Remove fields from previous step

I am trying to remove a field from the previous step used only in the choice task. I have this StepFunction

{
  "StartAt": "ConversionStep",
  "States": {
    "ConversionStep": {
      "Type": "Task",
      "TimeoutSeconds": 30,
      "Resource": "arn:aws:lambda:region:account-id:function:conversion-step",
      "Retry": [
        {
          "ErrorEquals": [
            "RetryError",
            "States.Timeout"
          ],
          "IntervalSeconds": 30,
          "MaxAttempts": 3
        }
      ],
      "Catch": [
        {
          "ErrorEquals": [
            "States.TaskFailed"
          ],
          "Next": "Failure"
        }
      ],
      "Next": "ChoiceStep"
    },
    "ChoiceStep": {
      "Type": "Choice",
      "Choices": [
        {
          "Variable": "$.processEvent",
          "BooleanEquals": true,
          "Next": "PublishStep"
        }
      ],
      "Default": "Failure"
    },
    "PublishStep": {
      "Type": "Task",
      "Resource": "arn:aws:states:::events:putEvents",
      "Parameters": {
        "Entries": [
          {
            "Detail.$": "$",
            "EventBusName": "example-event-bus",
            "DetailType": "example-detail-type",
            "Source": "EXAMPLE_SOURCE"
          }
        ]
      },
      "Retry": [
        {
          "ErrorEquals": [
            "RetryError",
            "States.Timeout"
          ],
          "IntervalSeconds": 30,
          "MaxAttempts": 3
        }
      ],
      "Catch": [
        {
          "ErrorEquals": [
            "States.TaskFailed"
          ],
          "Next": "Failure"
        }
      ],
      "Next": "Success"
    },
    "Success": {
      "Type": "Succeed"
    },
    "Failure": {
      "Type": "Fail"
    }
  }
}

In the Conversion step add the processEvent in the event. I need to send the conversion step result without this field in the publish step "Detail.$": "$",

Is there a way I can do it without creating another Lambda Step? The choice step does not have the Result Filter feature.

This is now possible with the recently launched support for JSONata using the Transform Operator.

You would need to update the state to use "QueryLanguage": "JSONata" (you can do this for your whole state machine or just for this state if you don't want to change the rest and replace the "$" (JSONPath input reference) to something like:

"{% $states.input ~> | ** | {}, ['processEvent'] | %}"