fix(server): application/json header for notifictions

.changeset/strong-hairs-study.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/server': patch
+---
+
+add application/json header for notifications

packages/server/src/server/webStandardStreamableHttp.ts

@@ -667,7 +667,12 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
                 for (const message of messages) {
                     this.onmessage?.(message, { authInfo: options?.authInfo, requestInfo });
                 }
-                return new Response(null, { status: 202 });
+                return new Response(null, {
+                    status: 202,
+                    headers: {
+                        'Content-Type': 'application/json'
+                    }
+                });
             }
 
             // The default behavior is to use SSE streaming

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

@@ -651,6 +651,7 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
             const response = await sendPostRequest(baseUrl, batchNotifications, sessionId);
 
             expect(response.status).toBe(202);
+            expect(response.headers.get('content-type')).toBe('application/json');
         });
 
         it('should handle batch request messages with SSE stream for responses', async () => {