Cartesia Sonic TTS API + MCP vs ElevenLabs Text to Speech API + MCP

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.

ElevenLabs Text to Speech API + MCP

Keys can be limited to chosen endpoints and given a credit quota, and service accounts hold keys that don't belong to a person. Content may be used for training unless you opt out under Data use, and the opt-out only applies going forward.

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.

ElevenLabs Text to Speech API + MCP

1. Call Eleven v4 through Text to Dialogue, not /v1/text-to-speech.
2. Spell out numbers yourself on eleven_flash_v2_5, which doesn't normalise them by default, and turning that on is Enterprise only.
3. On 429 read code, back off on rate_limit_exceeded and wait for running requests on concurrent_limit_exceeded.
4. Keep each request under 10,000 characters on v4 and Multilingual v2, 5,000 on v3.
5. Give the agent a key scoped to Text to Speech with a credit quota.