### Answer When using a custom LLM, dynamic variables are passed via the

retell_llm_dynamic_variables

parameter when registering or creating a call. [(1) ]() [(2) ]() For example, when registering a phone call:

json
{
  "retell_llm_dynamic_variables": {
    "customer_name": "John Doe"
  }
}

[(1) ]() These key-value pairs are then available to inject into your custom LLM's prompt and tool descriptions. [(1) ]() [(2) ]() With a custom LLM websocket integration, you would receive these variables as part of the call context and manually interpolate them into your prompt. The

RetellRequest

object sent to your websocket contains the transcript and interaction type, and you'd build your prompt using the dynamic variables you received at call registration time. [(3) ]() The actual injection into your prompt is your responsibility when using a custom LLM—you handle the variable substitution in your

PreparePrompt

function or equivalent.

​ To learn more, see the sources I considered: [[1] Register Phone Call](https://docs.retellai.com/api-references/register-phone-call) [[2] Create Phone Call](https://docs.retellai.com/api-references/create-phone-call) [[3] Integrate LLM](https://docs.retellai.com/integrate-llm/integrate-llm) ​

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