bug: Unexpected token 'o', "ok" is not valid JSON

Hello and thank you for this action Slack!

Already done a search in both open and closed issues but found none matching my problem. Also looked for open PRs and none fix it either.

When running the GitHub action to notify on failed workflows, as in:

name: Check-Workflow

on:
  workflow_run:
    workflows: ["*"]
    branches: [main]
    types:
      - completed

jobs:
  notify:
    runs-on: ubuntu-latest-small
    if: github.event.workflow_run.conclusion == 'failure'
    permissions:
      contents: read
    steps:
      - name: Slack alert for failure on main
        uses: slackapi/slack-github-action@dcb1066f776dd043e64d0e8ba94ca15cc7e1875d # v4.0.1
        with:
          errors: true
          webhook: ${{ secrets.WEBHOOK_URL }}
          webhook-type: webhook-trigger
          payload: |
            text: "valid text"

It fails with the following error: Error: SlackError: A request error occurred: Unexpected token 'o', "ok" is not valid JSON

Full stacktrace is:

Error: SlackError: A request error occurred: Unexpected token 'o', "ok" is not valid JSON

file:///home/runner/_work/_actions/slackapi/slack-github-action/dcb1066f776dd043e64d0e8ba94ca15cc7e1875d/src/webhook.js:57
      throw new SlackError(config.core, err.message);
^
SlackError: A request error occurred: Unexpected token 'o', "ok" is not valid JSON
    at Webhook.post (file:///home/runner/_work/_actions/slackapi/slack-github-action/dcb1066f776dd043e64d0e8ba94ca15cc7e1875d/src/webhook.js:57:1)
    at processTicksAndRejections (node:internal/process/task_queues:104:5)
    at post (file:///home/runner/_work/_actions/slackapi/slack-github-action/dcb1066f776dd043e64d0e8ba94ca15cc7e1875d/src/send.js:34:1)
    at send (file:///home/runner/_work/_actions/slackapi/slack-github-action/dcb1066f776dd043e64d0e8ba94ca15cc7e1875d/src/send.js:14:1)
    at file:///home/runner/_work/_actions/slackapi/slack-github-action/dcb1066f776dd043e64d0e8ba94ca15cc7e1875d/src/index.js:9:1
Error: A request error occurred: Unexpected token 'o', "ok" is not valid JSON
SlackError: A request error occurred: Unexpected token 'o', "ok" is not valid JSON
    at Webhook.post (file:///home/runner/_work/_actions/slackapi/slack-github-action/dcb1066f776dd043e64d0e8ba94ca15cc7e1875d/src/webhook.js:57:1)
    at processTicksAndRejections (node:internal/process/task_queues:104:5)
    at post (file:///home/runner/_work/_actions/slackapi/slack-github-action/dcb1066f776dd043e64d0e8ba94ca15cc7e1875d/src/send.js:34:1)
    at send (file:///home/runner/_work/_actions/slackapi/slack-github-action/dcb1066f776dd043e64d0e8ba94ca15cc7e1875d/src/send.js:14:1)
    at file:///home/runner/_work/_actions/slackapi/slack-github-action/dcb1066f776dd043e64d0e8ba94ca15cc7e1875d/src/index.js:9:1

Some small research with Claude mentioned the following on why it failed:

1. The Slack action posts to the webhook — Slack actually receives it and responds with ok (plain text).
2. But the action expects JSON back, parses "ok" as JSON → Unexpected token 'o', "ok" is not valid JSON.

Hope this is enough context 🤞

Thank you