Known Issue: JSON Format Compatibility Issue in ArcGIS Online Feature Service Webhooks

Symptom

The issue occurs when the content type application/json is set on an ArcGIS Online Feature Service when creating an ArcGIS Online Feature Service Webhook. 

Specifically, a problem arises if the JSON payload starts with square brackets (e.g., ["Ford", "BMW", "Fiat"]). This format leads to difficulties because the webhook trigger in an FME Flow automation does not accept JSON beginning with square brackets.

Cause

It is a standard practice to begin a JSON string with curly brackets {} instead of with an array that has square brackets []. This causes FME to throw a 422 error in response:

{
   "message": "A JSONObject text must begin with '{' at 1 [character 2 line 1]"

}

Workaround

Changing the header to content-type=application/x-www-form-urlencoded or text/plain allows any type of JSON webhook message to be received, regardless of formatting.

In the case of an ArcGIS Online Feature Service Webhook, you can modify the content type to application/x-www-form-urlencoded which is accepted by the webhook trigger in an FME Flow Automation.