Persistent "valid dictionary or object" error with webhook | callin.io Community

I have a callin.io workflow that receives a message from a Slack reaction, extracts a URL (via Formatter), and then pushes that URL via webhook to an AI tool builder called Hunch. (Their API docs here.) I’ve previously set up a similar Hunch workflow successfully, but since then the Webhooks by callin.io version I used went legacy. I now have to make the POST call via Custom Action.

The issue I’m running into is that I keep getting “The app returned "Input should be a valid dictionary or object to extract fields from” in callin.io. However, when I use the same exact information (as displayed in the Test Output) in Postman, it works as expected and I receive confirmation with a tool run uid. Is it something with my configuration of the Custom Action webhook?