What's free
$200 free credit on signup (no card, no expiration) — Nova speech-to-text and Aura text-to-speech at pay-as-you-go rates
Rate limits
STT pre-recorded up to 50 concurrent; STT streaming up to 150; TTS REST up to 15; TTS streaming up to 45
The catch
No card and no expiration on the credit. Data catch: the Model Improvement Program is opt-OUT — send mip_opt_out=true per request to keep your data out of training. Native REST/WebSocket API, not OpenAI-compatible.