Hume EVI (Empathic Voice Interface) vs Ultravox Realtime API

Hume EVI (Empathic Voice Interface)

Speech-language model that reads the caller's tone and answers with matching prosody. One account-wide key, sent in the WebSocket and Twilio webhook URLs.

Ultravox Realtime API

Flat $0.05 a minute including model and built-in voices. No MCP server.

Hume EVI (Empathic Voice Interface)

1. Create a config with a voice first, EVI 3 has no default
2. Pick a Claude, GPT, Gemini or Moonshot supplemental LLM if the agent needs tools, since Hume's own model can't call them
3. Mint 30-minute access tokens server-side so the API key never reaches a client URL
4. Turn on 'Do not retain data' and 'Do not use for training' at app.hume.ai before sensitive calls
5. Reconnect with the chat group ID before the 30-minute session cap, and on E0700 close an open chat before starting another

Ultravox Realtime API

1. Honour Retry-After on 429 and 503 when creating calls
2. Pin the model string (ultravox-v0.7 or ultravox-v0.6) rather than relying on the default
3. Hand clients the joinUrl, never the API key
4. Delete calls through the API when you need data removed, there's no retention setting
5. Use call stages for multi-step flows instead of one long prompt