When using the "Pre Token Generation Lambda Trigger Function", how do I set the "claimsToAddOrOverride" to return an array instead of a string

Question

I've written a Pre Token Generation Lambda Trigger function to split a custom string attribute into an array and assign to a new attribute, the string attribute would be in this format "FDVC443FD|HFVSD4434". The lambda function should then create the array ["FDVC443FD", "HFVSD4434"] and assigned it to the new attribute.

Expected Result Input -> "custom:eaid": "FDVC443FD|HFVSD4434" Output -> "eaid": ["FDVC443FD", "HFVSD4434"]

Pre Token Generation Lambda Trigger Function (Python)

def lambda_handler(event, context):
#This function handles adding a custom claim to the cognito ID token.# grab requestor's custom external id (eaid)
custom_eaid = event['request']['userAttributes']['custom:eaid']

# Split the custom attribute string into an array by the "|" seperator
custom_eaid = custom_eaid.split('|')

# placeholder variable
eaid = ''

# this allows us to override claims in the id token
# "claimsToAddOrOverride" is the important part 
event["response"]["claimsOverrideDetails"] = { 
    "claimsToAddOrOverride": { 
        "eaid": custom_eaid
    },
    "claimsToSuppress": ["custom:eaid"]
} 
     
# return modified ID token to Amazon Cognito 
return event

The Lambda function returns the following error:-

Error executing "InitiateAuth" on "https://cognito-idp.eu-west-1.amazonaws.com\ "; AWS HTTP error: Client error: POST https://cognito-idp.eu-west-1.amazonaws.com resulted in a 400 Bad Request response:\n{"__type":"InvalidLambdaResponseException","message":"Unrecognizable lambda output"}\n InvalidLambdaResponseException (client): Unrecognizable lambda output - {"__type":"InvalidLambdaResponseException","message":"Unrecognizable lambda output"}

Answer 1

According to the documentation , claimsToAddOrOverride expects a collection of string key/value pairs.

So as of now we can not pass a list as a value.

Maybe you can try, a comma separated string or set the custom:eaid as it is. Then whenever you need that as a list, extract that from the claim.