Does the record extension has specification?

[molikto]

Spec for msgpack https://github.com/msgpack/msgpack/blob/master/spec.md

[kriszyp]

The record extension is documented/specified here:
https://github.com/kriszyp/msgpackr#record-structure-extension-definition

However, if you value a more formal specification/standard, you might want to consider using CBOR (with my cbor-x package) instead of MessagePack. CBOR has gone through a full IETF standards track for its specification (MessagePack has not, it is just a spec posted in github), and the corresponding record extension for CBOR has a complete specification that has been submitted to the official CBOR IANA registry.