Cartesia Sonic TTS API + MCP vs Fish Audio TTS API

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.

Fish Audio TTS API

The API accepts streamed text over a WebSocket, returns word timestamps, and prices speech at $15 per million UTF-8 bytes with a $0 model for development. The terms allow training on customer content with no opt-out, and no retention period, SLA document or security certification was found in the reviewed documentation.

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.

Fish Audio TTS API

1. Send the model header on every request and check its spelling. A missing or unknown value is served and billed as s2.1-pro.
2. Budget by UTF-8 bytes, not characters. Chinese, Japanese and Korean text costs about three bytes a character.
3. Retry 429 and 5xx with exponential backoff. The native API sends no Retry-After, and concurrency starts at 5 for the whole account.
4. Use [bracket] cues on the S2 family. (parenthesis) tags from s1 are read aloud as text.
5. Pass the model explicitly in the JavaScript SDK, because npm release 0.1.0 defaults to s1.