Retell Receives Inbound Webhook Data, But {{first_name}} Is Not Resolving I... Retell AI #❓・ai-evy-support

Retell Receives Inbound Webhook Data, But {{first_...

manthu

06/04/2026, 8:06 AM

@AI Evy Retell Receives Inbound Webhook Data, But {{first_name}} Is Not Resolving In The Prompt Hi everyone, I'm currently in Section 5 #28 - Mastering Inbound AI Calls: Adding Personalization - Udemy of the course and I'm hoping someone can help me. I've been stuck on this for almost 6 hours and feel like I'm missing one small setting somewhere. I'm using an Inbound Call Webhook with Retell and Make.com. My flow is: Retell → Make Webhook → OpenAI → Google Sheets Search Row → Webhook Response → Retell The goal is to retrieve the caller's information from Google Sheets and use it inside the Retell prompt for personalization. Everything appears to be working correctly: ✅ Inbound webhook is triggered ✅ Phone number is formatted correctly and matches the Google Sheet record ✅ Google Sheets finds the correct row ✅ Webhook Response returns the caller information ✅ Retell Call History → Detail Logs shows the webhook response with all the correct data Example response:

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json
{
"first_name": "Mark",
"phone_number": "+61473503315",
"email_address": "mark@email.com",
"dental_service": "Teeth Cleaning"
}

I've already tried: * Confirming the correct Inbound Call Agent is attached to the number * Saving and republishing the agent * Matching variable names exactly (

first_name

) * Using variables in both the Welcome Message and Agent Prompt * Testing different webhook response structures * Hardcoding test values * Multiple test calls after each change The issue is that when I use:

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text
{{first_name}}

Retell literally says:

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text
{{first_name}}

instead of:

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text
Mark

What confuses me is that the Detail Logs clearly show Retell receiving the webhook response successfully. My question is: If Retell is receiving the inbound webhook response, how do I correctly access those values inside the prompt? Is there an additional mapping step, a different variable syntax, or something else that I'm missing?

AI Evy

06/04/2026, 8:06 AM

Thanks for tagging me @manthu. I'll look through relevant sources and get back to you shortly.

AI Evy

06/04/2026, 8:07 AM

### Answer Based on the information sources, the issue is that your Inbound Call Webhook response needs to be wrapped in a

retell_llm_dynamic_variables

object. [(1) ]() [(2) ]() Your current webhook response structure:

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json
{
  "first_name": "Mark",
  "phone_number": "+61473503315",
  "email_address": "mark@email.com",
  "dental_service": "Teeth Cleaning"
}

Should be changed to:

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json
{
  "retell_llm_dynamic_variables": {
    "first_name": "Mark",
    "phone_number": "+61473503315",
    "email_address": "mark@email.com",
    "dental_service": "Teeth Cleaning"
  }
}

The Inbound Call Webhook expects a JSON object in reply, consisting of key-value pairs of the variables you want to access within your prompt, but these must be nested inside the

retell_llm_dynamic_variables

object. [(2) ]() Anything within that object will be mapped, within the prompt, to

{{whatever_variable_name_you_used}}

. Once you update your Make.com webhook response module to return the data in this format, your

{{first_name}}

variable should resolve correctly to "Mark" instead of displaying literally as

{{first_name}}

. [(1) ]() [(3) ]()

AI Evy

06/04/2026, 8:07 AM

To learn more, see the sources I considered: [[1] Inbound webhook](https://docs.retellai.com/features/inbound-call-webhook) [[2] Discord message from @fyodor2572 on 2025-01-13](https://discord.com/channels/1186424376300679209/1186462578310910112/1328501089112424508) [[3] Dynamic Variables](https://docs.retellai.com/build/dynamic-variables)

AI Evy

06/04/2026, 8:07 AM

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

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