[cdk diff] Modifications to stack-level tags do not surface in cdk diff results, leading to unexpected updates

[norcroj]

Describe the bug

When you make a change to a stack-level tag in CDK, cdk diff returns There were no differences even though changes to stack-level tags do in fact results in updates to resources. This results in inaccurate diff results that can lead to unexpected updates.

In comparison - When you make a change to a stack-level tag and then create a CloudFormation ChangeSet, CloudFormation does report this as a change. It lists a Modify operation for all supported resources.

The cdk diff command creates (and then immediately deletes) a CloudFormation ChangeSet, and this ChangeSet does accurately report the Modify behavior, but the actual cdk diff results in the CDK CLI still returns no differences despite this. I have provided screenshots of this below.

CloudFormation ChangeSet:

CDK Diff Results:

In other words, this doesn't appear to be an issue or limitation with the CloudFormation ChangeSet behavior. This specifically looks like a limitation with how CDK diff parses and returns ChangeSet results when stack-level tags are involved.

Regression Issue

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

Last Known Working CDK Version

No response

Expected Behavior

CDK diff should accurately report changes to resources when changes are made to stack-level tags.

Current Behavior

CDK diff incorrectly returns "There were no differences" despite the CloudFormation ChangeSet reporting differences.

Reproduction Steps

1. Create a CDK stack. As an example, I made one that just creates an SNS topic:

import * as cdk from 'aws-cdk-lib/core';
import * as sqs from 'aws-cdk-lib/aws-sqs';
import { Construct } from 'constructs';


export class CdkDiffReplicationStack extends cdk.Stack {
  constructor(scope: Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    new sqs.Queue(this, 'Queue');
  }
}

2. Define one or more stack tags. For example:

#!/usr/bin/env node
import * as cdk from 'aws-cdk-lib/core';
import { CdkDiffReplicationStack } from '../lib/cdk_diff_replication-stack';

const app = new cdk.App();

new CdkDiffReplicationStack(app, 'CdkDiffReplicationStack', {
  // Specialize for the account/region from the current CLI configuration.
  env: {
    account: process.env.CDK_DEFAULT_ACCOUNT,
    region: process.env.CDK_DEFAULT_REGION,
  },

  // stack-level tag
  tags: {
    DiffReplicationTag: 'Original',
  },
});

3. Deploy the stack
4. Make a change to the stack-level tag in the CDK code:

  tags: {
    DiffReplicationTag: 'Changed', //<---Changed the value
  },

5. Run CDK diff - Observe that the command returns There were no differences.
6. In the CloudFormation console, manually create a CloudFormation ChangeSet. Use the same template but change the value of the stack-level tag. Observe that CloudFormation does report changes for these resources.

Possible Solution

No response

Additional Information/Context

No response

CDK CLI Version

2.1141.0

Framework Version

No response

Node.js Version

v24.14.0

OS

Windows 10

Language

TypeScript

Language Version

No response

Other information

No response