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

Cartesia Sonic TTS API + MCP vs HeyGen Voice

HeyGen Voice scores 71.3 (BB) on agent readiness against Cartesia Sonic TTS API + MCP's 63.8 (B), and leads in 5 of 7 scored categories. Cartesia Sonic TTS API + MCP leads on maintenance & community. Both do text-to-speech.

Best text-to-speech APIs for AI agents · All 66 tts comparisons

Cartesia Sonic TTS API + MCPCartesia · Model API B63.8/100#375 of 961 HeyGen VoiceHeyGen · Model API BB71.3/100#131 of 961

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

  • Maintenance & community, 79 against 65

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

HeyGen Voice BB

Best for Speech in a voice cloned from one short recording, for agents that already make HeyGen videos or need a cloned narrator with streaming and timestamps.

Ahead on

  • Reliability, 75 against 63
  • Schema & documentation, 94 against 86
  • Agent ergonomics, 83 against 65
  • Security & auth, 69 against 58

Also in its favour

  • Agent-ready, a grade of BB or better

Watch for

Speaks only voices cloned in the caller's workspace. Stock and designed voices go through a separate endpoint and engines

Score by category

CategoryWeight this runCartesia Sonic TTS API + MCPHeyGen VoiceEdge
Reliability16%206375HeyGen Voice +12
Performance10%pendingpendingpendingnot scored in this run
Schema & documentation13%16.28694HeyGen Voice +8
Agent ergonomics13%16.26583HeyGen Voice +18
Security & auth14%17.55869HeyGen Voice +11
Payments & pricing10%12.53030even
Task success10%pendingpendingpendingnot scored in this run
Maintenance & community7%8.87965Cartesia Sonic TTS API + MCP +14
Transparency & trust7%8.86769HeyGen Voice +2
Negative events≤1500
TotalB 63.8/100BB 71.3/100

Facts side by side

FactCartesia Sonic TTS API + MCPHeyGen Voice
KindModel APIModel API
VendorCartesiaHeyGen
Hosted endpointhttps://api.cartesia.aihttps://api.heygen.com
TransportsHTTP, Streamable HTTP, stdioHTTP
AuthOAuth or keyAPI key
PricingFreemiumPay per use
Price for text-to-speech$38 per 1M charactersnot published
x402nono
LicenceApache-2.0 (SDKs)Closed service under HeyGen's Terms of Service (HeyGen Technology, Inc., last updated 23 July 2026).
Tools exposed15none
Read-only variant documentednono
llms.txtyesyes
Last release2026-09-30none
Terms last updated2024-06-14
Privacy policy last updated2024-06-14
Customer content may train modelsyes, with an opt-out
Terms restrict automated accessyes
Terms restrict benchmarkingyes
Terms or service can change without noticenot found in the text
Arbitration or class-action waiveryes
Popularity133 stars, 166k npm/wk, 177k PyPI/wknone
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.

HeyGen Voice

HeyGen Voice has a typed OpenAPI contract, streaming with character timestamps, and a published price of $15 per million characters for instant voices, with failed requests not charged. It speaks only voices cloned in the caller's workspace, allows 30 requests a minute, and HeyGen may train on non-enterprise input unless the customer opts out by email.

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.

HeyGen Voice

  1. Create a voice first with POST /v3/models/audio/voices and mode, then poll GET /v3/models/audio/voices/{voice_id} until status is ACTIVE before calling speech
  2. Send expressiveness_boost only for instant voices and seed, speed, pitch_shift, pitch_variance or <break> tags only for professional voices. Mixing them returns 400 invalid_parameter
  3. On the stream, decode and play each base64 WAV part in part_index order. Do not concatenate the bytes
  4. Send an Idempotency-Key when creating a voice, and retry 502, 503 and 504 on speech, which the OpenAPI file marks as safe
  5. Use POST /v3/voices/speech for stock or designed voices. It is a different engine and price

Questions

Which is better for AI agents, Cartesia Sonic TTS API + MCP or HeyGen Voice?

HeyGen Voice scores 71.3 (BB) on agent readiness against Cartesia Sonic TTS API + MCP's 63.8 (B), and leads in 5 of 7 scored categories. Cartesia Sonic TTS API + MCP leads on maintenance & community.

Do Cartesia Sonic TTS API + MCP and HeyGen Voice need an API key?

Cartesia Sonic TTS API + MCP takes an API key or an OAuth sign-in. HeyGen Voice needs an API key.

Can an agent call Cartesia Sonic TTS API + MCP and HeyGen Voice without installing anything?

Yes. Cartesia Sonic TTS API + MCP has a hosted endpoint at https://api.cartesia.ai and HeyGen Voice at https://api.heygen.com.

Other comparisons with Cartesia Sonic TTS API + MCP or HeyGen Voice

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