audio and the full response time of the turn, p50 and p95, from your own region over a few hundred requests. Then price the result with the arithmetic above at your forecast volume. Most teams can finish this in two or three days, with results specific to their own traffic.