API Gateway Mapping Template to Kinesis Firehose – Unable to Include Header in Records, 500 InternalServerError

Hi AWS team,

I’m trying to send POST requests from API Gateway to a Kinesis Firehose delivery stream. The request body is a JSON array, and I also need to include a request header (source-system) in each Firehose record.

Here’s what I currently have working without the header:

{
  "DeliveryStreamName": "<delivery_stream_name>",
  "Records": [
    #foreach($elem in $input.path('$'))
        {
          #set($jsonPath = "$[$foreach.index]")
          "Data": "$util.base64Encode($input.json($jsonPath))"
        }#if($foreach.hasNext),#end
      #end
  ]
}

This works fine, each array element is sent to Firehose successfully.

However, when I try to add the header, like this:

 {
  "DeliveryStreamName": "<delivery_stream_name>",
  "Records": [
    #foreach($elem in $input.path('$'))
      {
        "Data": "$util.base64Encode(
          {
            \"source-system\": \"$input.params('source-system')\",
            \"event\": $input.json('$[$foreach.index]')
          }
        )"
      }#if($foreach.hasNext),#end
    #end
  ]
}

I get a 500 InternalServerError with logs: Execution failed due to configuration error: Unable to transform request

Request details:
HTTP Method: POST
Request body: JSON array of objects (see below)
Header: source-system: RM
Content type: application/json
Example request body:
[
  {
    "m_event": "e_KI",
    "m_time": 1754611200000,
    "a_name": "Mobile"
  }
]

Questions: How can I modify the mapping template so that each Firehose record contains both the event object and the source-system header?

Are there any common pitfalls in JSON formatting or escaping in mapping templates that could cause this 500 error?

Any guidance or example templates would be greatly appreciated.

Thanks, Yash