The controller asks the build role that has a holder, and hears both roles' outcomes (hq ADR 0190, the handover)

A controller that asked node-build-agent from its first run would queue every build where nothing
pulls, and the build that registers build-agent — the first holder — would be among them. So the
role is chosen at ask time from the catalogue: the current role when any assigned module claims it,
the retired one while only the builder does, the current one when neither. Outcomes are followed on
both seats, the controller may publish to both, and a build's log is read under whichever role did
it; a machine on the retired role is proven on the bus to take that role's asks. The switch order
is written where the role is named, and the retired half is marked for removal with the seat row.

internal/link/receive_nats.go

@@ -223,10 +223,11 @@ func kindOfSubject(subject string) (string, bool) {
 		return KindCatchUp, true
 	case broker.ControllerFollows[3]:
 		return KindSourceMoved, true
-	case BuildOutcome():
+	case BuildOutcome(), BuildOutcomeOf(TheBuildMachineBefore):
 		// A build's outcome is the role's event now, so it arrives on the events stream rather than
 		// the control branch — and is acted on by the same handler, because what the controller does
-		// with it did not change (novox/hq ADR 0121).
+		// with it did not change (novox/hq ADR 0121). From either build role while the handover
+		// runs (ADR 0190): the old builder still answers on the retired seat until it is unassigned.
 		return KindBuilt, true
 	}
 	return "", false