Webhook Tool Filler Messages

Filler messages let your AI assistant speak scripted phrases while synchronous webhook tools execute. Instead of silence during a database lookup or API call, callers hear natural phrases like “Let me look that up for you” at configurable intervals. In this guide, you will learn:

How filler messages work and when to use them
How to configure message types and timing
How to set them up via the API and Mission Control Portal

Video tutorial

Overview

When an AI assistant calls a synchronous webhook tool, it waits for the response before speaking again. If your backend takes several seconds to respond, the caller hears silence — creating an awkward experience. Filler messages solve this by playing scripted phrases at the start of the request and at timed intervals while the assistant waits. Messages are scripted per tool, not generated by the LLM — you control the exact wording, timing, and brand voice.

Filler messages work with synchronous webhook tools and MCP tool calls only. Async webhooks return immediately and don’t produce dead air. If your backend consistently takes more than a few seconds, consider async tools as an alternative.

Filler message types

There are two types of filler messages, each triggered at a different point during the webhook call.

Type	When it plays	`timing_ms` required
`request_start`	Immediately when the webhook call begins	No
`request_response_delayed`	After a specified delay if no response has arrived	Yes (100–120,000 ms)

You can configure multiple request_response_delayed messages at different delay thresholds to keep the caller engaged during longer operations.

Configuration

API schema

Add a filler_messages array to your webhook tool configuration:

{
  "type": "webhook",
  "webhook": {
    "name": "check_billing",
    "description": "Look up the customer's current billing information.",
    "url": "https://your-backend.com/billing",
    "method": "POST",
    "filler_messages": [
      {
        "type": "request_start",
        "content": "Let me look that up for you."
      },
      {
        "type": "request_response_delayed",
        "content": "Still working on this.",
        "timing_ms": 5000
      },
      {
        "type": "request_response_delayed",
        "content": "Almost there, just a moment.",
        "timing_ms": 15000
      }
    ],
    "body_parameters": {
      "type": "object",
      "properties": {
        "account_id": {
          "type": "string",
          "description": "The customer's account ID"
        }
      },
      "required": ["account_id"]
    }
  }
}

Key fields

Field	Description
`filler_messages`	Array of filler message objects on a webhook tool
`type`	`request_start` or `request_response_delayed`
`content`	The text the assistant speaks to the caller
`timing_ms`	Delay in milliseconds before speaking (100–120,000). Required for `request_response_delayed` messages only

Mission Control Portal

You can also configure filler messages through the Portal:

Navigate to AI > Assistants
Select your assistant and edit a webhook tool
Set the tool to Sync mode
Open the Filler Messages tab
Add your messages and timing thresholds

Filler Messages configuration in Mission Control Portal

Example: tiered filler messages

This example shows a billing lookup tool with three filler messages that escalate in reassurance as the wait grows:

{
  "filler_messages": [
    {
      "type": "request_start",
      "content": "Let me pull up your billing details."
    },
    {
      "type": "request_response_delayed",
      "content": "I'm still looking into that for you.",
      "timing_ms": 5000
    },
    {
      "type": "request_response_delayed",
      "content": "Just a bit longer, I want to make sure I have the latest information.",
      "timing_ms": 10000
    }
  ]
}

What the caller hears:

Caller: "What's my current balance?"

Agent: "Let me pull up your billing details."
       [5 seconds pass]
       "I'm still looking into that for you."
       [5 more seconds pass]
       "Just a bit longer, I want to make sure I have the latest information."
       [Backend responds]
       "Your current balance is $142.50, due on August 1st."

Use cases

Customer support: Query billing or CRM systems mid-call without silence
AI receptionists: Book appointments via webhook while keeping the caller engaged
Voice agents calling external LLMs: Fill silence while a third-party LLM or RAG pipeline processes a request
Appointment scheduling: Check availability across calendar systems with natural wait phrases

Filler messages vs. async tools

Both features address the same problem — slow backend responses — but take different approaches.

	Filler messages	Async tools
Approach	Keep the sync webhook, fill silence with scripted phrases	Make the webhook async, continue the conversation freely
Best for	Backends that respond in under ~30 seconds	Backends that take 30+ seconds or have unpredictable latency
Caller experience	Hears reassuring phrases at timed intervals	Assistant continues a natural conversation
Response handling	Assistant receives the webhook response directly	Backend injects results via the Add Messages API
Configuration	Add `filler_messages` to the tool	Set `async: true` and build an Add Messages callback

Use filler messages when your backend is reasonably fast and you want simple, predictable caller reassurance. Use async tools when backends are slow or you want the assistant to have a free-flowing conversation while waiting.

Create Assistant API Reference - Full webhook tool configuration options including filler_messages
Async Tools & Deferred Context - Alternative approach for long-running backend operations
Webhooks & Workflows - General webhook tool configuration

Guides

Missions

AI Insights

Assistants API

Missions API

Conversations API

For AI Agents

Webhook Tool Filler Messages

Video tutorial

Overview

Filler message types

Configuration

API schema

Key fields

Mission Control Portal

Example: tiered filler messages

Use cases

Filler messages vs. async tools

​Video tutorial

​Overview

​Filler message types

​Configuration

​API schema

​Key fields

​Mission Control Portal

​Example: tiered filler messages

​Use cases

​Filler messages vs. async tools

​Related resources

Video tutorial

Overview

Filler message types

Configuration

API schema

Key fields

Mission Control Portal

Example: tiered filler messages

Use cases

Filler messages vs. async tools

Related resources