Function Call Request

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.

Purpose

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.

Handling the message

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.

Turn confirmation

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.

Example payloads

Client-side function

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

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

The thought_signature field is optional. Certain Gemini models include it as an additional function call identifier. See Gemini Docs.

Server-side function

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

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

Fields

Related messages

• 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.