An Agent-Channel Frame for Identity-Keyed Fan-Out Delivery to Concurrent Sessions

This memo specifies an application-layer frame format and a delivery model by which the several concurrent agentic sessions of a single identity-bound principal, and the recognised members of an organisational identity substrate, exchange short structured messages. The frame, termed the agent-channel frame, is a transport envelope: it carries a closed-catalogue kind discriminator, a structured per-kind payload, an identity attribution pair, and an inline provenance block. Delivery is fan-out: a sender names a recipient scope rather than a single endpoint, and the scope is expanded at delivery time against the recipient's subscriptions. Recipients receive frames over a per-handle Server-Sent Events stream and MAY narrow what they receive with a subscribe-time filter expression. Frames are ephemeral routing units; the memo specifies only the wire envelope, the scope-expansion grammar, the subscribe filter grammar, and the delivery semantics. Frame persistence, where an implementation chooses to retain frames for replay, is out of scope and is not specified. The memo composes with the handle namespace of [IDPRONOUNS], the discovery surface of [MCPDNS], and the cross-organisational ceremony of [IDACCORD]; no new transport and no new handle category is introduced.