Hello, I am using custom TTS in assistant creation as mentioned here in the docs https://docs.vapi.ai/customization/custom-voices/custom-tts. While creating the assistant and testing the agent for the first time, it works; then, after that, It starts showing this error. There is no way to bypass this, as even if I try to update throught api, it will still appear, and once I click on publish agent, it will also append the Sarah voice id Please let me know how to fix that or if I am missing something. https://cdn.discordapp.com/attachments/1462447735696261202/1462447736136925366/Screenshot_2026-01-15_at_3.06.49_PM.png?ex=69796ed7&is=69781d57&hm=618a27522f75f9c1fb651642ea7ef2c9a81b7a80c6117b9e98d4d98a62850805& https://cdn.discordapp.com/attachments/1462447735696261202/1462447736912744750/Screenshot_2026-01-15_at_3.07.42_PM.png?ex=69796ed7&is=69781d57&hm=65639961875834880f17bd7a57348181934a0f069631f3535d14b03bcfbdf1d7& https://cdn.discordapp.com/attachments/1462447735696261202/1462447737826971658/Screenshot_2026-01-15_at_3.08.59_PM.png?ex=69796ed7&is=69781d57&hm=4fee06cd9457f36044bc96e7f90a36f0f14edc60d0e5ba6997df5eb807941a8c&

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To fix the error, ensure your assistant's voice configuration uses "provider": "custom-voice" and does not include a

voiceId

. Here’s the correct structure:

json
{
  "voice": {
    "provider": "custom-voice",
    "server": {
      "url": "https://your-tts-endpoint.com/api/synthesize",
      "secret": "your-webhook-secret",
      "timeoutSeconds": 45
    }
  }
}

Remove any

voiceId

property from your configuration before publishing. If the UI keeps adding a

voiceId

, try updating via the API with this structure and contact support if the issue persists. (Reference: [Vapi custom TTS documentation](https://docs.vapi.ai/customization/custom-voices/custom-tts)) Source: - [Vapi custom TTS documentation](https://docs.vapi.ai/customization/custom-voices/custom-tts)

i did the same when doing the integration but got the exact same error

{
  "voice": {
    "provider": "custom-voice",
    "server": {
      "url": "https://your-tts-endpoint.com/api/synthesize",
      "secret": "your-webhook-secret",
      "timeoutSeconds": 45
    }
  }
}

Hey, yeah, this is a known Vapi UI bug where it silently reverts to a default provider (Sarah) after first publish, especially with custom-voice configs. I’ve fixed this before by forcing the provider + voice lock via API and disabling UI overrides. I can help you patch this properly so it persists across saves and deploys. Quick question: are you creating the assistant via dashboard first or API-first workflow? @isoHunter

i am creating it with API

Got it, since you’re creating via API, this isn’t on your side. It’s a Vapi persistence bug where custom voices get overridden after save/publish. I’ve helped fix this exact issue by locking the voice config so the platform can’t inject Sarah again. Happy to walk you through it or just fix it for you. Want to connect privately and I’ll take a look at your setup? @isoHunter

Sure yes

Hi @isoHunter, Thank you for providing all the details. I managed to reproduce this issue and it appears to be a bug. This bug appears when custom TTS is selected. I have reported this to the team. And while they review this issue, I recommend using the API to make any updates to this particular assistant. We are sorry for any inconvenience that might have caused you.