What's free
20,000 credits/month (~27 min of Sonic TTS or ~1h51 of Ink speech-to-text)
Rate limits
2 concurrent TTS requests, 8 concurrent STT; 20,000 credits/month
The catch
Commercial use is NOT allowed on the free plan (the $5/mo Pro plan adds a commercial licence). Low-latency streaming voice over WebSocket/REST; own API (not OpenAI-shaped).