Are numeric indexes in the query parameter array representable in Swagger 2.0?

OpenAPI 2.0 (fka Swagger 2.0)

list[0], list[1], list[2] ... list[N] must be defined as separate query parameters. There's no way to define list as a single array-type parameter and have it serialized the way you want.

Note that this approach only works for fixed-sized arrays whose maximum size is known in advance.

parameters:
  - in: query
    name: list[0]
    type: string
  - in: query
    name: list[1]
    type: string

OpenAPI 3.0

One approach is to define list[0], list[1], etc. as separate query parameters (like in the OpenAPI 2.0 example above). This approach is suitable for fixed-sized arrays whose maximum size is known in advance.

parameters:
  - in: query
    name: list[0]
    schema:
      type: string
  - in: query
    name: list[1]
    schema:
      type: string

Another approach - suitable for arrays of unknown size - is to define such an array using free-form query parameters. The limitation of this approach is that it allows arbitrary parameter names - not just list[n] but also abcde. However, you can provide a description explaining that the parameter names must be in the format list[n], and also provide an example value for documentation purposes.

parameters:
  - in: query
    name: params  # arbitrary name, not used in the actual query string
    description: >-
      Parameter names must be in the format `list[n]`
      where `n` is an integer starting from 0.
    schema:
      type: object
    example:
      list[0]: x
      list[1]: y
      list[2]: z

In Swagger UI, enter the query parameters in the JSON object (not array) format, like so:

{
  "list[0]": "x",
  "list[1]": "y",
  "list[2]": "z"
}

Swagger UI will convert this into the following query string (before URL encoding):

?list[0]=x&list[1]=y&list[2]=z

OpenAPI 3.1

OAS 3.1 takes us one step forward. You would still use the free-form query approach but narrow it down by using the new patternProperties keyword to specify a regular expression for the allowed parameter names (e.g. ^list\[\d+\]$).

# openapi: 3.1.0

parameters:
  - in: query
    name: params  # arbitrary name, not used in the actual query string
    description: >-
      Parameter names must be in the format `list[n]`
      where `n` is an integer starting from 0.
    schema:
      type: object
      patternProperties:   # <--------
        ^list\[\d+\]$:
          type: string
      additionalProperties: false
    example:
      list[0]: x
      list[1]: y
      list[2]: z