How to produce one schema (rather than anyOf) when using union?

[FelixZY]

I use zod to parse query parameters in Next.JS:

async handler(req: NextApiRequest) {
  const parseResult = schema.parse(req.query);

My schema contains the query parameter tags which is an array of zero or more values.
However, the problem is that tags is only transformed into an array in req.query if it is used more than once in the url:

# { "tags": ["hello", "world"] }
https://example.com/a?tags=hello&tags=world

# { "tags": "hello" }
https://example.com/a?tags=hello

I have been able to get around this by using union:

tags: z
  .union([
    z.nativeEnum(IndividualTag),
    z.array(z.nativeEnum(IndividualTag)),
  ])
  .transform((it) => (Array.isArray(it) ? it : [it]))

However, this causes zod-to-openapi to produce a schema like this:

{
  "anyOf": [
    {
      "type": "string",
      "enum": [
        "musician",
        "songwriter",
        "instructor",
        "organizer",
        "photographer"
      ]
    },
    {
      "type": "array",
      "items": {
        "type": "string",
        "enum": [
          "musician",
          "songwriter",
          "instructor",
          "organizer",
          "photographer"
        ]
      }
    }
  ]
}

rather than the desired

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "musician",
      "songwriter",
      "instructor",
      "organizer",
      "photographer"
    ]
  }
}

Is there a way to make zod-to-openapi understand that I want the second output?

(Why is this important? Because it causes swagger ui to render two different styles of input)