credentials.yml with tts: name: cartesia and CARTESIA_API_KEY per Rasa’s speech-integrations reference. Start with their walkthrough, then use the reference for parameters (model_id, voice, multilingual language_map, etc.):
Tutorial: Build a Voice Agent with Rasa and Cartesia
Full tutorial for building a voice agent with Rasa and Cartesia.
Rasa > Docs > Speech integrations (Cartesia)
Rasa reference for Cartesia TTS in voice channels.