### Answer --- Based on the available documentation, dynamic variables like

{{call_id}}

can be used in tool configurations including custom function URLs, tool descriptions, and property descriptions [(1) ]() . The system variable

{{call_id}}

is automatically available for phone calls and represents the current call session ID . For custom functions, dynamic variables work in supported fields where you can use the variable picker by typing

{{

to see available variables . The documentation confirms that dynamic variables can be used in "Tool configurations: Custom function URLs, Tool descriptions, Property descriptions" . Regarding your specific questions: For Custom Function Parameters: The documentation shows that dynamic variables are supported in tool descriptions and property descriptions . You should be able to reference

{{call_id}}

in the parameter description, and the variable picker should assist with this . For MCP Tools: Based on a community report, there may be issues with how MCP tools handle response variables. One user reported that variables defined in MCP tool response mappings weren't being extracted correctly and didn't appear in

collected_dynamic_variables

[(2) ]() . This suggests there could be limitations or bugs with how MCP currently processes dynamic variables. The documentation doesn't specifically address setting default values for parameters in MCP tools, and the behavior you're experiencing (the field being dropped on save) isn't explicitly covered in the available sources. I'd recommend testing with a standard custom function first to confirm the

{{call_id}}

behavior works as expected, and consider reaching out to support about the MCP default parameter issue since it appears to be an edge case

not fully documented.

​ To learn more, see the sources I considered: [[1] Dynamic Variables](https://docs.retellai.com/build/dynamic-variables) [[2] mcp tool variables not extracted](https://discord.com/channels/1186424376300679209/1426340769924382791) ​

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