fix(core): preserve relatedRequestId 0 in debounce guard

.changeset/fix-related-request-id-zero-debounce.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/core': patch
+---
+
+Preserve `relatedRequestId: 0` when deciding whether notifications can be debounced. Request id `0` is valid, so request-associated notifications with that id now bypass debounce like other related notifications.

packages/core/src/shared/protocol.ts

@@ -1010,7 +1010,10 @@ export abstract class Protocol<ContextT extends BaseContext> {
         // A notification can only be debounced if it's in the list AND it's "simple"
         // (i.e., has no parameters and no related request ID or related task that could be lost).
         const canDebounce =
-            debouncedMethods.includes(notification.method) && !notification.params && !options?.relatedRequestId && !options?.relatedTask;
+            debouncedMethods.includes(notification.method) &&
+            !notification.params &&
+            options?.relatedRequestId === undefined &&
+            !options?.relatedTask;
 
         if (canDebounce) {
             // If a notification of this type is already scheduled, do nothing.

packages/core/test/shared/protocol.test.ts

@@ -768,6 +768,21 @@ describe('protocol tests', () => {
             expect(sendSpy).toHaveBeenCalledWith(expect.any(Object), { relatedRequestId: 'req-2' });
         });
 
+        it('should NOT debounce a notification that has relatedRequestId 0', async () => {
+            // ARRANGE
+            protocol = new TestProtocolImpl({ debouncedNotificationMethods: ['test/debounced_with_options'] });
+            await protocol.connect(transport);
+
+            // ACT
+            const firstNotification = protocol.notification({ method: 'test/debounced_with_options' }, { relatedRequestId: 0 });
+            const secondNotification = protocol.notification({ method: 'test/debounced_with_options' }, { relatedRequestId: 0 });
+            await Promise.all([firstNotification, secondNotification]);
+
+            // ASSERT
+            expect(sendSpy).toHaveBeenCalledTimes(2);
+            expect(sendSpy).toHaveBeenCalledWith(expect.any(Object), { relatedRequestId: 0 });
+        });
+
         it('should clear pending debounced notifications on connection close', async () => {
             // ARRANGE
             protocol = new TestProtocolImpl({ debouncedNotificationMethods: ['test/debounced'] });