Stalwart Labs LLC This document defines an extension to the JSON Meta Application Protocol (JMAP) that lets clients and servers attach metadata to objects of existing JMAP data types. Each data type the server lists gains two new properties, metadata and privateMetadata, whose values are objects keyed by _namespace identifier_. A namespace identifier is either a name registered with IANA or a domain name controlled by the vendor providing the namespace; the latter allows vendors and applications to extend the metadata schema without prior coordination. Because metadata is carried as a property on the related object, clients use the existing /get, /set, /changes, and /query methods to read, modify, and synchronize it.