feat(server): add keepAliveInterval to standalone GET SSE stream

Adds an opt-in keepAliveInterval option to
WebStandardStreamableHTTPServerTransportOptions that sends periodic
SSE comments (`: keepalive`) on the standalone GET SSE stream.

Reverse proxies commonly close connections that are idle for 30-60s.
With no server-initiated messages, the GET SSE stream has no traffic
during quiet periods, causing silent disconnections. This option lets
operators send harmless SSE comments at a configurable cadence to
keep the connection alive.

The timer is cleared on close(), closeStandaloneSSEStream(), and on
stream cancellation. Disabled by default; no behavior change for
existing deployments.

Addresses upstream #28, #876.

Author: rcdailey

.changeset/add-sse-keepalive.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/server': minor
+---
+
+Add optional `keepAliveInterval` to `WebStandardStreamableHTTPServerTransportOptions` that sends periodic SSE comments on the standalone GET stream to prevent reverse proxy idle timeout disconnections.

packages/server/src/server/streamableHttp.ts

@@ -141,6 +141,15 @@ export interface WebStandardStreamableHTTPServerTransportOptions {
      */
     retryInterval?: number;
 
+    /**
+     * Interval in milliseconds for sending SSE keepalive comments on the standalone
+     * GET SSE stream. When set, the transport sends periodic SSE comments
+     * (`: keepalive`) to prevent reverse proxies from closing idle connections.
+     *
+     * Disabled by default (no keepalive comments are sent).
+     */
+    keepAliveInterval?: number;
+
     /**
      * List of protocol versions that this transport will accept.
      * Used to validate the `mcp-protocol-version` header in incoming requests.
@@ -238,6 +247,8 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
     private _allowedOrigins?: string[];
     private _enableDnsRebindingProtection: boolean;
     private _retryInterval?: number;
+    private _keepAliveInterval?: number;
+    private _keepAliveTimer?: ReturnType<typeof setInterval>;
     private _supportedProtocolVersions: string[];
 
     sessionId?: string;
@@ -255,6 +266,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
         this._allowedOrigins = options.allowedOrigins;
         this._enableDnsRebindingProtection = options.enableDnsRebindingProtection ?? false;
         this._retryInterval = options.retryInterval;
+        this._keepAliveInterval = options.keepAliveInterval;
         this._supportedProtocolVersions = options.supportedProtocolVersions ?? SUPPORTED_PROTOCOL_VERSIONS;
     }
 
@@ -443,6 +455,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             },
             cancel: () => {
                 // Stream was cancelled by client
+                this._clearKeepAliveTimer();
                 this._streamMapping.delete(this._standaloneSseStreamId);
             }
         });
@@ -472,6 +485,19 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             }
         });
 
+        // Start keepalive timer to send periodic SSE comments that prevent
+        // reverse proxies from closing the connection due to idle timeouts
+        if (this._keepAliveInterval !== undefined) {
+            this._keepAliveTimer = setInterval(() => {
+                try {
+                    streamController!.enqueue(encoder.encode(': keepalive\n\n'));
+                } catch {
+                    // Controller is closed or errored, stop sending keepalives
+                    this._clearKeepAliveTimer();
+                }
+            }, this._keepAliveInterval);
+        }
+
         return new Response(readable, { headers });
     }
 
@@ -886,7 +912,16 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
         return undefined;
     }
 
+    private _clearKeepAliveTimer(): void {
+        if (this._keepAliveTimer !== undefined) {
+            clearInterval(this._keepAliveTimer);
+            this._keepAliveTimer = undefined;
+        }
+    }
+
     async close(): Promise<void> {
+        this._clearKeepAliveTimer();
+
         // Close all SSE connections
         for (const { cleanup } of this._streamMapping.values()) {
             cleanup();
@@ -918,6 +953,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
      * Use this to implement polling behavior for server-initiated notifications.
      */
     closeStandaloneSSEStream(): void {
+        this._clearKeepAliveTimer();
         const stream = this._streamMapping.get(this._standaloneSseStreamId);
         if (stream) {
             stream.cleanup();

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

@@ -765,4 +765,93 @@ describe('Zod v4', () => {
             await expect(transport.start()).rejects.toThrow('Transport already started');
         });
     });
+
+    describe('HTTPServerTransport - keepAliveInterval', () => {
+        let transport: WebStandardStreamableHTTPServerTransport;
+        let mcpServer: McpServer;
+
+        beforeEach(() => {
+            vi.useFakeTimers();
+        });
+
+        afterEach(async () => {
+            vi.useRealTimers();
+            await transport.close();
+        });
+
+        async function setupTransport(keepAliveInterval?: number): Promise<string> {
+            mcpServer = new McpServer({ name: 'test-server', version: '1.0.0' });
+
+            transport = new WebStandardStreamableHTTPServerTransport({
+                sessionIdGenerator: () => randomUUID(),
+                keepAliveInterval
+            });
+
+            await mcpServer.connect(transport);
+
+            const initReq = createRequest('POST', TEST_MESSAGES.initialize);
+            const initRes = await transport.handleRequest(initReq);
+            return initRes.headers.get('mcp-session-id') as string;
+        }
+
+        it('should send SSE keepalive comments periodically when keepAliveInterval is set', async () => {
+            const sessionId = await setupTransport(50);
+
+            const getReq = createRequest('GET', undefined, { sessionId });
+            const getRes = await transport.handleRequest(getReq);
+
+            expect(getRes.status).toBe(200);
+            expect(getRes.body).not.toBeNull();
+
+            const reader = getRes.body!.getReader();
+
+            // Advance past two intervals to accumulate keepalive comments
+            vi.advanceTimersByTime(120);
+
+            const { value } = await reader.read();
+            const text = new TextDecoder().decode(value);
+            expect(text).toContain(': keepalive');
+        });
+
+        it('should not send SSE comments when keepAliveInterval is not set', async () => {
+            const sessionId = await setupTransport(undefined);
+
+            const getReq = createRequest('GET', undefined, { sessionId });
+            const getRes = await transport.handleRequest(getReq);
+
+            expect(getRes.status).toBe(200);
+            expect(getRes.body).not.toBeNull();
+
+            const reader = getRes.body!.getReader();
+
+            // Advance time; no keepalive should be enqueued
+            vi.advanceTimersByTime(200);
+
+            // Close the transport to end the stream, then read whatever was buffered
+            await transport.close();
+
+            const chunks: string[] = [];
+            for (let result = await reader.read(); !result.done; result = await reader.read()) {
+                chunks.push(new TextDecoder().decode(result.value));
+            }
+
+            const allText = chunks.join('');
+            expect(allText).not.toContain(': keepalive');
+        });
+
+        it('should clear the keepalive interval when the transport is closed', async () => {
+            const sessionId = await setupTransport(50);
+
+            const getReq = createRequest('GET', undefined, { sessionId });
+            const getRes = await transport.handleRequest(getReq);
+
+            expect(getRes.status).toBe(200);
+
+            // Close the transport, which should clear the interval
+            await transport.close();
+
+            // Advancing timers after close should not throw
+            vi.advanceTimersByTime(200);
+        });
+    });
 });