When validating the query params of an endpoint with parameters of type array we get a validation error:

Example:

GET /api/users?ids[]=1&ids[]=2

Operation param defined as:

Operation.parameter(
:"ids[]",
:query,
%Schema{type: :array, items: %Schema{type: :integer}},
"IDs"
)

We get the following error:

[
  %OpenApiSpex.Cast.Error{
    reason: :invalid_type,
    value: "2",
    format: nil,
    type: :array,
    name: nil,
    path: [:ids],
    length: 0,
    meta: %{}
  }
]

This is because Plug.Conn.fetch_query_params removes the "[]" part by grouping the property and returning an array of results. Following the example above, the result of this function would be %{ids: [1,2]} which causes an error, because in the Schema "ids[]" has been defined as a possible parameter and not "ids".