# April 1, 2026

## New `thought_signature` field for Gemini function calling

The Voice Agent API now includes an optional `thought_signature` field in function call messages. Some Gemini models (3.0 and 3.1 families) require this as an additional function call identifier.

This field appears in two places:

- **Settings message** — in `agent.context.messages[].function_calls[]` when providing function call history
- **FunctionCallRequest** — in `functions[]` when the server requests a function call

### Example

```json
{
  "type": "FunctionCallRequest",
  "functions": [
    {
      "id": "fc_12345678-90ab-cdef-1234-567890abcdef",
      "name": "get_weather",
      "arguments": "{\"location\": \"Fremont, CA 94539\"}",
      "client_side": true,
      "thought_signature": "abc123"
    }
  ]
}
```

The `thought_signature` field is optional and only relevant when using Google Gemini models. This change addresses the degraded function calling performance that some users experienced with the Gemini 3.0 and 3.1 model families.

For more details, see the [Function Call Request](/guides/self-hosted-deployments-3-voice-agent-function-call-request) documentation, the [Voice Agent API Reference](/guides/self-hosted-deployments-2-reference-voice-agent-voice-agent), or [Gemini's Thought Signatures Documentation](https://ai.google.dev/gemini-api/docs/thought-signatures).

## New `volume` parameter for Cartesia TTS

The Voice Agent API now supports an optional `agent.speak.provider.volume` parameter when using Cartesia as the TTS provider. Valid values range from `0.5` to `2.0`.

For more details, see [Configure the Voice Agent](/guides/self-hosted-deployments-3-configure-voice-agent) or the [Cartesia volume, speed, and emotion documentation](https://docs.cartesia.ai/build-with-cartesia/sonic-3/volume-speed-emotion#volume-speed-and-emotion).

## Related pages

- [Amazon SageMaker](./amazon-sagemaker-index.md)
- [Aura](./aura-index.md)
- [Changelog](../changelog.md)
- [Custom Vocabulary](./custom-vocabulary-index.md)
- [Deepgram's Docs](../index.md)
- [Deployment](./deployment-index.md)
- [Docker/Podman](./docker-podman-index.md)
- [Features](./features-index.md)
- [Flux TTS](./flux-tts-index.md)
- [Formatting](./formatting-index.md)

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