[spec decoding] fix mrope_positions in draft extend (#28464)

python/sglang/srt/model_executor/forward_batch_info.py

@@ -781,6 +781,13 @@ class ForwardBatch(ForwardBatchDeepSeekMHAMixin):
                 and getattr(ret.spec_info, "positions", None) is not None
             ):
                 ret.compute_spec_mrope_positions(model_runner, batch)
+            elif ret.forward_mode.is_draft_extend_v2():
+                # Draft-extend tokens are uniform text continuation; reuse the
+                # spec mrope path with the input-consistent `ret.positions` rather
+                # than the per-request rebuild (which mis-sizes mm requests).
+                ret.compute_spec_mrope_positions(
+                    model_runner, batch, seq_positions=ret.positions
+                )
             else:
                 ret._compute_mrope_positions(model_runner, batch)
 
@@ -923,15 +930,18 @@ class ForwardBatch(ForwardBatchDeepSeekMHAMixin):
         )
 
     def compute_spec_mrope_positions(
-        self, model_runner: ModelRunner, batch: ScheduleBatch
+        self, model_runner: ModelRunner, batch: ScheduleBatch, seq_positions=None
     ):
         # TODO support batched deltas
         batch_size = self.seq_lens.shape[0]
         device = model_runner.device
         mm_inputs = batch.multimodal_inputs
 
-        # target_verify or draft_decode
+        # target_verify / draft_decode read spec_info.positions; draft_extend
-        seq_positions = batch.spec_info.positions.view(batch_size, -1)
+        # passes its own positions (uniform num_draft_tokens per request).
+        if seq_positions is None:
+            seq_positions = batch.spec_info.positions
+        seq_positions = seq_positions.view(batch_size, -1)
         # Split text-only and mixed batches here because SpecV2 text-only batches can avoid an extra D2H.
         if all(mm_input is None for mm_input in mm_inputs):
             mrope_delta_tensor = torch.zeros(