GPT Actions not sending payload to Slack Workflow triggers

GPT builders
1

Hi

I’m trying to create a GPT Action that will send a message to a slack webhook. I have defined this function:

openapi: 3.1.0
info:
  title: Slack Channel Message Trigger
  description: >
    Sends a message to a predefined Slack channel using a Slack Workflow Trigger Webhook.
    You must provide a non-empty `message` string.
  version: 1.0.1
servers:
  - url: https://hooks.slack.com/triggers/myurlishere
    description: Slack trigger webhook URL
paths:
  /:
    post:
      operationId: sendSlackChannelMessage
      summary: Send a message to a Slack channel via workflow
      description: >
        Triggers a Slack workflow by sending a JSON body with one required field: `message`.
        The workflow will post this message to a predefined channel.
      requestBody:
        required: true
        content:
          application/json:
            schema:
              type: object
              required:
                - message
              properties:
                message:
                  type: string
                  minLength: 1
                  description: >
                    The message text to send to the Slack channel.
                    Must be a plain-text string. Example: "Hello team, the deploy is live."
      responses:
        '200':
          description: Message sent successfully
        '400':
          description: Invalid input or missing message
        '404':
          description: Webhook URL invalid or expired

The agent will successfully call the function, and tell me that the correct payload is being sent to the slack webhook, and the webhook does receive a message, but the property “message” is blank. So - it sends an empty message.

I don’t think the issue is on the slack side - when I send the same message via curl, all is well.

When I tried troubleshooting with ChatGPT, it said:

:brain: The Problem (Summary):

  • You are sending:

json

CopyEdit

{ "message": "hello world" }
  • You confirmed with:
    • :white_check_mark: curl: works perfectly
    • :white_check_mark: Webhook.site: GPT sends the correct payload
    • :cross_mark: GPT → Slack webhook: Slack receives nothing, likely due to GPT platform filtering, stripping, or modifying body

Any ideas? I’ve been able to hit this webhook from a jupyter notebook, so I’m pretty convinced the issue is with the Action…