DL-303
The public SessionInjection.traceparent field (compass.proto, field 4) exposes the Server’s active-span W3C traceparent on the SubscribeAgentSession observation surface, so a public subscriber can join a SessionInjection to the message’s server-side trace (Matt ruled OQ2 = yes at the T4b freeze gate). Additive string scalar beside from_handle (a W3C trace-context token 00-<trace-id>-<span-id>-<flags>, not an identifier); EMPTY when the Server had no active span; trace machinery never blocks or fails a delivery. The internal control-op (agent.proto SteerControl/DeliverControl) and causal-link (agent_gateway.proto CommsCallRequest.trigger_traceparent) legs are internal-tree, not a public-API decision
Status: Active (Matt, 2026-08-27)