How to invoke a stepFunction who's ARN is in the Input

Question

I have a StepFunction (Parent) that needs to invoke another StepFunction (Child). I want to pass the ARN to the child step function as input to the child but doing so throws the following error

Invalid Arn: 'Invalid ARN prefix: $.StateMachine.child_ARN' (Service: AWSStepFunctions; Status Code: 400; Error Code: InvalidArn; Request ID: 53d74c73-a33d-48c4-bf56-6001629c40e8; Proxy: null)

The child function definition is;

{
  "Type": "Task",
  "Resource": "arn:aws:states:::states:startExecution",
  "Parameters": {
    "StateMachineArn": "$.StateMachine.child_ARN",
    "Input": {
      "$.input": "$.ExecutionInput"
    }
  },
  "End": true
}

And the parent's runtime input to the child is;

{
  "StartTime": "2023-03-11T14:05:42.010Z",
  "Region": "us-west-2",
  "ExecutionInput": {
    "orderID": "1234qwerasdfzxcv"
  },

  "StateMachine": {
    "ProcessAlert_ARN": "arn:aws:states:us-west-2:123456789:stateMachine:child"
  }
}

If I change the Parameters for the child function to be; arn:aws:states:us-west-2:123456789:stateMachine:child it invokes successfully ... but I need to variablelize it. Is that possible?

Answer 1

Append .$ to keys whose values have JSONPath substitutions, e.g. StateMachineArn.$:

{
  "Type": "Task",
  "Resource": "arn:aws:states:::states:startExecution",
  "Parameters": {
    "StateMachineArn.$": "$.StateMachine.child_ARN",
    "Input": {
      "input.$": "$.ExecutionInput"
    }
  },
  "End": true
}

When a key has no .$ suffix, the State Machine execution interprets the value literally - hence the error message. The relevant docs are here and here.