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

### Answer --- Yes, system default dynamic variables can be used in custom function parameters. Dynamic variables work in tool configurations including custom function URLs, tool descriptions, and property descriptions. [(1) ]() For the parameters field specifically, you can use the

"const"

property to inject dynamic variables. For example: [(2) ]()

json
{
  "type": "object",
  "properties": {
    "last_name": {
      "type": "string",
      "const": "{{last_name}}"
    }
  }
}

System default variables like

{{user_number}}

,

{{call_id}}

,

{{direction}}

,

{{current_time}}

, etc. can be used this way. [(1) ]() When to use each approach: - Use system default variables directly when you need built-in call context (like

{{user_number}}

for the caller's phone number on phone calls) . - Use the Extract Dynamic Variable tool when you need to capture information from the conversation that the LLM extracts from user speech (like a phone number the user verbally provides that differs from their caller ID) . [(3) ]() One important note: system variables like

{{user_number}}

only have values in the appropriate context—for example,

{{user_number}}

won't have a value during web call testing since there's no phone number involved.

​ To learn more, see the sources I considered: [[1] Dynamic Variables](https://docs.retellai.com/build/dynamic-variables) [[2] Custom Function](https://docs.retellai.com/build/conversation-flow/custom-function) [[3] Custom Function Not substituting variable](https://discord.com/channels/1186424376300679209/1450867178906714123) ​