XML and JSON tags for a Golang struct?

Go tags are space-separated. From the manual:

By convention, tag strings are a concatenation of optionally space-separated key:”value” pairs. Each key is a non-empty string consisting of non-control characters other than space (U+0020 ‘ ‘), quote (U+0022 ‘”‘), and colon (U+003A ‘:’). Each value is quoted using U+0022 ‘”‘ characters and Go string literal syntax.

So, what you want to write is:

type Foo struct {
    Id          int64       `xml:"id,attr" json:"id"`
    Version     int16       `xml:"version,attr" json:"version"`
}