Hi, it sounds like your trigger is reacting to itself. IE, the trigger is listening to a collection and modifying that same collection. That means that every time the trigger runs and updates the document it will cause another event to fire and update the document (and then another).

You will need to short-circuit this by not performing the write if the update being made is from this function code (you can examine the contents of the changeEvent / updateDescription for this)

Some pseudo-code would be:

if changeEvent.operationType === "update" && changeEvent.updateDescription.length === 1 && changeEvent.updateDescription["modifiedAt"] !== undefined  {
     return 
}

I saw your other post in which you are updating the “modifiedAt” field. So the idea is just that if we can identify that this is just a trigger running, then we can stop executing the function.