Several files

bidirekt publish takes any number of files and folders, and all the files of one publish are one contract. A folder stands for every .yaml and .yml file under it, subfolders included. The order does not matter, and the shell expands globs such as contracts/*.yaml. Reports name a file you passed by the path you typed, and a file found in a folder by the folder and its path inside it, such as contracts/rest/pets.yaml.

• One schema namespace. A schema declared in one file can be named from any other. The same name declared twice is a duplicate. A file that names schemas declared elsewhere is valid only when published together with that file.
• A provider declares each resource once. Files may split the endpoints, the methods or even the statuses of one method between them, but the same resource in two files is a duplicate. A file reached twice, such as a file passed on its own and again through its folder, is read once and is not a duplicate. A request has no status, so the request of an endpoint and method belongs in exactly one file, even when its responses are spread over several.
• A consumer merges by union. Each module of a consumer may declare the resources it reads in its own file. Fragments of the same resource merge: in a response, a property is optional only if every fragment allows it, and nullable only if every fragment accepts null; in a request, it is required only if every fragment sends it, and nullable if any fragment may send null. A property that one fragment leaves out together with its parent is not made optional: only the parent is. Two fragments that give one property different types are rejected.

A provider that keeps the 201 of POST /pets in one file and its 400 in another writes request in only one of them:

# pets.yaml
provides:
  rest:
    /pets:
      post:
        request: NewPet
        responses:
          201: Pet

# pets_errors.yaml
provides:
  rest:
    /pets:
      post:
        responses:
          400: Error

Writing request: NewPet in pets_errors.yaml too declares the request of POST /pets twice:

contract validation failed
  - pets_errors.yaml: duplicate resource "provides POST /pets request", also declared in pets.yaml

Two modules of the consumer petstore_web read the same GET /pets/*, each its own subset:

# web_list.yaml
consumes:
  petstore_api:
    rest:
      /pets/*:
        get:
          responses:
            200: PetRow

schemas:
  PetRow:
    type: object
    properties:
      petId:
        type: integer
      name:
        type: string

# web_card.yaml
consumes:
  petstore_api:
    rest:
      /pets/*:
        get:
          responses:
            200: PetCard

schemas:
  PetCard:
    type: object
    properties:
      petId:
        type: integer
      photoUrl:
        type: string
        optional: true

Published together, petstore_web reads petId and name as required and photoUrl as optional, so a provider that returns only petId and name is compatible. Remove optional: true from web_card.yaml and photoUrl becomes required, because one reader needs it.