(aws-amplify-alpha): Custom domain with blank prefix causes stack drift

[ajkerr]

Describe the bug

After deploying a CDK stack containing an Amplify app with a custom domain that has a blank prefix, the underlying CloudFormation stack is immediately in "drifted" state.

Regression Issue

• Select this option if this issue appears to be a regression.

Last Known Working CDK Library Version

No response

Expected Behavior

The underlying CloudFormation stack should not be drifted.

Current Behavior

The stack is in drifted state after deployment:

Expected:

{
  "SubDomainSettings": [
    {
      "BranchName": "master",
      "Prefix": ""
    }
  ],
  "AppId": "xxxyyyzzz",
  "AutoSubDomainIAMRole": "arn:aws:iam::xxxyyyzzz",
  "DomainName": "mysubdomain.mydomain.com",
  "EnableAutoSubDomain": false,
  "AutoSubDomainCreationPatterns": [
    "*",
    "pr*"
  ]
}

Actual:

{
  "SubDomainSettings": [
    {
      "BranchName": "master"
    }
  ],
  "AppId": "xxxyyyzzz",
  "AutoSubDomainIAMRole": "arn:aws:iam::xxxyyyzzz",
  "DomainName": "mysubdomain.mydomain.com",
  "EnableAutoSubDomain": false,
  "AutoSubDomainCreationPatterns": [
    "*",
    "pr*"
  ]
}

Reproduction Steps

Add a custom domain with a blank prefix like so:

amplify_app.add_domain('AppDomain',
    domain_name = 'mysubdomain.mydomain.com',
    sub_domains = [amplify.SubDomain(prefix = '', branch = 'master')]
)

As specified by the documentation for amplify.SubDomain, we: Use '' to map to the root of the domain.

Possible Solution

No response

Additional Information/Context

No response

AWS CDK Library version (aws-cdk-lib)

2.222.0-alpha.0

AWS CDK CLI version

2.1031.1

Node.js Version

v24.11.0

OS

macOS 15.7.1

Language

Python

Language Version

3.14.0

Other information

No response

[pahud]

Hi @ajkerr,

Thank you for reporting this issue. The analysis suggests this is a drift detection problem caused by how the AWS Amplify service handles empty string values in the Prefix property.

When you specify prefix='' in your CDK code, the synthesized CloudFormation template correctly includes "Prefix": "" in the SubDomainSettings. However, the AWS Amplify service backend omits properties with empty string values from the actual resource state. This causes CloudFormation's drift detection to flag a mismatch between the template (which has Prefix: "") and the actual state (which omits the Prefix property entirely).

The root cause is in the renderSubDomainSettings method which always includes the prefix property, even when it's an empty string.

aws-cdk/packages/@aws-cdk/aws-amplify-alpha/lib/domain.ts

Lines 200 to 206 in b298a21

	private renderSubDomainSettings() {
	return this.subDomains.map(s => ({
	branchName: s.branch.branchName,
	prefix: s.prefix ?? s.branch.branchName,
	}));
	}
	}

A potential fix would be to conditionally omit the prefix property from the SubDomainSettings when it's an empty string, aligning the CDK's template generation with AWS Amplify's actual behavior. This would prevent drift while maintaining the documented API where prefix='' maps to the root domain.

Would you be interested in contributing a PR to address this? The fix would involve modifying the renderSubDomainSettings method to only include the prefix property when it's not an empty string. I'd be happy to provide guidance on the implementation and testing approach.

[ajkerr]

@pahud I'd be happy to attempt a PR for this. I'll tag you once I have something up.

[ajkerr]

I don't think that this is fixable at the CDK level, given my PR attempt at #35963.

Based on https://docs.aws.amazon.com/AWSCloudFormation/latest/TemplateReference/aws-properties-amplify-domain-subdomainsetting.html#cfn-amplify-domain-subdomainsetting-prefix, the Prefix property is actually required at the CloudFormation level.

Perhaps it's a bug in the drift detection or CloudFormation itself that is causing the Prefix attribute to disappear?