diff --git a/lib/mcp/server/transports/streamable_http_transport.rb b/lib/mcp/server/transports/streamable_http_transport.rb
index fd45b332..bd395696 100644
--- a/lib/mcp/server/transports/streamable_http_transport.rb
+++ b/lib/mcp/server/transports/streamable_http_transport.rb
@@ -517,6 +517,11 @@ def handle_post(request)
             end
 
             if notification?(body)
+              # Reject a notification carrying an unknown or expired session ID instead of
+              # dispatching it against the shared `Server`. Mirrors the response and regular-request
+              # branches; without it a custom notification handler could run without a live session.
+              return session_not_found_response if !@stateless && !session_active?(session_id)
+
               dispatch_notification(body_string, session_id)
               handle_accepted
             elsif response?(body)
diff --git a/test/mcp/server/transports/streamable_http_transport_test.rb b/test/mcp/server/transports/streamable_http_transport_test.rb
index d36e1b07..3b79ce85 100644
--- a/test/mcp/server/transports/streamable_http_transport_test.rb
+++ b/test/mcp/server/transports/streamable_http_transport_test.rb
@@ -525,6 +525,40 @@ def string
           assert_equal "Missing session ID", body["error"]["message"]
         end
 
+        test "rejects notification with an unknown session ID in stateful mode" do
+          request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json", "HTTP_MCP_SESSION_ID" => "does-not-exist" },
+            { jsonrpc: "2.0", method: "notifications/initialized" }.to_json,
+          )
+
+          response = @transport.handle_request(request)
+          assert_equal 404, response[0]
+          body = JSON.parse(response[2][0])
+          assert_equal "Session not found", body["error"]["message"]
+        end
+
+        test "accepts a notification carrying a live session ID in stateful mode" do
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init", params: initialize_params }.to_json,
+          )
+          session_id = @transport.handle_request(init_request)[1]["Mcp-Session-Id"]
+
+          notification = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json", "HTTP_MCP_SESSION_ID" => session_id },
+            { jsonrpc: "2.0", method: "notifications/initialized" }.to_json,
+          )
+
+          response = @transport.handle_request(notification)
+          assert_equal 202, response[0]
+        end
+
         test "rejects response without session ID in stateful mode" do
           request = create_rack_request(
             "POST",