Cannot convert created field to integer

Elastic Stack Logstash
6

That will work for the add_field because the decorator calls sprintf on the fieldname. The convert function just does a get of the field. Since it does not sprintf you cannot use %{} in the convert.

You would have to use a ruby filter to do it. The following

input { generator { count => 1 lines => [ '' ] } }
filter {
    mutate { add_field => { "fieldName" => "foo" "foo" => "1" } }
    ruby {
        code => '
            name = event.get("fieldName")
            event.set(name, event.get(name).to_i)
        '
    }
}
output { stdout { codec => rubydebug { metadata => false } } }

will produce

       "foo" => 1