Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

Running a cloudformation template with Fn::Select with a function to return the list eg Fn::Split is being shown as invalid by the API:
api error ValidationError: Template error: Fn::Select requires a list argument with two elements: an integer index and a list
This is also happening with the Fn::Cidr so I think it's not just one function under Fn::Select.

Expected Behavior

The cloudformation template should deploy as designed.

How are you starting LocalStack?

With the localstack script

Steps To Reproduce

How are you starting localstack (e.g., bin/localstack command, arguments, or docker-compose.yml)

DEBUG=1 localstack start

Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

aws --endpoint-url=http://localhost:4566 cloudformation create-stack --stack-name debug --template-url https://.s3.amazonaws.com/debug/template.yaml

template attached

template.yaml

Environment

- OS: Mac 15.6.1
- LocalStack:
  LocalStack version: 4.8.2.dev1
LocalStack build date: 2025-09-17
LocalStack build git hash: e1aa8f420

Anything else?

No response