The Extended JSON spec says:

A parser that accepts Legacy Extended JSON MUST be configurable such that a JSON text of a MongoDB query filter containing the type query operator can be parsed, e.g.:

{ "zipCode" : { $type : 2 } }
or:

{ "zipCode" : { $type : "string" } }

But libbson returns an error when parsing a $type in any of the JSON parsing functions:

• bson_json_reader_read
• bson_init_from_json
• bson_new_from_json

Though it does accept the legacy binary format, e.g.

{ "$binary": "AQIDBAU=", "$type": "80" }

I see no way to configure parsing to interpret $type as just a normal key (not a legacy binary type).