This is working as intended. false is the zero value of booleans, and your json struct tag has omitempty. As you can see from t2, if you don't use omitempty, the value isn't omitted.

Actually that's slightly misleading: it's not that omitempty omits precisely zero values; the set of omitted values is listed in the Marshal documentation:

The empty values are false, 0, any nil pointer or interface value, and any array, slice, map, or string of length zero.

If you need to make a distinction between a non-existent boolean and false boolean, use a *bool in your struct, like this:

type t struct {
    Autoplay *bool `json:"autoplay,omitempty"`
}

You'll have to do the extra work of checking that t.Autoplay is not nil, but that's unavoidable - it sounds like you want this ternary logic.

@cespare Considering golang's design where primitive types are always initialized to empty values, your comment makes perfect sense. So I realize this is not a bug, but it's also somewhat confusing and perhaps even unexpected. When writing go code, this isn't a big deal. But in the case of JSON data being passed from client to server (one of the largest use cases of JSON) , this makes go somewhat inconsistent with the rest of the world (clients and servers), and can confuse a lot of people. Not sure what a good solution would be.

@spenczar that's a great idea, thanks. That's exactly what I was after.

A boolean only has two values. You can either send false or not, depending on what tag you put on the variable. If you need a three-valued boolean, that's not a boolean. As @spenczar suggested, *bool is a good choice in this case.