cdk diff (and cdk deploy) complains about short functions in existing yaml

I have a stack deployed with an authored YAML template that uses short functions like !GetAtt. I am trying to convert this template to CDK, but cdk diff strips them.

Reproduction Steps

1. Deploy a stack with a YAML file that uses !GetAtt authored outside of CDK
2. Re-create that stack using CDK, generating the same Fn::GetAtt references
3. Run cdk diff my-stack

Error Log

(node:2590) YAMLWarning: The tag !GetAtt is unavailable, falling back to tag:yaml.org,2002:str (node:2590) YAMLWarning: The tag !Ref is unavailable, falling back to tag:yaml.org,2002:str (node:2590) YAMLWarning: The tag !Ref is unavailable, falling back to tag:yaml.org,2002:str (node:2590) YAMLWarning: The tag !GetAtt is unavailable, falling back to tag:yaml.org,2002:str (node:2590) YAMLWarning: The tag !ImportValue is unavailable, falling back to tag:yaml.org,2002:str (node:2590) YAMLWarning: The tag !Ref is unavailable, falling back to tag:yaml.org,2002:str (node:2590) YAMLWarning: The tag !GetAtt is unavailable, falling back to tag:yaml.org,2002:str (node:2590) YAMLWarning: The tag !Sub is unavailable, falling back to tag:yaml.org,2002:str (node:2590) YAMLWarning: The tag !Ref is unavailable, falling back to tag:yaml.org,2002:str (node:2590) YAMLWarning: The tag !ImportValue is unavailable, falling back to tag:yaml.org,2002:str (node:2590) YAMLWarning: The tag !ImportValue is unavailable, falling back to tag:yaml.org,2002:str (node:2590) YAMLWarning: The tag !FindInMap is unavailable, falling back to tag:yaml.org,2002:seq

The !Fn is stripped, so the diff acts as if Prop: !GetAtt Thing.myAtt is actually the string Prop: Thing.myAtt

 │      [-]     "value": "MyKey.SecretAccessKey",
 │      [+]     "value": {
 │      [+]       "Fn::GetAtt": [
 │      [+]         "MyKey",
 │      [+]         "SecretAccessKey"
 │      [+]       ]
 │      [+]     },

Environment

• CLI Version :1.26.0
• Framework Version:
• OS :MacOS
• Language :Typescript

Other

Here’s my workaround for this.

Use cfn-flip to flip it to JSON:

aws cloudformation get-template --stack-name my-stack | jq ".TemplateBody" -r | cfn-flip -i yaml > cdk.out/my-stack.orig.json

Then, use cdk diff to compare to that:

 diff my-stack --template ./cdk.out/my-stack.orig.json

---

This is 🐛 Bug Report

[skinny85]

@comcalvi has actually added support for CloudFormation short-forms in this PR: #8746 .

We’ll probably have to extract his logic after that PR is merged to use the same YAML parsing in the cdk diff command.

[jfrconley]

Encountering this same issue when using CfnInclude on an existing template. Issue persists even after using cfn-flip to convert to JSON beforehand. Seems like the yaml generator used internally is not setup with the required custom tags for cloudformation.