fix: speculative draft worker clobbering target attention backend (#28559)

python/sglang/srt/model_executor/model_runner.py

@@ -2415,6 +2415,14 @@ class ModelRunner(ModelRunnerKVCacheMixin):
         else:
             self.attn_backend = self._get_attention_backend()
 
+        # Record resolved per-mode backends on the backend for model dispatch.
+        self.attn_backend.prefill_attention_backend_str = (
+            self.prefill_attention_backend_str
+        )
+        self.attn_backend.decode_attention_backend_str = (
+            self.decode_attention_backend_str
+        )
+
     def _get_attention_backend(self, init_new_workspace: bool = False):
         """Init attention kernel backend."""
         draft_attn_backend = self.server_args.speculative_draft_attention_backend
@@ -2422,6 +2430,9 @@ class ModelRunner(ModelRunnerKVCacheMixin):
             logger.warning(
                 f"Overriding draft attention backend to {draft_attn_backend}."
             )
+            # Single backend for all draft modes (no prefill/decode split).
+            self.prefill_attention_backend_str = draft_attn_backend
+            self.decode_attention_backend_str = draft_attn_backend
             return self._get_attention_backend_from_str(
                 draft_attn_backend,
                 init_new_workspace=init_new_workspace,
@@ -2463,10 +2474,6 @@ class ModelRunner(ModelRunnerKVCacheMixin):
                 init_new_workspace=init_new_workspace,
             )
 
-        (
-            get_global_server_args().prefill_attention_backend,
-            get_global_server_args().decode_attention_backend,
-        ) = (self.prefill_attention_backend_str, self.decode_attention_backend_str)
         return attn_backend
 
     def _get_attention_backend_from_str(