fix: replay request SSE responses after disconnect

packages/middleware/node/test/streamableHttp.test.ts

@@ -1954,6 +1954,99 @@ describe('Zod v4', () => {
             toolResolve!();
         });
 
+        it('should replay terminal response after request-scoped SSE stream is closed', async () => {
+            const result = await createTestServer({
+                sessionIdGenerator: () => randomUUID(),
+                eventStore: createEventStore(),
+                retryInterval: 1000
+            });
+            server = result.server;
+            transport = result.transport;
+            baseUrl = result.baseUrl;
+            mcpServer = result.mcpServer;
+
+            let streamCloseCalled = false;
+            let toolResolve: () => void;
+            const toolCompletePromise = new Promise<void>(resolve => {
+                toolResolve = resolve;
+            });
+
+            mcpServer.registerTool('close-and-finish-tool', { description: 'Closes and then finishes' }, async ctx => {
+                ctx.http?.closeSSE?.();
+                streamCloseCalled = true;
+                await toolCompletePromise;
+                return { content: [{ type: 'text', text: 'finished after reconnect' }] };
+            });
+
+            const initResponse = await sendPostRequest(baseUrl, TEST_MESSAGES.initialize);
+            sessionId = initResponse.headers.get('mcp-session-id') as string;
+            expect(sessionId).toBeDefined();
+
+            const toolCallRequest: JSONRPCMessage = {
+                jsonrpc: '2.0',
+                id: 101,
+                method: 'tools/call',
+                params: { name: 'close-and-finish-tool', arguments: {} }
+            };
+
+            const postResponse = await fetch(baseUrl, {
+                method: 'POST',
+                headers: {
+                    'Content-Type': 'application/json',
+                    Accept: 'text/event-stream, application/json',
+                    'mcp-session-id': sessionId,
+                    'mcp-protocol-version': '2025-11-25'
+                },
+                body: JSON.stringify(toolCallRequest)
+            });
+
+            expect(postResponse.status).toBe(200);
+
+            const reader = postResponse.body?.getReader();
+            const priming = await reader!.read();
+            const primingText = new TextDecoder().decode(priming.value);
+            const idMatch = primingText.match(/id: ([^\n]+)/);
+            expect(idMatch).toBeTruthy();
+            const lastEventId = idMatch![1]!;
+
+            await new Promise(resolve => setTimeout(resolve, 100));
+            expect(streamCloseCalled).toBe(true);
+
+            const { done } = await reader!.read();
+            expect(done).toBe(true);
+
+            toolResolve!();
+            await new Promise(resolve => setTimeout(resolve, 100));
+
+            const reconnectResponse = await fetch(baseUrl, {
+                method: 'GET',
+                headers: {
+                    Accept: 'text/event-stream',
+                    'mcp-session-id': sessionId,
+                    'mcp-protocol-version': '2025-11-25',
+                    'last-event-id': lastEventId
+                }
+            });
+
+            expect(reconnectResponse.status).toBe(200);
+
+            const reconnectReader = reconnectResponse.body?.getReader();
+            let replayedText = '';
+            const timeout = setTimeout(() => reconnectReader!.cancel(), 2000);
+            try {
+                while (!replayedText.includes('finished after reconnect')) {
+                    const { value, done } = await reconnectReader!.read();
+                    if (done) break;
+                    replayedText += new TextDecoder().decode(value);
+                }
+            } finally {
+                clearTimeout(timeout);
+            }
+
+            expect(replayedText).toContain('finished after reconnect');
+            expect(replayedText).toContain('"id":101');
+        });
+
         it('should provide closeSSEStream callback in ctx when eventStore is configured', async () => {
             const result = await createTestServer({
                 sessionIdGenerator: () => randomUUID(),

packages/server/src/server/streamableHttp.ts

@@ -985,14 +985,12 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
 
         const stream = this._streamMapping.get(streamId);
 
-        if (!this._enableJsonResponse && stream?.controller && stream?.encoder) {
-            // For SSE responses, generate event ID if event store is provided
-            let eventId: string | undefined;
+        let eventId: string | undefined;
+        if (!this._enableJsonResponse && this._eventStore) {
+            eventId = await this._eventStore.storeEvent(streamId, message);
+        }
 
-            if (this._eventStore) {
-                eventId = await this._eventStore.storeEvent(streamId, message);
-            }
-            // Write the event to the response stream
+        if (!this._enableJsonResponse && stream?.controller && stream?.encoder) {
             this.writeSSEEvent(stream.controller, stream.encoder, message, eventId);
         }
 
@@ -1005,6 +1003,13 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
 
             if (allResponsesReady) {
                 if (!stream) {
+                    if (!this._enableJsonResponse && this._eventStore) {
+                        for (const id of relatedIds) {
+                            this._requestResponseMap.delete(id);
+                            this._requestToStreamMapping.delete(id);
+                        }
+                        return;
+                    }
                     throw new Error(`No connection established for request ID: ${String(requestId)}`);
                 }
                 if (this._enableJsonResponse && stream.resolveJson) {