🚨 Retell AI — Dynamic Variable Not Substituting in Whisper & Three-Way Debrief Messages

Ryan Smith

8d • Support Needed 💻

Hey everyone — hitting a wall with Retell AI and hoping someone here has solved this before.

The Setup:

I have an inbound voice agent (Amber) handling calls for a restoration company. The flow is:

Caller calls in → Amber answers and collects info

Amber calls a Custom Function (get_oncall_routing) that hits a Relay.app webhook

Relay looks up who's on call, returns data back to Retell

Amber does a Warm Transfer to the on-call PM's phone number

Amber delivers a Whisper Debrief to the PM and a Three-Way Handoff to both parties

What Works Perfectly:

{{oncall_phone}} substitutes correctly in the Dynamic Routing field — Amber routes to the right phone number every single time ✅

{{caller_name}} and {{emergency_summary}} are being collected correctly during the call ✅

The full Relay JSON response (Step 6) includes both oncall_name and oncall_phone ✅

The Relay JSON response looks like this:

json{

"assignedPerson": "Ryan Smith",

"assignedPhone": "+12089707024",

"region": "EAST",

"oncall_name": "Ryan Smith",

"oncall_phone": "+12089707024",

"fallbackPerson": "Blake Hansen",

"fallbackPhone": "+12085898887",

"zipLookupSucceeded": true

}

Store Fields as Variables mapping in get_oncall_routing:

oncall_name → oncall_name

oncall_phone → oncall_phone

caller_name → caller_name

emergency_summary → emergency_summary

assignedPerson → assignedPerson

region → oncall_region

The Problem:

{{oncall_name}} is NOT substituting in either the Whisper Debrief Message or the Three-Way Debrief Message.

In Prompt mode — Amber literally says "Oncall name" out loud. The LLM reads the placeholder as literal text.

In Static Sentence mode — the variable doesn't substitute at all. Amber either skips it or says nothing where the name should be.

What We've Tried:

✅ Confirmed oncall_name is in the Relay JSON response

✅ Confirmed Store Fields mapping is correct — oncall_name → oncall_name

✅ Tried Prompt mode with {{oncall_name}} directly in the sentence

✅ Tried Static Sentence mode with {{oncall_name}}

✅ Tried removing the example sentence from the prompt (thought Amber was pattern-matching off "Hi Kaden" in the example)

✅ Tried adding oncall_name explicitly to the main agent prompt as a declared variable

✅ Published Relay workflow and Retell agent after every change

✅ Consulted ChatGPT, Grok, Gemini, and Manus — all agree Static Sentence should work but it's not

The Weird Part:

{{oncall_phone}} works perfectly in the Dynamic Routing field. Same variable pipeline, same JSON, same mapping. But {{oncall_name}} fails in the Whisper and Three-Way fields regardless of mode.

This suggests the issue is specific to how Retell handles dynamic variable substitution inside Whisper and Three-Way Debrief message fields — not the data pipeline itself.

Current Workaround:

We've temporarily removed the PM name from both messages entirely. The whisper still delivers caller name, location, and emergency summary — just skips the PM greeting by name. Handoff works the same way. Functional but not ideal.

What We're Hoping to Find:

Has anyone successfully used a custom dynamic variable (not a system variable) inside a Whisper or Three-Way Debrief message?

Is there a specific syntax or configuration required for these fields that's different from other prompt fields?

Is this a known Retell limitation or bug?

Any workarounds that preserve the dynamic PM name greeting?

Using GPT-4.1, Warm Transfer mode, custom function hitting Relay.app webhook. Agent is a single prompt agent, not conversation flow.

Any help appreciated — this is the last piece of an otherwise fully working AI receptionist system! 🙏

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