Tool parameters define what gets passed to your backend function when the tool is called. When creating a tool, parameters are defined as one of three types:
1

Dynamic

The model will choose which values to pass. These are the parameters you’d use for a single-generation LLM API.
2

Static

This value is known when the tool is defined and is unconditionally set on invocations. The parameter is not exposed to or set by the model.
3

Automatic

Like “Static”, except that the value may not be known when the tool is defined but will instead be populated by the system when the tool is invoked.

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Dynamic Parameters

Dynamic parameters will have their values set by the model. Creating a dynamic parameter on a tool looks like this:
Adding a dynamic parameter to a tool
// Adding a dynamic parameter to a stock price tool
// The parameter will be named 'symbol' and will be passed as a query parameter
{
  "name": "stock_price",
  "description": "Get the current stock price for a given symbol",
  "dynamicParameters": [
    {
      "name": "symbol",
      "location": "PARAMETER_LOCATION_QUERY",
      "schema": {
        "type": "string",
        "description": "Stock symbol (e.g., AAPL for Apple Inc.)"
      },
      "required": true
    }
  ]
}

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Parameter Overrides

You can choose to set static values for dynamic parameters when you create an agent or start a call. The model won’t see any parameters that you override. When creating a call simply pass in the overrides with each tool, as below. You should also consider overriding the tool name or description to give the model a more specific understanding of what the tool will do in this case.
Overriding a dynamic parameter with a static value
// Overriding dynamic parameter when starting a new call
// Always set the stock symbol to 'NVDA'
{
  "systemPrompt": ...
  "selectedTools": [
    "toolName": "stock_price",
    "nameOverride": "nvidia_stock_price",
    "descriptionOverride": "Looks up the current stock price for Nvidia."
    "parameterOverrides": {
      "symbol": "NVDA"
    }
  ]
}

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Static Parameters

If you have parameters that are known at the time you create the tool, static parameters can be used. Static parameters are not exposed to or set by the LLM.
Adding a static parameter to a tool
// Adding a static parameter that always sends utm=ultravox
{
  "name": "stock_price",
  "description": "Get the current stock price for a given symbol",
  "staticParameters": [
    {
      "name": "utm",
      "location": "PARAMETER_LOCATION_QUERY",
      "value": "ultravox"
    }
  ]
}

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Parameter Overrides

Static parameters can also be overridden when you create an agent or start a call. This is most useful with built-in tools. For example, the built-in queryCorpus tool allows you to statically override max_results. See queryCorpus Tool → for more.

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Automatic Parameters

Automatic parameters are used when you want a consistent, predictable value (not generated by the model) but you don’t know the value when the tool is created. Here are some of the most common automatic parameters:
knownValueDescription
KNOWN_PARAM_CALL_IDUsed for sending the current Ultravox call ID to the tool.
KNOWN_PARAM_CONVERSATION_HISTORYIncludes the full conversation history leading up to this tool call. Typically should be in the body of a request.
KNOWN_PARAM_CALL_STATEIncludes arbitrary state previously set by tools. See Guiding Agents.
More details can be found in the Tool Definition Schema →
Adding an automatic parameter to a tool
// Adding automatic parameters to a profile creation tool
// There are two parameters added:
// 'call_id' which is sent as a query param
// 'conversation_history' which is sent in the request body
{
  "name": "create_profile",
  "description": "Creates a profile for the current caller",
  "automaticParameters": [
    {
      "name": "call_id",
      "location": "PARAMETER_LOCATION_QUERY",
      "knownValue": "KNOWN_PARAM_CALL_ID"
    },
    {
      "name": "conversation_history",
      "location": "PARAMETER_LOCATION_BODY",
      "knownValue": "KNOWN_PARAM_CONVERSATION_HISTORY"
    }
  ]
}

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Required Parameter Overrides

Sometimes your tool will require a parameter to function that you need to have defined when the call is created instead of having the model come up with a value. In these cases, you can require that the parameter be overridden at call creation. For example, the built-in queryCorpus tool requires the corpus id to be specified during call creation. More advanced information can be found in Parameter Overrides →