fix(server): keep idle GET SSE streams alive

packages/server/src/server/streamableHttp.ts

@@ -438,6 +438,14 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
 
         const encoder = new TextEncoder();
         let streamController: ReadableStreamDefaultController<Uint8Array>;
+        let keepAliveInterval: ReturnType<typeof setInterval> | undefined;
+
+        const clearKeepAlive = () => {
+            if (keepAliveInterval !== undefined) {
+                clearInterval(keepAliveInterval);
+                keepAliveInterval = undefined;
+            }
+        };
 
         // Create a ReadableStream with a controller we can use to push SSE events
         const readable = new ReadableStream<Uint8Array>({
@@ -446,6 +454,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             },
             cancel: () => {
                 // Stream was cancelled by client
+                clearKeepAlive();
                 this._streamMapping.delete(this._standaloneSseStreamId);
             }
         });
@@ -466,6 +475,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             controller: streamController!,
             encoder,
             cleanup: () => {
+                clearKeepAlive();
                 this._streamMapping.delete(this._standaloneSseStreamId);
                 try {
                     streamController!.close();
@@ -475,6 +485,14 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             }
         });
 
+        keepAliveInterval = setInterval(() => {
+            try {
+                streamController!.enqueue(encoder.encode(': keep-alive\n\n'));
+            } catch {
+                clearKeepAlive();
+            }
+        }, 15_000);
+
         return new Response(readable, { headers });
     }
 

packages/server/test/server/streamableHttp.test.ts

@@ -360,6 +360,28 @@ describe('Zod v4', () => {
                 expect(response.headers.get('mcp-session-id')).toBe(sessionId);
             });
 
+            it('should send keep-alive comments on idle standalone SSE streams', async () => {
+                vi.useFakeTimers();
+                try {
+                    sessionId = await initializeServer();
+
+                    const request = createRequest('GET', undefined, { sessionId });
+                    const response = await transport.handleRequest(request);
+                    const reader = response.body!.getReader();
+                    const read = reader.read();
+
+                    await vi.advanceTimersByTimeAsync(15_000);
+
+                    const { value, done } = await read;
+                    expect(done).toBe(false);
+                    expect(new TextDecoder().decode(value)).toBe(': keep-alive\n\n');
+
+                    await reader.cancel();
+                } finally {
+                    vi.useRealTimers();
+                }
+            });
+
             it('should reject GET without Accept: text/event-stream', async () => {
                 sessionId = await initializeServer();