fix: handle cancellation for request id zero

Author: he-yufeng

.changeset/request-id-zero-cancel.md

@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/core": patch
+---
+
+fix: handle cancellation for request id zero

packages/core/src/shared/protocol.ts

@@ -409,11 +409,12 @@ export abstract class Protocol<ContextT extends BaseContext> {
     protected abstract buildContext(ctx: BaseContext, transportInfo?: MessageExtraInfo): ContextT;
 
     private async _oncancel(notification: CancelledNotification): Promise<void> {
-        if (!notification.params.requestId) {
+        const requestId = notification.params.requestId;
+        if (requestId === undefined) {
             return;
         }
         // Handle request cancellation
-        const controller = this._requestHandlerAbortControllers.get(notification.params.requestId);
+        const controller = this._requestHandlerAbortControllers.get(requestId);
         controller?.abort(notification.params.reason);
     }
 

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

@@ -2319,6 +2319,39 @@ describe('Request Cancellation vs Task Cancellation', () => {
             expect(wasAborted).toBe(true);
         });
 
+        test('should abort request handler for requestId 0', async () => {
+            await protocol.connect(transport);
+
+            let wasAborted = false;
+            protocol.setRequestHandler('ping', async (_request, ctx) => {
+                await new Promise(resolve => setTimeout(resolve, 100));
+                wasAborted = ctx.mcpReq.signal.aborted;
+                return {};
+            });
+
+            transport.onmessage?.({
+                jsonrpc: '2.0',
+                id: 0,
+                method: 'ping',
+                params: {}
+            });
+
+            await new Promise(resolve => setTimeout(resolve, 10));
+
+            transport.onmessage?.({
+                jsonrpc: '2.0',
+                method: 'notifications/cancelled',
+                params: {
+                    requestId: 0,
+                    reason: 'User cancelled'
+                }
+            });
+
+            await new Promise(resolve => setTimeout(resolve, 150));
+
+            expect(wasAborted).toBe(true);
+        });
+
         test('should NOT automatically cancel associated tasks when notifications/cancelled is received', async () => {
             await protocol.connect(transport);