April 1, 2026

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

{
  "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 documentation, the Voice Agent API Reference, or Gemini's Thought Signatures Documentation.

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 or the Cartesia volume, speed, and emotion documentation.