Encoding of request bodies as x-www-form-urlencoded, and handling of arrays/explode

[jdegre]

Hi,
A, hopefully, straightforward question on the correct interpretation of the OpenAPI spec.

Regarding the x-www-form-urlencoded encoding of request bodies, such as this example:

      requestBody:
        content:
          application/x-www-form-urlencoded:
            schema:
              type: object
              properties:
                foo:
                  type: array
                  items:
                    type: object
                    properties:
                      a:
                        type: integer
                      b:
                        type: integer

How is the foo array supposed to be encoded in the request body? As an exploded array?, such as:

foo={"a":1,"b":2}&foo={"a":3,"b":4}

or not exploded? such as:

foo={"a":1,"b":2},{"a":3,"b":4}

I'm aware that the explode behavior can be explicitly indicated by adding an encoding section with style: form and explode: true/false, but I cannot find in the OpenAPI spec a clear indication of what is the expected behavior when the encoding section is NOT provided.

Out of curiosity, I have tried with Swagger UI and it follows the non-exploded approach (2nd example), but please note that this is not a question about Swagger UI or any other tool. I'm interested in the correct interpretation of the OpenAPI spec.

Thanks in advance!