Support aborting requests via a Rack::Response#finish

I have a post endpoint where I'd like to conditionally return a custom Rack::Response, like so:

post do
  if some_condition?
    hash_of_contextually_relevant_goodness = { foo: 'bar' }
    Rack::Response.new(
      [hash_of_contextually_relevant_goodness], 
      418, 
      { 'Content-Type' => 'application/json' }
    ).finish
  else
    # do some object creation stuff
  end
end

However, this returns the following response body:

[
    418,
    {
        "Content-Length": "13",
        "Content-Type": "application/json"
    },
    {
        "block": {},
        "body": {
            "block": null,
            "body": [
                "{:foo=>\"bar\"}"
            ],
            "chunked": false,
            "header": {
                "Content-Length": "13",
                "Content-Type": "application/json"
            },
            "length": 13,
            "status": 418,
            "writer": {}
        },
        "closed": false
    }
]

It looks like the response is doubling up. What is the correct way to return a custom Rack::Response from my endpoint?

Start by tracing how the endpoint handles the returned Rack::Response and how Rack::Response#finish is interpreted. Reproduce the post endpoint example, then inspect the request/response handling path for where the response becomes nested. Done means a custom Rack::Response can be returned without its status, headers, and body being serialized again.