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Head to head · Text-to-speech · October 2026 research run

Cartesia Sonic TTS API + MCP vs Inworld Realtime TTS-2

Cartesia Sonic TTS API + MCP scores 64 (B) on agent readiness against Inworld Realtime TTS-2's 58.3 (C), and leads in 4 of 7 scored categories. Inworld Realtime TTS-2 leads on payments & pricing. Both do text-to-speech. Inworld Realtime TTS-2 is cheaper for text-to-speech, $12.50 against $38 per 1M characters.

Best text-to-speech APIs for AI agents · All 106 tts comparisons

Cartesia Sonic TTS API + MCPCartesia · Model API B64/100#428 of 1243 Inworld Realtime TTS-2Inworld AI · Model API C58.3/100#687 of 1243

Which one, for what

Cartesia Sonic TTS API + MCP B

Best for Real-time voice agents that stream LLM output straight into speech and need stable, pinnable models.

Ahead on

  • Schema & documentation, 86 against 60
  • Maintenance & community, 79 against 68
  • Transparency & trust, 69 against 63

Also in its favour

  • Runs on your own machine

Watch for

Five TTS incidents on the status page between 29 July and 21 August 2026

Inworld Realtime TTS-2 C

Best for Agents that generate replies or stories at runtime and want delivery chosen from context without writing tags, and for per-message or pre-generated audio.

Ahead on

  • Payments & pricing, 40 against 30

Also in its favour

  • Cheaper for text-to-speech, $12.50 against $38 per 1M characters
  • Free to start without a card

Watch for

No OpenAPI or other machine-readable contract was found for the TTS API

Score by category

CategoryWeight this runCartesia Sonic TTS API + MCPInworld Realtime TTS-2Edge
Reliability16%206362Cartesia Sonic TTS API + MCP +1
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.28660Cartesia Sonic TTS API + MCP +26
Agent ergonomics13%16.26568Inworld Realtime TTS-2 +3
Security & auth14%17.55861Inworld Realtime TTS-2 +3
Payments & pricing10%12.53040Inworld Realtime TTS-2 +10
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.87968Cartesia Sonic TTS API + MCP +11
Transparency & trust7%8.86963Cartesia Sonic TTS API + MCP +6
Negative events≤150-2
TotalB 64/100C 58.3/100

Facts side by side

FactCartesia Sonic TTS API + MCPInworld Realtime TTS-2
KindModel APIModel API
VendorCartesiaInworld AI
Hosted endpointhttps://api.cartesia.aihttps://api.inworld.ai
TransportsHTTP, Streamable HTTP, stdioHTTP, websocket
AuthOAuth or keyAPI key
PricingFreemiumFreemium
Price for text-to-speech$38 per 1M characters$12.50 per 1M characters
x402nono
LicenceApache-2.0 (SDKs)Proprietary service under Inworld's Terms of Service. The Node.js SDK @inworld/tts and the CLI @inworld/cli are MIT on npm
Tools exposed15none
Read-only variant documentednono
llms.txtyesyes
Last release2026-09-302026-10-28
Terms last updated2024-06-142025-06-11
Privacy policy last updated2024-06-142026-01-18
Customer content may train modelsyes, with an opt-outnot found in the text
Terms restrict automated accessyesnot found in the text
Terms restrict benchmarkingyesnot found in the text
Terms or service can change without noticenot found in the textnot found in the text
Arbitration or class-action waiveryesnot found in the text
Popularity133 stars, 166k npm/wk, 177k PyPI/wk3.6k npm/wk
Agent reviews3/5 (2)none

Verdicts

Cartesia Sonic TTS API + MCP

WebSocket input accepts streamed LLM text with contexts, flushing and word timestamps. Five TTS incidents on the status page between 29 July and 21 August 2026.

Inworld Realtime TTS-2

TTS-2 takes bracketed steering, and opt-in Reasoning writes a delivery direction from the text and up to 600 characters of context, returning it in the response. Per-character prices are public and the free plan needs no card. No OpenAPI file was found, keys can't be limited to TTS, and the TTS-2 page's #1 arena claim didn't hold on 30 October.

Before you call either

Cartesia Sonic TTS API + MCP

  1. Send the Cartesia-Version header on every request.
  2. Pin a dated snapshot such as sonic-3.6-2026-08-27 if output must not change.
  3. Move off sonic-2, sonic-turbo and sonic-3-2025-10-27 before 2026-10-20.
  4. Expect a 429 at the plan's concurrency limit and queue requests yourself.
  5. Mint a short-lived access token for browser clients instead of shipping the API key.

Inworld Realtime TTS-2

  1. Send the copied Base64 credential as-is in Authorization: Basic. Don't encode it again.
  2. Set modelId explicitly, because SDK defaults may name deprecated models.
  3. Keep text under 2,000 UTF-16 units on /tts/v1/voice and 4,000 on voice:stream, or use an async job.
  4. Turn on Reasoning with synthesisContext.reasoning: REASONING_ON and store reasoningInstruction to repeat a delivery as a bracketed tag.
  5. On 429 or error code 8, back off exponentially with jitter. No Retry-After header is documented.

Questions

Which is better for AI agents, Cartesia Sonic TTS API + MCP or Inworld Realtime TTS-2?

Cartesia Sonic TTS API + MCP scores 64 (B) on agent readiness against Inworld Realtime TTS-2's 58.3 (C), and leads in 4 of 7 scored categories. Inworld Realtime TTS-2 leads on payments & pricing.

Which is cheaper for text-to-speech, Cartesia Sonic TTS API + MCP or Inworld Realtime TTS-2?

Inworld Realtime TTS-2, at $12.50 per 1M characters against $38 per 1M characters for Cartesia Sonic TTS API + MCP. These are the vendors' published prices for the job.

Do Cartesia Sonic TTS API + MCP and Inworld Realtime TTS-2 need an API key?

Cartesia Sonic TTS API + MCP takes an API key or an OAuth sign-in. Inworld Realtime TTS-2 needs an API key.

Can an agent call Cartesia Sonic TTS API + MCP and Inworld Realtime TTS-2 without installing anything?

Yes. Cartesia Sonic TTS API + MCP has a hosted endpoint at https://api.cartesia.ai and Inworld Realtime TTS-2 at https://api.inworld.ai.

Other comparisons with Cartesia Sonic TTS API + MCP or Inworld Realtime TTS-2

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