Objects

properties maps each member name to its own node, nested as deep as the body is:

Pet:
  type: object
  properties:
    owner:
      type: object
      properties:
        name:
          type: string

A property name or a schema name must not be empty, must not contain ., [, ], {, } or ;, and must not contain control characters. The six characters are reserved for the paths the broker compares and shows: ., [] and {} write $.owner.name, $[].photoUrl and $.pets{}.name. Any other name is valid, such as content-type, @type, $ref or first name:

contract validation failed
  - petstore_api.yaml: invalid property name "user.name" at schemas Pet properties, must not contain ".", "[", "]", "{", "}" or ";"

An empty name ends with must not be empty, and a name with a control character with must not contain control characters. A property spelled differently from the body is renamed to the key the body has. A key the body really has with one of these characters is left out of the contract when it is fixed, and a property no side declares is not checked (Which side decides). Keys that vary go in a map, whose keys are not property names (Maps).

An object without properties, or with properties: {}, is rejected:

contract validation failed
  - petstore_web.yaml: object schema without properties at schemas Pet properties metadata, declare the keys you read, or use "type: map" with "values" if the keys vary

The fix is to declare the keys that arrive in the body, except a fixed key with a reserved character in its name, which stays out. The side that reads the body marks each key its code does not read with optional: true, so the side that produces the body is free to drop it. The side that produces the body declares the keys its code sends. When the keys vary, both sides write a map with values instead (Maps).

Here petstore_web only counts a pet's vaccinations. Each vaccination arrives with a name and a date, and the code reads neither:

Pet:
  type: object
  properties:
    vaccinations:
      type: array
      items:
        type: object
        properties:
          name:
            type: string
            optional: true
          date:
            type: string
            optional: true