diff --git a/run_agent.py b/run_agent.py
index fcef3057..67a18758 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -2443,7 +2443,18 @@ class AIAgent:
                 "Pre-call sanitizer: added %d stub tool result(s)",
                 len(missing_results),
             )
-
+        # 3. Strip trailing empty assistant messages to prevent prefill rejection.
+        # These can leak from Responses API reasoning-only turns (Codex/MiniMax)
+        # where an empty assistant message is required by the Responses API but
+        # must NOT be sent to Chat Completions or Anthropic Messages API providers.
+        while (
+            messages
+            and messages[-1].get("role") == "assistant"
+            and not (messages[-1].get("content") or "").strip()
+            and not messages[-1].get("tool_calls")
+        ):
+            logger.debug("Pre-call sanitizer: removed trailing empty assistant message")
+            messages = messages[:-1]
         return messages
 
     @staticmethod