fix(server-runtime): preserve explicit empty data destinations

packages/server-runtime/src/middlewares/route.test.ts

@@ -99,11 +99,11 @@ describe('route middleware', () => {
 
     expect(collectDestinations(event)).toEqual(['label:env=prod'])
   })
-  it('respects explicit empty destinations as an override', () => {
+  it('treats an explicit empty route destination list as the override', () => {
     const event = createSparkNotifyEvent({
       data: {
-        id: 'evt-3',
-        eventId: 'spark-3',
+        id: 'evt-override',
+        eventId: 'spark-override',
         kind: 'ping',
         urgency: 'soon',
         headline: 'hello',
@@ -115,22 +115,36 @@ describe('route middleware', () => {
     expect(collectDestinations(event)).toEqual([])
   })
 
-  it('treats an explicit empty route destination list as the override', () => {
+  it('treats an explicit empty data destination list as the override', () => {
     const event = createSparkNotifyEvent({
       data: {
-        id: 'evt-override',
-        eventId: 'spark-override',
+        id: 'evt-data-empty',
+        eventId: 'spark-data-empty',
         kind: 'ping',
         urgency: 'soon',
         headline: 'hello',
-        destinations: ['module:character'],
+        destinations: [],
       },
-      route: { destinations: [] },
+      route: undefined,
     })
 
     expect(collectDestinations(event)).toEqual([])
   })
 
+  it('ignores primitive data payloads when checking destinations', () => {
+    const event = {
+      type: 'spark:notify',
+      data: 'not-an-object',
+      metadata: {
+        source: { kind: 'plugin', plugin: { id: 'server-runtime' }, id: 'test' },
+        event: { id: 'evt-primitive' },
+      },
+      route: undefined,
+    } as unknown as WebSocketBaseEvent<'spark:notify', WebSocketEvents['spark:notify'], any>
+
+    expect(collectDestinations(event)).toBeUndefined()
+  })
+
   it('matches destinations by label selector', () => {
     const peer = createPeer({
       id: 'peer-2',

packages/server-runtime/src/middlewares/route.ts

@@ -25,6 +25,8 @@ export interface RouteContext {
 
 export type RouteMiddleware = (context: RouteContext) => RouteDecision | void
 
+type DestinationList = Array<string | RouteTargetExpression>
+
 function getPeerLabels(peer: AuthenticatedPeer) {
   return {
     ...peer.identity?.plugin?.labels,
@@ -118,25 +120,29 @@ export function createPolicyMiddleware(policy: RoutingPolicy): RouteMiddleware {
 }
 
 /**
- * Resolves the destinations attached to an event.
+ * Collects explicit route destinations from the route envelope or event payload.
  *
  * Use when:
- * - Route-level destinations should override payload-level destinations
- * - Delivery logic needs to distinguish between "broadcast" and "explicitly send nowhere"
+ * - Routing middleware needs the effective destination override for a websocket event
+ * - Callers must preserve explicit empty destination lists instead of falling back to broadcast
  *
  * Expects:
- * - An explicit empty `route.destinations` array is a meaningful override
+ * - A websocket event whose `route.destinations` or `data.destinations` may be present
+ * - `data.destinations` is only treated as valid when it is an array-shaped override
  *
  * Returns:
- * - The route destinations, payload destinations, or `undefined` when the event is unrestricted
+ * - The explicit destination list when present
+ * - `undefined` when no destination override was provided
  */
-export function collectDestinations(event: WebSocketEvent | (Omit<WebSocketEvent, 'metadata'> & Partial<Pick<WebSocketEvent, 'metadata'>>)) {
+export function collectDestinations(
+  event: WebSocketEvent | (Omit<WebSocketEvent, 'metadata'> & Partial<Pick<WebSocketEvent, 'metadata'>>),
+): DestinationList | undefined {
   if (event.route && 'destinations' in event.route) {
     return event.route.destinations
   }
 
-  const data = event.data as { destinations?: Array<string | RouteTargetExpression> } | undefined
-  if (data?.destinations?.length) {
+  const data = event.data as unknown
+  if (typeof data === 'object' && data !== null && 'destinations' in data && Array.isArray(data.destinations)) {
     return data.destinations
   }