fix(server): add keepalive timer in replayEvents() and fix test

The replayEvents() code path (client reconnects with Last-Event-ID) was

missing keepalive timer setup, so reconnecting clients would lose

keepalive protection and get dropped again at the next proxy idle

timeout.

Also fixes the cleanup test to actually prove that close() clears the

timer by asserting vi.getTimerCount() drops to 0, instead of relying

on the catch fallback which would self-clear anyway.

Addresses PR review feedback from @felixweinberger on PR #1726.

Author: rcdailey

packages/server/src/server/streamableHttp.ts

@@ -551,6 +551,7 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
                 },
                 cancel: () => {
                     // Stream was cancelled by client
+                    this._clearKeepAliveTimer();
                     // Cleanup will be handled by the mapping
                 }
             });
@@ -582,6 +583,18 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
                 }
             });
 
+            // Start keepalive timer for the replayed stream so reconnecting
+            // clients remain protected from proxy idle timeouts
+            if (this._keepAliveInterval !== undefined) {
+                this._keepAliveTimer = setInterval(() => {
+                    try {
+                        streamController!.enqueue(encoder.encode(': keepalive\n\n'));
+                    } catch {
+                        this._clearKeepAliveTimer();
+                    }
+                }, this._keepAliveInterval);
+            }
+
             return new Response(readable, { headers });
         } catch (error) {
             this.onerror?.(error as Error);

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

@@ -1037,12 +1037,12 @@ describe('Zod v4', () => {
             const getRes = await transport.handleRequest(getReq);
 
             expect(getRes.status).toBe(200);
+            expect(vi.getTimerCount()).toBe(1);
 
             // Close the transport, which should clear the interval
             await transport.close();
 
-            // Advancing timers after close should not throw
-            vi.advanceTimersByTime(200);
+            expect(vi.getTimerCount()).toBe(0);
         });
     });
 });