<p>Leaving unused definitions in your model is harmful in several ways:</p>

<ul>
  <li>it clutters your Swagger file and makes it less readable for humans;</li>
  <li>it lets code generators create useless classes that spread into your users code;</li>
  <li>it must be maintained over time and adds useless complexity to your code;</li>
</ul>

<p>Make sure your clean up your contract definition to remove any unsused reference.<br/>
Note: in the special case of OpenAPI 2 discriminators (see https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#fixed-fields-13),
this rule assumes that the discriminator uses the enum model in the base object, where the values of the enum are the names
of the derived models, as in the following example:
</p>

<pre>
definitions:
  Pet:
    type: object
    discriminator: petType
    properties:
      name:
        type: string
      petType:
        type: string
        enum: [Cat, Dog]
    required: [name, petType]
  Cat:
    type: object
    allOf:
    - $ref: "#/definitions/Pet"
    - type: object
      properties:
        huntingSkills:
          type: string
      required:
      - huntingSkills
  Dog:
    type: object
    allOf:
    - $ref: "#/definitions/Pet"
    - type: object
      properties:
        master:
          type: string
      required:
      - master
</pre>


<h2>Noncompliant Code Example (OpenAPI 2)</h2>
<pre>
swagger: "2.0"
info:
  version: 1.0.0
  title: Swagger Petstore
paths:
  /pets:
    post:
      parameters:
      - $ref: "#/parameters/Used"
      responses:
        default:
          description: the default response
          schema:
            $ref: "#/definitions/Used"
        '201':
          $ref: "#/responses/Used"
definitions:
  Unused:                 # Noncompliant {{Unused schema}}
    type: object
  Used:
    type: object
parameters:
  Unused:                 # Noncompliant {{Unused parameter}}
    type: string
    in: query
    name: toto
  Used:
    type: string
    in: query
    name: toto
responses:
  Unused:                 # Noncompliant {{Unused response}}
    description: some unused response
  Used:
    description: some referenced response
</pre>
<h2>Compliant Solution (OpenAPI 2)</h2>
<pre>
swagger: "2.0"
info:
  version: 1.0.0
  title: Swagger Petstore
paths:
  /pets:
    post:
      parameters:
      - $ref: "#/parameters/Used"
      responses:
        default:
          description: the default response
          schema:
            $ref: "#/definitions/Used"
        '201':
          $ref: "#/responses/Used"
definitions:
  Used:
    type: object
parameters:
  Used:
    type: string
    in: query
    name: toto
responses:
  Used:
    description: some referenced response
</pre>