diff --git a/rdagent/oai/backend/base.py b/rdagent/oai/backend/base.py
index f7703ae2..20853cad 100644
--- a/rdagent/oai/backend/base.py
+++ b/rdagent/oai/backend/base.py
@@ -720,7 +720,13 @@ class APIBackend(ABC):
 
             if finish_reason is None or finish_reason != "length":
                 break  # we get a full response now.
-            new_messages.append({"role": "assistant", "content": response})
+            # Merge into the previous assistant message if there already is one at the end.
+            # Appending a second consecutive assistant message causes llama-server to return 400
+            # ("Cannot have 2 or more assistant messages at the end of the list").
+            if new_messages and new_messages[-1]["role"] == "assistant":
+                new_messages[-1]["content"] += response
+            else:
+                new_messages.append({"role": "assistant", "content": response})
         else:
             raise RuntimeError(f"Failed to continue the conversation after {try_n} retries.")
 
diff --git a/rdagent/utils/workflow/loop.py b/rdagent/utils/workflow/loop.py
index fe54d986..d4bbc25c 100644
--- a/rdagent/utils/workflow/loop.py
+++ b/rdagent/utils/workflow/loop.py
@@ -270,6 +270,11 @@ class LoopBase:
                         msg = "We have reset the loop instance, stop all the routines and resume."
                         raise self.LoopResumeError(msg) from e
                     else:
+                        # Do NOT advance step_idx for unhandled exceptions (e.g. LoopResumeError
+                        # propagating from _propose). Keeping step_idx at the current step lets
+                        # kickoff_loop retry step 0 on the next resume instead of permanently
+                        # corrupting the loop with a missing direct_exp_gen result.
+                        step_forward = False
                         raise  # re-raise unhandled exceptions
                 finally:
                     # No matter the execution succeed or not, we have to finish the following steps