diff --git a/.changeset/fix-in-memory-event-store-underscore-streams.md b/.changeset/fix-in-memory-event-store-underscore-streams.md
new file mode 100644
index 000000000..aa7651a4f
--- /dev/null
+++ b/.changeset/fix-in-memory-event-store-underscore-streams.md
@@ -0,0 +1,6 @@
+---
+"@modelcontextprotocol/examples-server": patch
+"@modelcontextprotocol/test-integration": patch
+---
+
+fix: resume in-memory event streams with underscored ids
diff --git a/examples/server/src/inMemoryEventStore.ts b/examples/server/src/inMemoryEventStore.ts
index 604b84d39..2c98baecf 100644
--- a/examples/server/src/inMemoryEventStore.ts
+++ b/examples/server/src/inMemoryEventStore.ts
@@ -15,14 +15,6 @@ export class InMemoryEventStore implements EventStore {
         return `${streamId}_${Date.now()}_${Math.random().toString(36).slice(2, 10)}`;
     }
 
-    /**
-     * Extracts the stream ID from an event ID
-     */
-    private getStreamIdFromEventId(eventId: string): string {
-        const parts = eventId.split('_');
-        return parts.length > 0 ? parts[0]! : '';
-    }
-
     /**
      * Stores an event with a generated event ID
      * Implements EventStore.storeEvent
@@ -45,18 +37,17 @@ export class InMemoryEventStore implements EventStore {
             return '';
         }
 
-        // Extract the stream ID from the event ID
-        const streamId = this.getStreamIdFromEventId(lastEventId);
+        const streamId = this.events.get(lastEventId)?.streamId ?? '';
         if (!streamId) {
             return '';
         }
 
         let foundLastEvent = false;
 
-        // Sort events by eventId for chronological ordering
-        const sortedEvents = [...this.events.entries()].toSorted((a, b) => a[0].localeCompare(b[0]));
-
-        for (const [eventId, { streamId: eventStreamId, message }] of sortedEvents) {
+        // Map preserves insertion order, which is the event creation order. The
+        // generated IDs include a random suffix, so lexicographic sorting can
+        // reorder events created in the same millisecond.
+        for (const [eventId, { streamId: eventStreamId, message }] of this.events) {
             // Only include events from the same stream
             if (eventStreamId !== streamId) {
                 continue;
diff --git a/test/integration/test/taskResumability.test.ts b/test/integration/test/taskResumability.test.ts
index f7b4174d1..bf0aa0eb1 100644
--- a/test/integration/test/taskResumability.test.ts
+++ b/test/integration/test/taskResumability.test.ts
@@ -26,12 +26,11 @@ class InMemoryEventStore implements EventStore {
         { send }: { send: (eventId: string, message: JSONRPCMessage) => Promise<void> }
     ): Promise<string> {
         if (!lastEventId || !this.events.has(lastEventId)) return '';
-        const streamId = lastEventId.split('_')[0] ?? '';
+        const streamId = this.events.get(lastEventId)?.streamId ?? '';
         if (!streamId) return '';
 
         let found = false;
-        const sorted = [...this.events.entries()].toSorted((a, b) => a[0].localeCompare(b[0]));
-        for (const [eventId, { streamId: sid, message }] of sorted) {
+        for (const [eventId, { streamId: sid, message }] of this.events) {
             if (sid !== streamId) continue;
             if (eventId === lastEventId) {
                 found = true;
@@ -158,6 +157,40 @@ describe('Zod v4', () => {
             await transport.close();
         });
 
+        it('should replay events for standalone SSE stream IDs', async () => {
+            const nowSpy = vi.spyOn(Date, 'now').mockReturnValue(1_777_506_150_663);
+            const randomSpy = vi.spyOn(Math, 'random').mockReturnValueOnce(0.9).mockReturnValueOnce(0.1);
+
+            const message1: JSONRPCMessage = {
+                jsonrpc: '2.0',
+                method: 'notifications/message',
+                params: { level: 'info', data: 'first' }
+            };
+            const message2: JSONRPCMessage = {
+                jsonrpc: '2.0',
+                method: 'notifications/message',
+                params: { level: 'info', data: 'second' }
+            };
+            const replayed: Array<{ eventId: string; message: JSONRPCMessage }> = [];
+
+            try {
+                const firstEventId = await eventStore.storeEvent('_GET_stream', message1);
+                const secondEventId = await eventStore.storeEvent('_GET_stream', message2);
+
+                const streamId = await eventStore.replayEventsAfter(firstEventId, {
+                    send: async (eventId, message) => {
+                        replayed.push({ eventId, message });
+                    }
+                });
+
+                expect(streamId).toBe('_GET_stream');
+                expect(replayed).toEqual([{ eventId: secondEventId, message: message2 }]);
+            } finally {
+                nowSpy.mockRestore();
+                randomSpy.mockRestore();
+            }
+        });
+
         it('should have session ID functionality', async () => {
             // The ability to store a session ID when connecting
             const client = new Client({