RISE AB IMT Atlantique IRISA Consultant This document defines how to compress Constrained Application Protocol (CoAP) headers using the Static Context Header Compression and fragmentation (SCHC) framework. SCHC defines a header compression mechanism adapted for constrained devices, and it uses a static description of the header to reduce the header's redundancy and size. While RFC 8724 describes the SCHC compression and fragmentation framework and its application for IPv6 and UDP headers, this document applies SCHC to CoAP headers. The CoAP header structure differs from that of IPv6 and UDP headers, since CoAP uses a flexible header with a variable number of options that are in turn of variable length. The CoAP message format is asymmetric, i.e., request messages have a header format different from that of response messages. This specification gives guidance on applying SCHC to flexible headers and on leveraging the message format asymmetry for defining more efficient compression Rules. This document replaces and obsoletes RFC 8824.