fix(server): complete empty listen subscriptions

.changeset/complete-empty-listen-subscriptions.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/server': patch
+---
+
+Complete subscriptions that do not honor any requested notification types instead of retaining an idle SSE connection.

packages/server/src/server/listenRouter.ts

@@ -213,6 +213,15 @@ export function createListenRouter(options: ListenRouterOptions): ListenRouter {
                 );
                 writeNotification(ack.method, ack.params);
 
+                // A stream that honoured no notification types can never
+                // deliver an event. Acknowledge that outcome, then complete
+                // it instead of retaining a socket, bus subscription, and
+                // keep-alive timer indefinitely.
+                if (Object.keys(honored).length === 0) {
+                    teardown(true);
+                    return;
+                }
+
                 // Only after the ack frame is enqueued does delivery activate.
                 unsubscribe = bus.subscribe(event => {
                     if (closed || !listenFilterAccepts(honored, event)) return;
@@ -232,7 +241,7 @@ export function createListenRouter(options: ListenRouterOptions): ListenRouter {
             }
         });
 
-        if (signal !== undefined) {
+        if (!closed && signal !== undefined) {
             if (signal.aborted) {
                 teardown(false);
             } else {

packages/server/test/server/createMcpHandlerListen.test.ts

@@ -170,6 +170,32 @@ describe('createMcpHandler — subscriptions/listen', () => {
         await handler.close();
     });
 
+    it('acknowledges and completes an empty honored filter without opening a subscription', async () => {
+        vi.useFakeTimers();
+        try {
+            const subscribe = vi.fn(() => () => {});
+            const bus: ServerEventBus = { publish() {}, subscribe };
+            const handler = createMcpHandler(() => new McpServer({ name: 'no-listen-capabilities', version: '1.0.0' }), {
+                bus,
+                keepAliveMs: 1000
+            });
+
+            const response = await handler.fetch(listenRequest(12, { toolsListChanged: true }));
+            const text = await response.text();
+
+            expect(text).toContain('notifications/subscriptions/acknowledged');
+            expect(text).toContain('"notifications":{}');
+            expect(text).toContain('"resultType":"complete"');
+            expect(text.indexOf('notifications/subscriptions/acknowledged')).toBeLessThan(text.indexOf('"resultType":"complete"'));
+            expect(subscribe).not.toHaveBeenCalled();
+            expect(vi.getTimerCount()).toBe(0);
+
+            await handler.close();
+        } finally {
+            vi.useRealTimers();
+        }
+    });
+
     it('delivers only opted-in change types, each stamped with the subscription id', async () => {
         const handler = createMcpHandler(trivialFactory(), { keepAliveMs: 0 });
         const response = await handler.fetch(listenRequest(7, { toolsListChanged: true, resourceSubscriptions: ['file:///a'] }));

test/e2e/scenarios/subscriptions.test.ts

@@ -276,7 +276,7 @@ verifies('subscriptions:listen:capacity-guard', async () => {
                     jsonrpc: '2.0',
                     id,
                     method: 'subscriptions/listen',
-                    params: { _meta: modernEnvelopeMeta(), notifications: {} }
+                    params: { _meta: modernEnvelopeMeta(), notifications: { toolsListChanged: true } }
                 })
             })
         );