The exists predicate is primarily for object data types, like HTTP headers and query parameters. It works on string fields by simply returning true if the exists value is true and the string if non-empty. Setting the exists value to false inverts the meaning.

POST /imposters HTTP/1.1
Host: localhost:<%= port %>
Accept: application/json
Content-Type: application/json

{
  "port": 4551,
  "protocol": "http",
  "stubs": [
    {
      "responses": [{ "is": { "body": "first response" } }],
      "predicates": [
        {
          "exists": {
            "query": {
              "q": true,
              "search": false
            },
            "headers": {
              "Accept": true,
              "X-Rate-Limit": false
            }
          }
        }
      ]
    },
    {
      "responses": [{ "is": { "body": "second response" } }],
      "predicates": [
        {
          "exists": {
            "method": true,
            "body": false
          }
        }
      ]
    },
    {
      "responses": [{ "is": { "body": "third response" } }],
      "predicates": [
        {
          "exists": { "body": true }
        }
      ]
    }
  ]
}

The first stub matches if the querystring includes q, but not if it includes search, and if the headers include Accept, but not if they include X-Rate-Limit.

GET /?q=mountebank HTTP/1.1
Host: localhost:4551
Accept: text/plain

HTTP/1.1 200 OK
Connection: close
Date: Thu, 09 Jan 2014 02:30:31 GMT
Transfer-Encoding: chunked

first response

The second stub matches if the request method is a non-empty string (always true), and if the body is empty.

GET / HTTP/1.1
Host: localhost:4551

HTTP/1.1 200 OK
Connection: close
Date: Thu, 09 Jan 2014 02:30:31 GMT
Transfer-Encoding: chunked

second response

The last stub matches if the body is non-empty:

POST / HTTP/1.1
Host: localhost:4551

non-empty body

HTTP/1.1 200 OK
Connection: close
Date: Thu, 09 Jan 2014 02:30:31 GMT
Transfer-Encoding: chunked

third response