### Answer Yes, dynamic variables are supported in functions and tools. You can use dynamic variables in custom function configurations, including in fields like Event Type ID. [(1) ]() [(2) ]() Dynamic variables work in several places within tool configurations: [(1) ]() - Custom function URLs - Tool descriptions - Property descriptions - Request headers (can be static or include dynamic variables) [(2) ]() - Query parameters (both description and const value can be dynamic variables) For your Cal.com use case, you can define a dynamic variable like

{{event_type_id}}

in the Event Type ID field of your Check Calendar Availability function. [(1) ]() Then pass the actual value when creating the call through the

retell_llm_dynamic_variables

field in the API request. For example, when making an outbound call, you'd include:

json
{
  "retell_llm_dynamic_variables": {
    "event_type_id": "your_event_type_value"
  }
}

All values in

retell_llm_dynamic_variables

must be strings. Regarding MCP tools specifically, the documentation confirms you can set query parameters for MCP configurations, [(3) ]() which would support dynamic variable usage in that context as well.

​ To learn more, see the sources I considered: [[1] Dynamic Variables](https://docs.retellai.com/build/dynamic-variables) [[2] Integrate any system with custom function](https://docs.retellai.com/build/single-multi-prompt/custom-function) [[3] MCP](https://docs.retellai.com/build/single-multi-prompt/mcp) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.