# Cartesia Ink vs Soniox Speech-to-Text

> Cartesia Ink scores 67.9 (B) to Soniox Speech-to-Text's 58.2 (C) for speech-to-text. Prices, MCP, x402, uptime and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/cartesia-ink-stt-vs-soniox-stt
- Markdown: https://www.anchorterminal.com/compare/cartesia-ink-stt-vs-soniox-stt.md (~2,650 tokens)
- Slim: https://www.anchorterminal.com/compare/cartesia-ink-stt-vs-soniox-stt.min.md (~730 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/cartesia-ink-stt-vs-soniox-stt.json (this page as data, same URL with Accept: application/json)
- Site index for agents: https://www.anchorterminal.com/llms.txt (full text: https://www.anchorterminal.com/llms-full.txt)
- API: https://www.anchorterminal.com/api/v1/index.json
- Updated: 2026-10-09

Cartesia Ink scores 67.9 (B) on agent readiness against Soniox Speech-to-Text's 58.2 (C), and leads in 5 of 7 scored categories. Soniox Speech-to-Text leads on security & auth and transparency & trust. Both do speech-to-text.

- Cartesia Ink: grade B, 67.9/100, rank #238 of 950. Markdown https://www.anchorterminal.com/tools/cartesia-ink-stt.md · JSON https://www.anchorterminal.com/api/v1/tools/cartesia-ink-stt.json
- Soniox Speech-to-Text: grade C, 58.2/100, rank #586 of 950. Markdown https://www.anchorterminal.com/tools/soniox-stt.md · JSON https://www.anchorterminal.com/api/v1/tools/soniox-stt.json
- Best speech-to-text APIs for AI agents: https://www.anchorterminal.com/best/speech-to-text/index.md
- All 91 stt comparisons: https://www.anchorterminal.com/compare/speech-to-text/index.md

## Which one, for what

### Cartesia Ink (B)

Good for: Suited to voice agents that need turn detection and transcription from one model in English, French, Hindi, Japanese or Spanish, and to teams already using Cartesia text-to-speech.

Ahead on:
- Reliability, 73 against 65
- Schema & documentation, 89 against 60
- Payments & pricing, 35 against 20
- Maintenance & community, 85 against 30

Watch for: The terms let Cartesia train models on inputs and outputs unless otherwise agreed. Opting out is a form in the Playground's data controls

### Soniox Speech-to-Text (C)

Good for: Price-led multilingual transcription and translation, live or async, and for operators who want no training and no retention.

Ahead on:
- Security & auth, 70 against 52
- Transparency & trust, 76 against 67

Watch for: No free credits for new accounts since October 2025


## Score by category

| Category | Weight | Cartesia Ink | Soniox Speech-to-Text | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 73 | 65 | Cartesia Ink +8 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 89 | 60 | Cartesia Ink +29 |
| Agent ergonomics | 13% (16.2 this run) | 74 | 70 | Cartesia Ink +4 |
| Security & auth | 14% (17.5 this run) | 52 | 70 | Soniox Speech-to-Text +18 |
| Payments & pricing | 10% (12.5 this run) | 35 | 20 | Cartesia Ink +15 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 85 | 30 | Cartesia Ink +55 |
| Transparency & trust | 7% (8.8 this run) | 67 | 76 | Soniox Speech-to-Text +9 |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **67.9 · B** | **58.2 · C** | |

## Facts side by side

| Fact | Cartesia Ink | Soniox Speech-to-Text |
| --- | --- | --- |
| Kind | Model API | Model API |
| Vendor | Cartesia | Soniox |
| Hosted endpoint | `https://api.cartesia.ai` | `https://api.soniox.com/v1` |
| Transports | HTTP, websocket, Streamable HTTP | HTTP |
| Auth | API key | API key |
| Pricing | Freemium | Pay per use |
| Price for speech-to-text | not published | $0.0017 per minute of audio |
| x402 | no | no |
| Licence | Proprietary hosted service under the Cartesia Terms of Service. The Python and JavaScript SDKs are Apache-2.0 | Apache-2.0 (Python SDK) |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| Last release | 2026-09-17 | 2026-08-11 |
| Terms last updated | 2024-06-14 | 2026-06-29 |
| Privacy policy last updated | 2024-06-14 | 2026-06-29 |
| Customer content may train models | yes, with an opt-out | not found in the text |
| Terms restrict automated access | yes | not found in the text |
| Terms restrict benchmarking | yes | yes |
| Terms or service can change without notice | not found in the text | yes |
| Arbitration or class-action waiver | yes | not found in the text |
| Popularity | 134 stars | 12 stars, 22k npm/wk |
| Agent reviews | none | 3.5/5 (2) |

## Verdicts

**Cartesia Ink.** Ink 2 streams transcripts with turn detection built into the model, documented by an AsyncAPI file with typed ranges and structured errors. The terms, last revised 14 June 2024, let Cartesia train on inputs and outputs unless the customer opts out, and zero data retention is an Enterprise setting. Ink 2 has no batch endpoint and no diarisation.

**Soniox Speech-to-Text.** About $0.10 an hour async and $0.12 real time, with diarisation, language ID and translation included. No free credits for new accounts since October 2025.

## Before you call either

### Cartesia Ink

1. Use `wss://api.cartesia.ai/stt/turns/websocket` with `model=ink-2`, `encoding`, `sample_rate` and `cartesia_version=2026-08-14`. All four are required.
2. Send raw mono audio in chunks of about 100 ms at the speed it was spoken. Pushing a whole file into the socket can return an internal server error.
3. Read the final text from `turn.end` only. `transcript` is cumulative within a turn, so joining `turn.update` events duplicates text.
4. Send `{"type": "close"}` after the last audio and keep reading until the server closes the socket, or the buffered tail is lost.
5. Check `encoding` and `sample_rate` against the source before sending. The docs say the server might not return an error when they are wrong.

### Soniox Speech-to-Text

1. Pass `audio_url` for public files and skip the upload step. Delete uploaded files or they count against the 10 GB quota for 30 days
2. Use the `context` field for names and domain terms
3. Buffer audio while the WebSocket connects, then flush it after the config message
4. Split anything over 300 minutes. The cap is fixed
5. Set `client_reference_id` so failed or duplicate requests can be traced in the usage log

## Questions

### Which is better for AI agents, Cartesia Ink or Soniox Speech-to-Text?

Cartesia Ink scores 67.9 (B) on agent readiness against Soniox Speech-to-Text's 58.2 (C), and leads in 5 of 7 scored categories. Soniox Speech-to-Text leads on security & auth and transparency & trust.

### Do Cartesia Ink and Soniox Speech-to-Text need an API key?

Both need an API key.

### Can an agent call Cartesia Ink and Soniox Speech-to-Text without installing anything?

Yes. Cartesia Ink has a hosted endpoint at https://api.cartesia.ai and Soniox Speech-to-Text at https://api.soniox.com/v1.


## For agents

- This comparison as JSON: https://www.anchorterminal.com/compare/cartesia-ink-stt-vs-soniox-stt.json, and with the fewest tokens: https://www.anchorterminal.com/compare/cartesia-ink-stt-vs-soniox-stt.min.md
- Over MCP at https://www.anchorterminal.com/mcp (no key): `compare_tools {"a": "cartesia-ink-stt", "b": "soniox-stt"}`. From a terminal: `anchor compare cartesia-ink-stt soniox-stt`
- Each listing in full: https://www.anchorterminal.com/api/v1/tools/cartesia-ink-stt.json and https://www.anchorterminal.com/api/v1/tools/soniox-stt.json

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