Universität Bremen TZI This document formalizes and consolidates the definition of the Extended Diagnostic Notation (EDN) of the Concise Binary Object Representation (CBOR), addressing implementer experience. Replacing EDN's previous informal descriptions, it updates RFC 8949, obsoleting its Section 8, and RFC 8610, obsoleting its Appendix G. It also specifies registry-based extension points and uses them to support text representations such as of epoch-based dates/times and of IP addresses and prefixes. // (This cref will be removed by the RFC editor:) The present -21 // incorporates pull request #84, including more integrated parsers // for using application extensions with raw strings, as well as // various cleanup. -21 is intended for use at the 2026-04-01 CBOR // interim, but contains no known April fools pranks.