Webhook tool

Slack AI voice agents that tell your team what just happened

The agent answers the phone, and when a call needs a person it posts the caller, reason and urgency to a Slack channel with chat.postMessage.

Front desk agent

Answers calls and alerts the team in Slack.

Instructions

Answer hours, location and refund policy questions from your knowledge. If the caller reports something urgent or asks for a callback, post a short message to Slack with their name, the reason and how urgent it is. If it is an emergency, post first, then transfer to the on-call line.

Handoff
On-call line

Connected tools

  • Post to SlackWebhook
  • Transfer to on-callTransfer

What a Slack AI voice agent can do

Open one to watch a call. The grey lines are the tools the agent calls mid-sentence.

An urgent call posts to the channel your team watches, with the caller's number and what they need from you.

Tool definitions

Webhook tools go to POST /v1/agents/tools. Transfers are a system tool on the agent. Replace the placeholder values in capitals with your own.

Webhook

Post to Slack

Posts a message to one Slack channel.

Show the JSON definition
post_to_slack
{
  "type": "webhook",
  "name": "post_to_slack",
  "description": "Posts a message about this call to the team's Slack channel. Use it for urgent calls, callback requests and the call summary.",
  "pre_tool_speech": "auto",
  "execution_mode": "async",
  "response_timeout_secs": 10,
  "api_schema": {
    "url": "https://slack.com/api/chat.postMessage",
    "method": "POST",
    "request_body_schema": {
      "type": "object",
      "properties": {
        "channel": {
          "type": "string",
          "constant_value": "YOUR_CHANNEL_ID"
        },
        "text": {
          "type": "string",
          "description": "The message: caller name, what they need, how urgent it is, and any promise made, then: Called from {{system__caller_id}}. Plain sentences, under 500 characters."
        }
      },
      "required": [
        "text"
      ]
    },
    "authentication": {
      "mode": "bearer",
      "token": {
        "type": "secret",
        "secret_value": "YOUR_SLACK_BOT_TOKEN"
      }
    }
  }
}

Transfer

Transfer to on-call

Cold transfer to the on-call number.

Show the JSON definition
transfer_to_on_call
{
  "config": {
    "system_tools": {
      "transfer_to_number": {
        "description": null,
        "pre_tool_speech": "auto",
        "transfers": [
          {
            "destination": {
              "type": "phone",
              "phone_number": "+14155550131"
            },
            "condition": "The caller reports an emergency, after the Slack message is posted."
          }
        ]
      }
    }
  }
}

Set it up

No native connector. The tool calls Slack's Web API with a bot token from a Slack app you create, stored as a secret. Vendor details read in November 2026.

  1. Create a Slack app

    Create an app for your workspace, add the chat:write bot scope, install it, and copy the bot token. It starts with xoxb-.

  2. Invite the app to the channel

    Add the app to the channel it posts in, and copy the channel ID from the channel details.

  3. Add the tool

    Store the bot token as a bearer secret and set the channel ID as a constant, so the agent can only post to that channel.

  4. Test an escalation

    Call the agent, report something urgent, and check the message in Slack.

Before you build

Slack reports errors in the body

A failed post comes back with ok set to false and an error code such as not_in_channel or channel_not_found. If nothing shows up in Slack, look for that code in the tool's response.

Why not an incoming webhook

Slack's incoming webhooks also post to a channel, but their URL is the credential. A webhook tool stores its URL as plain configuration and hides only secrets, so this page uses a bot token.

One message per second

Slack allows about one message per second to a channel, with short bursts. One post per call stays well inside that, but don't have the agent post on every turn.

Post-call summaries

To post a summary after each call, including calls where the agent never used the tool, send Cartesia's post-call webhook to a service of yours that calls chat.postMessage.

Enterprise-grade security

  • HIPAA compliant badge

    HIPAA compliant

  • SOC 2 Type 2 badge

    SOC 2 Type 2

  • GDPR badge

    GDPR

Slack AI voice agent FAQs

Can an AI voice agent send a Slack message?

Yes. The agent on this page answers the phone, and when a call needs a person it posts the caller, the reason and the urgency to a Slack channel with chat.postMessage, using a bot token from a Slack app you create. It doesn't read or answer Slack messages.

How do I connect Slack to an AI voice agent?

Create a Slack app with the chat:write bot scope, install it, and copy the xoxb- bot token. Invite the app to the channel, copy the channel ID, and add the webhook tool with the token as a bearer secret and the channel ID as a constant.

Can the agent post a summary of each call to Slack?

Yes, through Cartesia's post-call webhook rather than a tool. The webhook payload includes a summary, so a service of yours can post it with chat.postMessage, including for calls where the agent never used the tool.

Can it post to more than one Slack channel?

Yes, with one tool per channel, each with its own channel ID as a constant. Keeping the channel fixed stops the agent from posting somewhere you did not choose.

Why doesn't the agent use a Slack incoming webhook?

An incoming webhook's URL is the credential, and a webhook tool stores its URL as plain configuration and hides only secrets. A bot token stored as a secret keeps the credential hidden.

What does a phone agent that alerts Slack cost?

Cartesia bills the agent at $0.03/min, plus LLM usage and telephony. Phone minutes depend on the number. Cartesia-provisioned numbers are US only and carry Cartesia's telephony fee. Numbers from your Twilio account or a SIP trunk pay your carrier's rates instead.

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