diff --git a/api/src/test/java/org/opensearch/sql/api/UnifiedQueryPlannerTest.java b/api/src/test/java/org/opensearch/sql/api/UnifiedQueryPlannerTest.java
index bb2d1e4a53f..008121e8377 100644
--- a/api/src/test/java/org/opensearch/sql/api/UnifiedQueryPlannerTest.java
+++ b/api/src/test/java/org/opensearch/sql/api/UnifiedQueryPlannerTest.java
@@ -159,6 +159,18 @@ public void unsupportedFeatureIsRethrownAsSemanticCheckException() {
         .assertErrorMessageContains("unsupported in Calcite");
   }
 
+  @Test
+  public void unsupportedWindowFunctionIsRethrownAsSemanticCheckException() {
+    // Window functions outside WINDOW_FUNC_MAPPING reach
+    // CalciteRexNodeVisitor#visitWindowFunction's
+    // orElseThrow. The throw site emits CalciteUnsupportedException so this path normalizes to a
+    // 4xx SemanticCheckException rather than escaping as a 500.
+    givenInvalidQuery("source = catalog.employees | eventstats rank()")
+        .assertErrorType(SemanticCheckException.class)
+        .assertCauseType(CalciteUnsupportedException.class)
+        .assertErrorMessageContains("Unexpected window function: rank");
+  }
+
   @Test
   public void assertionErrorIsWrappedAsSemanticCheckException() {
     // Remove when the underlying Calcite assertion is fixed.
diff --git a/core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java b/core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java
index 3c37a11ba5b..849b615f970 100644
--- a/core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java
+++ b/core/src/main/java/org/opensearch/sql/calcite/CalciteRexNodeVisitor.java
@@ -705,7 +705,7 @@ public RexNode visitWindowFunction(WindowFunction node, CalcitePlanContext conte
                       node.getWindowFrame());
             })
         .orElseThrow(
-            () -> new UnsupportedOperationException("Unexpected window function: " + funcName));
+            () -> new CalciteUnsupportedException("Unexpected window function: " + funcName));
   }
 
   private List<RexNode> translateOrderKeys(