Representing ISO 8601 year-month dates documentation with Swagger

How to make Swagger core generate openapi.json documentation with a Year-Month field represented as String?

In code I already tried @JsonFormat and @JsonbDateFormat.

Returned entity is correctly represented as String, with correct format, but documentation always is generated as Object.

Environment:

• io.swagger.core.v3.swagger-jaxrs2 2.1.9
• wildfly 21.0.2
• java 11

Here a snippet:

@JsonbDateFormat(value = "uuuu-MM") // or "yyyy-MM", or @JsonFormat(shape = Shape.STRING, pattern = "uuuu-MM")
private YearMonth reference;

Entity return:

{
    "reference": "2020-11",
}

Documentation generated:

{
  "openapi" : "3.0.1",
  ...
  "components" : {
    "schemas" : {
      "Entity" : {
        "type" : "object",
        "properties" : {
          "reference" : {
            "type" : "object",
            "properties" : {
              "year" : {
                "type" : "integer",
                "format" : "int32"
              },
              "month" : {
                "type" : "string",
                "enum" : [ "JANUARY", "FEBRUARY", "MARCH", "APRIL", "MAY", "JUNE", "JULY", "AUGUST", "SEPTEMBER", "OCTOBER", "NOVEMBER", "DECEMBER" ]
              },
              "monthValue" : {
                "type" : "integer",
                "format" : "int32"
              },
              "leapYear" : {
                "type" : "boolean"
              }
            }
          }
        }
      }
    }
  }
}