Fix extended thinking bug by reordering tool messages

src/agents/extensions/models/litellm_model.py

@@ -23,6 +23,7 @@
     ChatCompletionChunk,
     ChatCompletionMessageCustomToolCall,
     ChatCompletionMessageFunctionToolCall,
+    ChatCompletionMessageParam,
 )
 from openai.types.chat.chat_completion_message import (
     Annotation,
@@ -267,6 +268,10 @@ async def _fetch_response(
             input, preserve_thinking_blocks=preserve_thinking_blocks
         )
 
+        # Fix for interleaved thinking bug: reorder messages to ensure tool_use comes before tool_result  # noqa: E501
+        if preserve_thinking_blocks:
+            converted_messages = self._fix_tool_message_ordering(converted_messages)
+
         if system_instructions:
             converted_messages.insert(
                 0,
@@ -379,6 +384,112 @@ async def _fetch_response(
         )
         return response, ret
 
+    def _fix_tool_message_ordering(
+        self, messages: list[ChatCompletionMessageParam]
+    ) -> list[ChatCompletionMessageParam]:
+        """
+        Fix the ordering of tool messages to ensure tool_use messages come before tool_result messages.
+
+        This addresses the interleaved thinking bug where conversation histories may contain
+        tool results before their corresponding tool calls, causing Anthropic API to reject the request.
+        """  # noqa: E501
+        if not messages:
+            return messages
+
+        # Collect all tool calls and tool results
+        tool_call_messages = {}  # tool_id -> (index, message)
+        tool_result_messages = {}  # tool_id -> (index, message)
+        other_messages = []  # (index, message) for non-tool messages
+
+        for i, message in enumerate(messages):
+            if not isinstance(message, dict):
+                other_messages.append((i, message))
+                continue
+
+            role = message.get("role")
+
+            if role == "assistant" and message.get("tool_calls"):
+                # Extract tool calls from this assistant message
+                tool_calls = message.get("tool_calls", [])
+                if isinstance(tool_calls, list):
+                    for tool_call in tool_calls:
+                        if isinstance(tool_call, dict):
+                            tool_id = tool_call.get("id")
+                            if tool_id:
+                                # Create a separate assistant message for each tool call
+                                single_tool_msg = cast(dict[str, Any], message.copy())
+                                single_tool_msg["tool_calls"] = [tool_call]
+                                tool_call_messages[tool_id] = (
+                                    i,
+                                    cast(ChatCompletionMessageParam, single_tool_msg),
+                                )
+
+            elif role == "tool":
+                tool_call_id = message.get("tool_call_id")
+                if tool_call_id:
+                    tool_result_messages[tool_call_id] = (i, message)
+                else:
+                    other_messages.append((i, message))
+            else:
+                other_messages.append((i, message))
+
+        # Create the fixed message sequence
+        fixed_messages: list[ChatCompletionMessageParam] = []
+        used_indices = set()
+
+        # Add messages in their original order, but ensure tool_use → tool_result pairing
+        for i, original_message in enumerate(messages):
+            if i in used_indices:
+                continue
+
+            if not isinstance(original_message, dict):
+                fixed_messages.append(original_message)
+                used_indices.add(i)
+                continue
+
+            role = original_message.get("role")
+
+            if role == "assistant" and original_message.get("tool_calls"):
+                # Process each tool call in this assistant message
+                tool_calls = original_message.get("tool_calls", [])
+                if isinstance(tool_calls, list):
+                    for tool_call in tool_calls:
+                        if isinstance(tool_call, dict):
+                            tool_id = tool_call.get("id")
+                            if (
+                                tool_id
+                                and tool_id in tool_call_messages
+                                and tool_id in tool_result_messages
+                            ):
+                                # Add tool_use → tool_result pair
+                                _, tool_call_msg = tool_call_messages[tool_id]
+                                _, tool_result_msg = tool_result_messages[tool_id]
+
+                                fixed_messages.append(tool_call_msg)
+                                fixed_messages.append(tool_result_msg)
+
+                                # Mark both as used
+                                used_indices.add(tool_call_messages[tool_id][0])
+                                used_indices.add(tool_result_messages[tool_id][0])
+                            elif tool_id and tool_id in tool_call_messages:
+                                # Tool call without result - add just the tool call
+                                _, tool_call_msg = tool_call_messages[tool_id]
+                                fixed_messages.append(tool_call_msg)
+                                used_indices.add(tool_call_messages[tool_id][0])
+
+                used_indices.add(i)  # Mark original multi-tool message as used
+
+            elif role == "tool":
+                # Skip - these will be handled as part of tool pairs above
+                used_indices.add(i)
+
+            else:
+                # Regular message - add it normally
+                fixed_messages.append(original_message)
+                used_indices.add(i)
+
+        return fixed_messages
+
     def _remove_not_given(self, value: Any) -> Any:
         if isinstance(value, NotGiven):
             return None

src/agents/models/chatcmpl_converter.py

@@ -533,7 +533,7 @@ def ensure_assistant_message() -> ChatCompletionAssistantMessageParam:
 
                 if content_items and preserve_thinking_blocks:
                     # Reconstruct thinking blocks from content and signature
-                    pending_thinking_blocks = []
+                    reconstructed_thinking_blocks = []
                     for content_item in content_items:
                         if (
                             isinstance(content_item, dict)
@@ -546,7 +546,11 @@ def ensure_assistant_message() -> ChatCompletionAssistantMessageParam:
                             # Add signatures if available
                             if signatures:
                                 thinking_block["signature"] = signatures.pop(0)
-                            pending_thinking_blocks.append(thinking_block)
+                            reconstructed_thinking_blocks.append(thinking_block)
+
+                    # Store thinking blocks as pending for the next assistant message
+                    # This preserves the original behavior
+                    pending_thinking_blocks = reconstructed_thinking_blocks
 
             # 8) If we haven't recognized it => fail or ignore
             else: