AWS Step Function Lambda error catch clause custom error with new fields - amazon-web-services

Question

The setup is an AWS Step Function with Lambdas which throw errors in a catch clause and should add them to the event payload for an Error Handler Lambda at the end of the chain.
This is accomplished with adding a result path like
"Catch": [ {
"ErrorEquals": [ "States.ALL" ],
"ResultPath": "$.error-info",
"Next": "Error Handler"
}]
as described in the docs: https://docs.aws.amazon.com/step-functions/latest/dg/concepts-error-handling.html.
I now need to create custom errors that include new fields lets say a new field called "lambdaName".
For that I would customize an error class like this:
class SFLambdaError extends Error {
constructor(message,lambdaName){
super(message)
this.lambdaName = lambdaName
}
}
Testing the Lambda directly this outputs the desired new field and looks fine:
Invoke Error {"errorType":"SFLambdaError","errorMessage":"someNumber.replace is not a function","lambdaName":"testLambdaName","stack": (...)}
But when implemented in the flow of a Step Function and outputed into "error-info" of the event, the new fields got cut of like following:
"error-info": {
"Error": "SFLambdaError",
"Cause": "{\"errorType\":\"SFLambdaError\",\"errorMessage\":\"someNumber.replace is not a function\",\"trace\":[\"SFLambdaError: someNumber.replace is not a function\",\" at Runtime.exports.handler (/var/task/index.js:27:23)\",\" at Runtime.handleOnce (/var/runtime/Runtime.js:66:25)\"]}"
So it seems that SF error result path just allowes for the standard error class fields to be given out.
Work arounds I tested are to pass all desired fields in errorMessage via stringifying and parsing at the other end in the error handler. But I hope there must be a better way.
So my question: is there a thing I've overseen in the docs or a cleaner workaroun?
Thanks for any help!

Answer 1

Unfortunately there is no handy way to handle such case. A workaround is using an extra state with builtin intrinsic function States.StringToJson:
"Catch": [ {
"ErrorEquals": [ "States.ALL" ],
"ResultPath": "$.error-info",
"Next": "Error Handler Preprocessing"
}]
...
"Error Handler Preprocessing": {
"Type": "Pass",
"Parameters": {
"Error.$": "$.error-info.Error",
"Cause.$": "States.StringToJson($.error-info.Cause)"
},
"ResultPath": "$.error-info",
"Next": "Error Handler"
},
"Error Handler": {
"Type": "Task",
...
}
References
https://docs.aws.amazon.com/step-functions/latest/dg/amazon-states-language-intrinsic-functions.html