AWS Step Functions - include all step input but add a parameter

Let's say I have an AWS step function that simply passes the output from the first step to the second step. But now let's say I need to add an additional input value for that second step.

How do I set the parameters for the second step to keep ALL input fields (without specifying them individually) and add a new input value?

The closest I can get is setting the Parameters like this:

"Second Step": {
  "Type": "Task",
  "Resource": "arn:aws:lambda:blahblahblah",
  "InputPath": "$",
  "Parameters": {
    "Input.$":"$",
    "additionalValue": "ABC"
  }
}

But that results in pushing all of the input values under that new "Input" key, where I really just want them in the root of the dictionary. I could swear there was some magic expression I saw once that made this work the way I want it to, but now I can't find it.

AWS now has a simulator you can try this in. Set InputPath to $ and Parameters to {"Input.$":"$","additionalValue":"ABC"} to see an example of this situation.

Solution

You can have this kind of manipulation with ResultSelector. But it can only be applied at ouput step. With this approach you need to either

Alter the output of previous step with ResultSelector
Create a pass-through step that add the static additional value you wanted