Fastmail The JMAP base protocol ([JMAP-CORE]) provides the Foo/set method for creating, updating, and destroying objects. It offers a single concurrency control, the "ifInState" argument, which guards an entire object type: if any object of that type has changed, the whole method is rejected. This extension adds a finer, per-object conditional mechanism. A client may require that an individual update or destroy proceed only if the target object still matches a set of expected property values, expressed using the JMAP PatchObject already defined for updates. This provides optimistic concurrency control scoped to a single object — the equivalent of an HTTP "If-Match" precondition — for any JMAP data type. This extension also defines an optional "atomic" argument that applies an entire Foo/set as a single unit: either every change it requests takes effect, or none does. Combined with the per-object precondition, this lets a client express a multi-object change that is safe only when applied together — such as an atomic rename that exchanges two names.