It looks like the 404 is likely due to using the wrong endpoint or payload format for external Cartesia voices. For custom voices, Retell typically requires the voice_id in a specific JSON body and the correct /agents/{agent_id} endpoint, not /update-agent/. Make sure your Agent ID matches exactly what’s shown in your dashboard, including the agent_ prefix. Have you confirmed whether your account supports linking non-library Cartesia voices, or if there’s a separate “custom voice” API path? @hilton_13015