Propogating error message through Fail state in aws Step Functions

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.

I too want to use a JSON path in a Fail state to dynamically set the Cause and Error fields. In my case, I have a known set of errors so for each one I created a separate Fail state. Each one of those fail states corresponded with a catch object where Next is set to the appropriate fail state.

{
  "TaskState": {
    "Type": "Task",
    ...
    "Catch": [
      {
         "ErrorEquals": ["ErrorA"],
         "Next": "FailureA"
      },
      {
         "ErrorEquals": ["ErrorB"],
         "Next": "FailureB"
      }
    ]
  },
  "FailureA": {
    "Type": "Fail",
    "Error": "FailureA",
    "Cause": "This failed because of A"
  },
  "FailureB": {
    "Type": "Fail",
    "Error": "FailureB",
    "Cause": "This failed because of B"
  }
}

I was able to achieve something close to a desired behavior by creating a "Fail-Me" Lambda that fails with an unhandled exception, dynamically choosing exception class based on the Error and Cause provided as its payload. If "Error" was a name of a built-in exception class it uses it, otherwise it creates its own class.

In State Machine, use an Invoke Lambda state calling this "Fail-Me" lambda with no retrier and no catcher.

import inspect, sys

# Expected payload (event), example:
#      {
#           "Error":"RuntimeError",
#           "Cause":"No Cause"
#       }

def lambda_handler(event, context):
    
    ErrorType  = event.get("Error") or ""
    ErrorCause = event.get("Cause") or ""


    if not ErrorType.isidentifier():
        ErrorCause = "{}: {}".format(ErrorType, ErrorCause)
        ErrorType = "OtherError"

    DynamicExceptionClass = type(ErrorType, (BaseException,), {})

    for name, obj in inspect.getmembers(sys.modules["builtins"]):
        if inspect.isclass(obj) and issubclass (obj, BaseException):
            if name==ErrorType:              #ErrorType is an existing built-in exception - use it
                raise obj(ErrorCause)


    #Create our own dynamic class and raise it

    DynamicExceptionClass = type(ErrorType, (BaseException,), {})

    raise DynamicExceptionClass(ErrorCause)

This has been added with the latest update for step functions.

Enhanced Error Handling AWS Step Functions

Example with CDK:

new Fail(this, "Failed Step", {
  cause: JsonPath.stringAt("$.payload.Cause"),
  error: JsonPath.stringAt("$.customErrorLocation"),
});