Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

​ To learn more, see the sources I considered: [[1] Issue when sending

voice_model

parameter

elevenlabs_turbo_v2_5

](https://discord.com/channels/1186424376300679209/1367836412778512545) [[2] Update Voice Agent](https://docs.retellai.com/api-references/update-agent) ​

### Answer --- This error occurs because the agent previously had an ElevenLabs voice model set (

eleven_turbo_v2_5

) , and sending

null

via API doesn't reset it the same way the dashboard does. [(1) ]() When switching to a Cartesia voice (

cartesia-Mia

) , the incompatible ElevenLabs model persists. Workaround: You need to reset the voice settings in a separate API call before changing the voice ID. This ensures the voice model is cleared first, then you can set the new voice. The available

voice_model

options are provider-specific: [(2) ]() - ElevenLabs:

eleven_turbo_v2

,

eleven_flash_v2

,

eleven_turbo_v2_5

,

eleven_flash_v2_5

,

eleven_multilingual_v2

- Cartesia:

sonic-2

,

sonic-3

,

sonic-turbo

- OpenAI:

tts-1

,

gpt-4o-mini-tts

For a Cartesia voice, try explicitly setting a compatible Cartesia model like

sonic-2

or

sonic-turbo

instead of

null

.