diff --git a/packages/server/src/server/streamableHttp.ts b/packages/server/src/server/streamableHttp.ts
index fd3563a077..bdf703dfdc 100644
--- a/packages/server/src/server/streamableHttp.ts
+++ b/packages/server/src/server/streamableHttp.ts
@@ -231,7 +231,10 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
     private _requestResponseMap: Map<RequestId, JSONRPCMessage> = new Map();
     private _initialized: boolean = false;
     private _enableJsonResponse: boolean = false;
-    private _standaloneSseStreamId: string = '_GET_stream';
+    private get _standaloneSseStreamId(): string {
+        const hardcode = '_GET_stream';
+        return this.sessionId === undefined ? hardcode : `${hardcode}_${this.sessionId}`;
+    }
     private _eventStore?: EventStore;
     private _onsessioninitialized?: ((sessionId: string) => void | Promise<void>) | undefined;
     private _onsessionclosed?: ((sessionId: string) => void | Promise<void>) | undefined;
diff --git a/packages/server/test/server/streamableHttp.test.ts b/packages/server/test/server/streamableHttp.test.ts
index 7a23dd56bb..549d908914 100644
--- a/packages/server/test/server/streamableHttp.test.ts
+++ b/packages/server/test/server/streamableHttp.test.ts
@@ -707,6 +707,78 @@ describe('Zod v4', () => {
         });
     });
 
+    describe('HTTPServerTransport - Standalone SSE Stream Isolation', () => {
+        // 2 transports sharing one EventStore must record distinct `streamId` when
+        // server-initiated notifications take the standalone-SSE branch in `send()`
+        // (streamableHttp.ts: `requestId === undefined` -> `storeEvent(this._standaloneSseStreamId, message)`).
+
+        const standaloneNotification: JSONRPCMessage = {
+            jsonrpc: '2.0',
+            method: 'notifications/tools/list_changed'
+        };
+
+        const makeRecordingEventStore = (): {
+            store: EventStore;
+            calls: { streamId: StreamId; message: JSONRPCMessage }[];
+        } => {
+            const calls: { streamId: StreamId; message: JSONRPCMessage }[] = [];
+            const store: EventStore = {
+                async storeEvent(streamId, message) {
+                    calls.push({ streamId, message });
+                    return `${streamId}_${calls.length}`;
+                },
+                async replayEventsAfter() {
+                    return '';
+                }
+            };
+            return { store, calls };
+        };
+
+        const makeTransport = (sessionIdValue: string | undefined, store: EventStore): WebStandardStreamableHTTPServerTransport => {
+            const transport = new WebStandardStreamableHTTPServerTransport({
+                sessionIdGenerator: sessionIdValue === undefined ? undefined : () => sessionIdValue,
+                eventStore: store
+            });
+            // Model the post-initialized state without running the full handshake;
+            // `send()` does not require `start()`.
+            if (sessionIdValue !== undefined) {
+                transport.sessionId = sessionIdValue;
+            }
+            return transport;
+        };
+
+        const HARDCODED_STREAM_ID = '_GET_stream';
+
+        it('uses distinct streamIds for 2 sessions sharing 1 EventStore', async () => {
+            const { store, calls } = makeRecordingEventStore();
+            const transportA = makeTransport('sess-A', store);
+            const transportB = makeTransport('sess-B', store);
+            try {
+                await transportA.send(standaloneNotification);
+                await transportB.send(standaloneNotification);
+                expect(calls).toHaveLength(2);
+                const streamIds = new Set(calls.map(c => c.streamId));
+                expect(streamIds.size).toBe(2);
+                expect(streamIds.has(HARDCODED_STREAM_ID)).toBe(false);
+            } finally {
+                await transportA.close();
+                await transportB.close();
+            }
+        });
+
+        it('falls back to the literal "_GET_stream" in stateless mode', async () => {
+            const { store, calls } = makeRecordingEventStore();
+            const transport = makeTransport(undefined, store);
+            try {
+                await transport.send(standaloneNotification);
+                expect(calls).toHaveLength(1);
+                expect(calls[0]!.streamId).toBe(HARDCODED_STREAM_ID);
+            } finally {
+                await transport.close();
+            }
+        });
+    });
+
     describe('HTTPServerTransport - Protocol Version Validation', () => {
         let transport: WebStandardStreamableHTTPServerTransport;
         let mcpServer: McpServer;