fix: correct SSE retry timing test to track GET requests only

The test was incorrectly measuring timing between all requests (POST and GET),
but the retry field only applies to GET SSE stream reconnections.

Changes:
- Only track timestamps and Last-Event-ID for GET requests
- Return 202 Accepted for notifications to trigger client GET stream
- Add eventsource-parser dependency

Author: felixweinberger

package-lock.json

@@ -45,6 +45,7 @@
   "dependencies": {
     "@modelcontextprotocol/sdk": "^1.22.0",
     "commander": "^14.0.2",
+    "eventsource-parser": "^3.0.6",
     "express": "^5.1.0",
     "lefthook": "^2.0.2",
     "zod": "^3.25.76"

package.json

@@ -76,14 +76,15 @@ export class SSERetryScenario implements Scenario {
     req: http.IncomingMessage,
     res: http.ServerResponse
   ): void {
-    const timestamp = performance.now();
-    this.connectionTimestamps.push(timestamp);
+    if (req.method === 'GET') {
+      // Track timing and Last-Event-ID only for GET requests
+      // since retry field only applies to SSE stream reconnections
+      const timestamp = performance.now();
+      this.connectionTimestamps.push(timestamp);
 
-    // Track Last-Event-ID header
-    const lastEventId = req.headers['last-event-id'] as string | undefined;
-    this.lastEventIds.push(lastEventId);
+      const lastEventId = req.headers['last-event-id'] as string | undefined;
+      this.lastEventIds.push(lastEventId);
 
-    if (req.method === 'GET') {
       // Handle SSE stream request
       this.handleSSEStream(req, res);
     } else if (req.method === 'POST') {
@@ -174,6 +175,11 @@ export class SSERetryScenario implements Scenario {
           setTimeout(() => {
             res.end();
           }, 100);
+        } else if (request.id === undefined) {
+          // Notifications (no id) - return 202 Accepted
+          // This triggers the client to start a GET SSE stream
+          res.writeHead(202);
+          res.end();
         } else {
           // For other requests, send a simple JSON response
           res.writeHead(200, { 'Content-Type': 'application/json' });