Some types of AWS resources require the use of TagSpecifications in the AWS API and CLI. While I was attempting to tag newly created resources through SDK calls in Step Functions, I had a hard time figuring out, how to overcome the following error:

Cannot deserialize value of type java.util.ArrayList<software.amazon.awssdk.services.ec2.model.TagSpecification$BuilderImpl> from Object value (token JsonToken.START_OBJECT) /States/CreateTransitGateway/Arguments

This parameter for the AWS CLI seems to have confused already others. Due to the lack of publicly documented examples of using TagSpecifications in AWS Step Functions state machines, this is a minimal working example for creating Transit Gateway:

{
  "QueryLanguage": "JSONata",
  "Comment": "A description of my state machine",
  "StartAt": "CreateTransitGateway",
  "States": {
    "CreateTransitGateway": {
      "Type": "Task",
      "Resource": "arn:aws:states:::aws-sdk:ec2:createTransitGateway",
      "Arguments": {
        "TagSpecifications": [
          {
            "ResourceType": "transit-gateway",
            "Tags": [
              {
                "Key": "Name",
                "Value": "Step Functions rock!"
              }
            ]
          }
        ]
      },
      "End": true
    }
  }
}

In contrast to the AWS CLI, where we can supply the --tag-specifications argument multiple times, we have to supply a list of TagSpecifications to the SDK call.