Vapi Integration

KugelAudio has a built-in Vapi custom TTS endpoint, so no proxy server is needed. Point your Vapi assistant at our API and you’re done.

Setup

1. Get your KugelAudio API key and a voice ID

API key: open the KugelAudio dashboard, then go to Settings → API Keys.
Voice ID: open the KugelAudio dashboard, go to Voices, pick a voice, and copy its numeric ID.

2. Configure your Vapi assistant

Vapi API (recommended)
Vapi Dashboard

Use PATCH /assistant/{id} to update an existing assistant, or include the voice field when creating a new one with POST /assistant:

{
  "voice": {
    "provider": "custom-voice",
    "server": {
      "url": "https://api.kugelaudio.com/vapi/synthesize?voice_id=YOUR_VOICE_ID",
      "secret": "YOUR_KUGELAUDIO_API_KEY",
      "timeoutSeconds": 30
    }
  }
}

voice_id: the numeric voice ID from Step 1.
server.secret: your KugelAudio API key. Vapi sends it as the x-vapi-secret header on every request, and KugelAudio authenticates it like any API key. This keeps the key out of the URL, which Vapi stores in the assistant config and may log.

Open the assistant

In the Vapi dashboard, open the assistant you want to use.

Go to Voice → Provider

Click Voice, then Provider.

Pick Custom Provider

Scroll down in the provider list and select Custom Provider.

Paste the URL

https://api.kugelaudio.com/vapi/synthesize?voice_id=YOUR_VOICE_ID&api_key=YOUR_KUGELAUDIO_API_KEY

Save

Save the assistant. The voice is now powered by KugelAudio.

The dashboard flow puts the API key in the URL. Prefer the API flow with server.secret for production assistants.

The “Voice ID” field inside Vapi’s UI has no effect when using a custom provider: the voice_id query param in the URL is what selects the voice.

No code, no extra server, no proxy.

To select the model explicitly, add &model_id=kugel-3 to the URL.

How it works

Vapi sends one POST per phrase to /vapi/synthesize:

{
  "message": {
    "type": "voice-request",
    "text": "Hello, how can I help you today?",
    "sampleRate": 24000
  }
}

KugelAudio streams back raw PCM16 at the requested sample rate, which is what Vapi expects. message.sampleRate must be one of 8000, 16000, 22050, 24000, or 44100; unsupported rates are rejected with 400.

Audio format

Parameter	Value
Format	Raw PCM (no WAV/container header)
Bit depth	16-bit signed, little-endian
Channels	1 (mono)
Sample rate	Matches `message.sampleRate` from Vapi’s request

Troubleshooting

401: authentication failed

Check the key in server.secret (or the api_key query parameter). For an eu- key, point the URL at https://api.eu.kugelaudio.com/vapi/synthesize and use the key without the eu- prefix: only the KugelAudio SDKs strip it for you.

400: unsupported sample rate

Vapi’s message.sampleRate must be 8000, 16000, 22050, 24000 or 44100. Change the assistant’s audio settings in Vapi.

The assistant speaks with the wrong voice

The voice comes from the voice_id query parameter in the server URL, not from Vapi’s Voice ID field. Check the ID in Voices.

Next steps

Voices

Find and pick a voice ID

Voice prompting

Prompt patterns that make an LLM sound good when spoken

Errors

Status codes and error bodies

Get started

Build a voice agent

Generate audio

Voices

Enhance audio

Hosting and deployment

SDK Reference

API Reference

Setup

1. Get your KugelAudio API key and a voice ID

2. Configure your Vapi assistant

How it works

Audio format

Troubleshooting

Next steps

Voices

Voice prompting

Errors

​Setup

​1. Get your KugelAudio API key and a voice ID

​2. Configure your Vapi assistant

​How it works

​Audio format

​Troubleshooting

​Next steps

Voices

Voice prompting

Errors

Setup

1. Get your KugelAudio API key and a voice ID

2. Configure your Vapi assistant

How it works

Audio format

Troubleshooting

Next steps