fix(disagg): correct DSA/SWA state-page transfer mismatch in PD disaggregation (#27004)

python/sglang/srt/disaggregation/common/utils.py

@@ -98,9 +98,19 @@ def group_concurrent_contiguous(
     src_indices: npt.NDArray[np.int32], dst_indices: npt.NDArray[np.int32]
 ) -> Tuple[List[npt.NDArray[np.int32]], List[npt.NDArray[np.int32]]]:
     """Vectorised NumPy implementation."""
-    if src_indices.size == 0:
+    # src/dst indices are transferred pairwise, so an empty side means there is
+    # nothing to transfer. Guarding both sides (not just src) avoids a cryptic
+    # NumPy broadcast error from np.diff() below when only one side is empty, e.g.
+    # a non-empty prefill DSA/SWA state list paired with an empty decode registration.
+    if src_indices.size == 0 or dst_indices.size == 0:
         return [], []
 
+    if src_indices.size != dst_indices.size:
+        raise ValueError(
+            "group_concurrent_contiguous requires equal-length src/dst index arrays, "
+            f"got {src_indices.size} and {dst_indices.size}"
+        )
+
     brk = np.where((np.diff(src_indices) != 1) | (np.diff(dst_indices) != 1))[0] + 1
     src_groups = np.split(src_indices, brk)
     dst_groups = np.split(dst_indices, brk)

python/sglang/srt/disaggregation/prefill.py

@@ -947,7 +947,13 @@ class SchedulerDisaggregationPrefillMixin:
         if last_chunk:
             self.disagg_metadata_buffers.set_buf(req)
 
-            seq_len = len(req.fill_ids)
+            # fill_ids includes the token sampled during prefill, but decode
+            # registers state pages over origin_input_ids (DecodePreallocQueue)
+            # and the main pool send is clamped to end_idx above. Matching that
+            # length here avoids emitting an extra state page when the sampled
+            # token crosses a page boundary, which mismatched src/dst lengths in
+            # group_concurrent_contiguous.
+            seq_len = min(len(req.fill_ids), len(req.origin_input_ids))
 
             def _mamba_payload():
                 return [