If you're sending to a Slack incoming webhook from a Prefect

Webhook

block /

Call webhook

action, your headers should usually be:

Content-Type: application/json

And that's typically it. A few important details: Slack does not need an Authorization header for incoming webhooks. The secret is embedded in the webhook URL itself. So in your Prefect

Webhook

block, headers can be either:

{
  "Content-Type": "application/json"
}

or possibly empty if you're calling the block directly with a Python

dict

payload. But if you're using the Call webhook automation action, the payload is rendered as a string, so it's safest to set:

{
  "Content-Type": "application/json"
}

Example Slack payload body:

{
  "text": "Hello from Prefect"
}

So your setup should look roughly like: URL

<https://hooks.slack.com/services/>...

Method

POST

Headers

{
  "Content-Type": "application/json"
}

Payload

{
  "text": "Flow run failed"
}

Gotcha: make sure the payload is valid JSON, especially if you use Jinja templating in the

Call webhook

action. Example:

{
  "text": "Flow {{ flow_run.name }} failed"
}

If you want, I can also show you the exact

Call webhook

payload template to use for a Slack message in a Prefect automation. Relevant docs: - Events and automations - Slack incoming webhooks