fix(nsa): guard cp split to extend batches

test/registered/unit/layers/test_nsa_cp_utils.py

@@ -77,6 +77,31 @@ class TestPageAlignedCacheExtent(unittest.TestCase):
 
 
 class TestNSAInSeqCPUtils(unittest.TestCase):
+    def test_can_cp_split_ignores_decode_batch_without_extend_lens(self):
+        forward_batch = SimpleNamespace(
+            forward_mode=ForwardMode.DECODE,
+            extend_seq_lens_cpu=None,
+            uses_cp_shared_kv=True,
+        )
+
+        self.assertFalse(
+            can_cp_split(
+                seq_len=200, cp_size=8, use_nsa=True, forward_batch=forward_batch
+            )
+        )
+
+    def test_can_cp_split_fails_extend_batch_without_extend_lens(self):
+        forward_batch = SimpleNamespace(
+            forward_mode=ForwardMode.EXTEND,
+            extend_seq_lens_cpu=None,
+            uses_cp_shared_kv=True,
+        )
+
+        with self.assertRaisesRegex(RuntimeError, "missing_extend_seq_lens_cpu"):
+            can_cp_split(
+                seq_len=200, cp_size=8, use_nsa=True, forward_batch=forward_batch
+            )
+
     def test_contiguous_valid_cp_query_count(self):
         from sglang.srt.layers.attention.nsa.nsa_indexer import (
             _compute_contiguous_valid_cp_query_count,