Pass a immutable custom attribute value in pre signup lambda trigger

[gursheeshk]

I have some bussiness rules that i need to check for the user trying to signup into my app, if the requirements are met, i return the event object, else throw an error.

if len_obj==1:
    logger.info(f"event before setting attributes: {event}")
    event['request']['userAttributes']['custom:guid'] = 'guid-testing-12348765435678978656721'
    event['request']['userAttributes']['given_name'] = 'Name'
    logger.info(f"event after setting attributes: {event}")
    return event
else:
    logger.info(f"User {email} does not exist")
    raise Exception("User isn't a member, don't create an account")

The logger message before print:
'request': {'userAttributes': {'email': '[email protected]'}, 'validationData': None}

The logger message after print:
'request': {'userAttributes': {'email': '[email protected]', 'custom:datasteward_guid': 'guid-testing-12348765435678978656721', 'given_name': 'Name'}, 'validationData': None}

But the user that gets created does not have these attributes, what's happening, i see no errors in logs.

My custom attribute is immutable so i can't set it up in post signup lambdas, and interestingly if i use cli and create a account from there using aws cognito-idp sign-up, for the same app client and pass the attributes here, the created user does contain the attributes

I really need help!