Note: This is not a question an integration response that is a VTL template, gateway response is not a VTL template, per AWS docs.

How do I get access to the the incoming body to in a API Gateway BAD_REQUEST_BODY?

In the documentation for API Gateway responses it states that certain gatweay errors can return non-VTL templates but the gateway response can access $context, $stageVariables, and method.request.<x>. But I can only get access to some of the $context variables.

I would like to access the incoming request body, just pull one property from the incoming request and return that property in the BAD_REQUEST_BODY response

I see more info in the API GW Logs which makes me assume this information is accessible:

Execution log for request xxxxx-2541-4e8e-8214-xxxx
Mon May 13 16:26:22 UTC 2024 : Starting execution for request: xxx-2541-4e8e-8214-xxxxx
Mon May 13 16:26:22 UTC 2024 : HTTP Method: PUT, Resource Path: /test-endpoint
Mon May 13 16:26:22 UTC 2024 : Method request path: {}
Mon May 13 16:26:22 UTC 2024 : Method request query string: {}
Mon May 13 16:26:22 UTC 2024 : Method request headers: {}
Mon May 13 16:26:22 UTC 2024 : Method request body before transformations: {
    "pk": "COMP123",
    "colB": "USA"
}
Mon May 13 16:26:22 UTC 2024 : Request body does not match model schema for content type application/json: [instance type (string) does not match any allowed primitive type (allowed: ["boolean"])] 
Mon May 13 16:26:22 UTC 2024 : Gateway response type: BAD_REQUEST_BODY with status code: 400
Mon May 13 16:26:22 UTC 2024 : Gateway response body: {
    "key": {
       "colA": "<How do I access $input.body($.pk)>"
    },
    "rowAccepted": false,
    "errors": [
        "BAD_REQUEST_BODY",
        "[instance type (string) does not match any allowed primitive type (allowed: ["boolean"])]",
        ""
    ],
    ,
}

Current Gateway Response template:

{
    "key": {
        "pk": "$context.requestOverride.path.body.pk"
    },
    "rowAccepted": false,
    "errors": [
        "$context.error.responseType",
        "$context.error.validationErrorString",
        "$context.integration.error"
    ]
}