Propogating error message through Fail state in aws Step Functions

Question

I am using aws Step Functions to manage a workflow. I am using Fail states to handle errors within the workflow. I would like to propagate some of the json from the Step Function workflow so that a user can easily identify the source of their error. So for example, if the json input to a Fail state looked like this:

{
    "error": "some error text",
    "other_stuff": {...}
}

Then I would like to pull the source of the error. I have set up my Fail state like so:

FailState:
    Type: Fail
    Cause: States.Format($.error)
    Error: Failure Here

However, this simply produces the literal string States.Format($.error) as the Cause for the Fail state. How can I use aws states language and the Fail state to show the actual error as a part of the output of the Fail state? Any solution that can successfully propagate the error text from Step Input to Step Output for the Fail state would be sufficient to solve this problem.

Answer 1

If anyone else stumbles on this question, I contacted AWS support and this is what they told me:

"The ‘Cause’ and ‘Error’ fields in this state only accept the string type values. This is why you are getting the literal string as a response. However, the good news is that, we already have an existing feature request, pending with Step Functions Development team, to implement a feature for sending JSON Path(like $.error) into the Fail state."

So for some reason AWS step functions does not allow you to pass dynamic error messages. They did offer some workarounds such as changing the failure state to success and propagating the error message that way, or creating an sns topic in the case of statemachine failure to post to. I personally just updated the status polling API to grab the state at index [-2] to propagate the error to the user. In any case, to use this functionality currently some workaround should be employed, and hopefully AWS can get this feature out quickly.