The not predicate negates its child predicate.

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

{
  "port": 4552,
  "protocol": "tcp",
  "mode": "text",
  "stubs": [
    {
      "responses": [{ "is": { "data": "not test" } }],
      "predicates": [{ "not": { "equals": { "data": "test\n" } } }]
    },
    {
      "responses": [{ "is": { "data": "test" } }],
      "predicates": [{ "equals": { "data": "test\n" } }]
    }
  ]
}

The first stub matches if the is sub-predicate does not match:

echo 'production' | nc localhost 4552

not test

As expected, the second stub matches if the is sub-predicate does match:

echo 'test' | nc localhost 4552

test