application/vnd.msgpack

"It's like JSON, but fast and small" is the project's own pitch, and for once the marketing is accurate: MessagePack keeps JSON's data model but encodes it in binary, routinely shaving payloads by a third. The bureaucratic story is stranger than the format — MessagePack shipped around 2008, yet the IANA registration only landed in May 2024, and in the vendor tree at that. Until then the wild ran on the unregistered application/x-msgpack and application/msgpack, both of which still circulate; treat all three as the same bytes under different names. The spec lives on GitHub, not in an RFC.

Defined by MessagePack specification · IANA registration. Registry facts from the IANA media-types registry via mime-db.