diff --git a/src/Protocol.php b/src/Protocol.php
index 918885e..7870a49 100644
--- a/src/Protocol.php
+++ b/src/Protocol.php
@@ -120,7 +120,12 @@ public function processMessage(Request|Notification|BatchRequest $message, strin
         $session = $this->sessionManager->getSession($sessionId);
 
         if ($session === null) {
-            $error = Error::forInvalidRequest('Invalid or expired session. Please re-initialize the session.', $message->id);
+            if ($message instanceof Notification) {
+                $this->logger->warning('Notification received for invalid or expired session. Ignoring.', ['sessionId' => $sessionId, 'method' => $message->method]);
+                return;
+            }
+
+            $error = Error::forInvalidRequest('Invalid or expired session. Please re-initialize the session.', $message->getId());
             $messageContext['status_code'] = 404;
 
             $this->transport->sendMessage($error, $sessionId, $messageContext)
diff --git a/tests/Unit/ProtocolTest.php b/tests/Unit/ProtocolTest.php
index 4bc0b15..0975930 100644
--- a/tests/Unit/ProtocolTest.php
+++ b/tests/Unit/ProtocolTest.php
@@ -271,6 +271,31 @@ function expectSuccessResponse(mixed $response, mixed $expectedResult, string|in
     $this->session->shouldNotHaveReceived('save');
 });
 
+it('ignores notification if session is not found', function () {
+    $notification = createNotification('notifications/initialized');
+    $this->sessionManager->shouldReceive('getSession')->with('unknown-client')->andReturn(null);
+
+    $this->transport->shouldNotReceive('sendMessage');
+    $this->dispatcher->shouldNotReceive('handleNotification');
+
+    $this->protocol->processMessage($notification, 'unknown-client', ['is_initialize_request' => false]);
+    $this->session->shouldNotHaveReceived('save');
+});
+
+it('sends error response with first request id if session is not found for batch', function () {
+    $batchRequest = new BatchRequest([createNotification('notifications/initialized'), createRequest('tools/list', [], 'batch-req-1')]);
+    $this->sessionManager->shouldReceive('getSession')->with('unknown-client')->andReturn(null);
+
+    $this->transport->shouldReceive('sendMessage')->once()
+        ->with(Mockery::on(function (Error $error) {
+            expectErrorResponse($error, \PhpMcp\Schema\Constants::INVALID_REQUEST, 'batch-req-1');
+            return true;
+        }), 'unknown-client', Mockery::any())
+        ->andReturn(resolve(null));
+
+    $this->protocol->processMessage($batchRequest, 'unknown-client');
+});
+
 it('sends error response if session is not initialized for non-initialize request', function () {
     $request = createRequest('tools/list');
     $this->session->shouldReceive('get')->with('initialized', false)->andReturn(false);