diff --git a/models/openai_compatible_bot.py b/models/openai_compatible_bot.py
index 1fcad7d7..aba5b327 100644
--- a/models/openai_compatible_bot.py
+++ b/models/openai_compatible_bot.py
@@ -249,24 +249,49 @@ class OpenAICompatibleBot:
     def _convert_messages_to_openai_format(self, messages):
         """
         Convert messages from Claude format to OpenAI format
-        
-        Claude uses content blocks with types like 'tool_use', 'tool_result'
-        OpenAI uses 'tool_calls' in assistant messages and 'tool' role for results
+
+        Claude content blocks (tool_use / tool_result / thinking) → OpenAI
+        tool_calls / tool role / reasoning_content. Some thinking-mode
+        providers require reasoning_content on assistant messages after a
+        tool_call appears in history; back-fill with empty string when the
+        trace was not captured.
         """
         if not messages:
             return []
-        
+
+        # Detect any prior tool-call turn — gates reasoning_content back-fill below.
+        has_tool_call_history = False
+        for msg in messages:
+            if msg.get("role") != "assistant":
+                continue
+            if msg.get("tool_calls"):
+                has_tool_call_history = True
+                break
+            inner = msg.get("content")
+            if isinstance(inner, list) and any(
+                isinstance(b, dict) and b.get("type") == "tool_use" for b in inner
+            ):
+                has_tool_call_history = True
+                break
+
         openai_messages = []
-        
+
         for msg in messages:
             role = msg.get("role")
             content = msg.get("content")
-            
+
             # Handle string content (already in correct format)
             if isinstance(content, str):
-                openai_messages.append(msg)
+                if (role == "assistant" and has_tool_call_history
+                        and isinstance(msg, dict)
+                        and "reasoning_content" not in msg):
+                    patched = dict(msg)
+                    patched["reasoning_content"] = ""
+                    openai_messages.append(patched)
+                else:
+                    openai_messages.append(msg)
                 continue
-            
+
             # Handle list content (Claude format with content blocks)
             if isinstance(content, list):
                 # Check if this is a tool result message (user role with tool_result blocks)
@@ -305,14 +330,17 @@ class OpenAICompatibleBot:
 
                 # Check if this is an assistant message with tool_use blocks
                 elif role == "assistant":
-                    # Separate text content and tool_use blocks
                     text_parts = []
                     tool_calls = []
+                    reasoning_parts = []
 
                     for block in content:
-                        if block.get("type") == "text":
+                        if not isinstance(block, dict):
+                            continue
+                        btype = block.get("type")
+                        if btype == "text":
                             text_parts.append(block.get("text", ""))
-                        elif block.get("type") == "tool_use":
+                        elif btype == "tool_use":
                             tool_id = block.get("id") or ""
                             if not tool_id:
                                 logger.warning(f"[OpenAICompatible] tool_use missing id for '{block.get('name')}'")
@@ -324,6 +352,8 @@ class OpenAICompatibleBot:
                                     "arguments": json.dumps(block.get("input", {}))
                                 }
                             })
+                        elif btype == "thinking":
+                            reasoning_parts.append(block.get("thinking", ""))
 
                     # Build OpenAI format assistant message
                     openai_msg = {
@@ -334,6 +364,13 @@ class OpenAICompatibleBot:
                     if tool_calls:
                         openai_msg["tool_calls"] = tool_calls
 
+                    # Round-trip reasoning_content; empty string when missing
+                    # after a tool-call turn keeps strict providers happy.
+                    if reasoning_parts:
+                        openai_msg["reasoning_content"] = "\n".join(reasoning_parts)
+                    elif has_tool_call_history:
+                        openai_msg["reasoning_content"] = ""
+
                     if msg.get("_gemini_raw_parts"):
                         openai_msg["_gemini_raw_parts"] = msg["_gemini_raw_parts"]