diff --git a/backend/src/main/java/com/algorithmrace/visualizer/RateLimitFilter.java b/backend/src/main/java/com/algorithmrace/visualizer/RateLimitFilter.java
index 27fe72e..fa5f3eb 100644
--- a/backend/src/main/java/com/algorithmrace/visualizer/RateLimitFilter.java
+++ b/backend/src/main/java/com/algorithmrace/visualizer/RateLimitFilter.java
@@ -139,7 +139,7 @@ private static class SlidingWindow {
     private final java.util.Deque<Long> timestamps =
         new java.util.concurrent.ConcurrentLinkedDeque<>();
 
-    boolean tryAcquire(int limit) {
+    synchronized boolean tryAcquire(int limit) {
       long now = System.currentTimeMillis();
       long cutoff = now - WINDOW_MS;