Function Call Request

Voice Agent

The Voice Agent server sends FunctionCallRequest to request a function call. The client_side flag determines whether the server executes the function or expects the client to.

This message is used to trigger either a built-in server-side function or a custom function defined by the client.

• When client_side is false, the server will handle the function using built-in logic.
• When client_side is true, the client must handle the function and respond with a FunctionCallResponse.
• The optional thought_signature field may be present when using certain Gemini models that require an additional function call identifier. See Gemini Docs for details.

A request can arrive before the user’s turn is confirmed. See Turn confirmation below.

When your client receives a FunctionCallRequest:

1. Check the client_side field.
2. If it’s true, call the appropriate client-defined function.
3. Return a FunctionCallResponse message with the function result.
4. If it’s false, no client action is needed; the server will handle it internally.

The agent begins building a reply before speech-to-text confirms the user has finished speaking, so a FunctionCallRequest can reach you inside that speculative window. If the user keeps speaking, the turn resumes and the call is cancelled.

• By default, a call is dispatched as soon as the LLM emits it. You may receive a FunctionCallCancelled for it, in which case you should stop work on that id and send no response.
• To stop a function from being requested speculatively at all, set defer_until_eot to true on its definition in Settings. The request is then only sent once the turn is confirmed.

defer_until_eot is a Settings property, not a field on this message. Requests carry only the fields listed below.

For the behavior in full, see Speculative Replies & Turn Confirmation.

The server asks the client to execute get_weather and reply with a FunctionCallResponse.

{
  "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. Certain Gemini models include it as an additional function call identifier. See Gemini Docs.

The server executes the function internally and notifies the client. The client takes no action.

{
  "type": "FunctionCallRequest",
  "functions": [
    {
      "id": "fc_aabbccdd-eeff-0011-2233-445566778899",
      "name": "end_call",
      "arguments": "{\"reason\": \"completed\"}",
      "client_side": false
    }
  ]
}

• FunctionCallResponse: The expected response from the client when client_side is true.
• FunctionCallCancelled: Sent when a request you already received is cancelled because the user started speaking again, either inside the speculative window or after the turn was confirmed.