Fix LiteLLM thinking models with tool calling across providers

.gitignore

@@ -143,3 +143,6 @@ cython_debug/
 # PyPI configuration file
 .pypirc
 .aider*
+
+# Claude Code settings
+.claude/settings.local.json

src/agents/extensions/models/litellm_model.py

@@ -231,6 +231,10 @@ async def _fetch_response(
         stream: bool = False,
     ) -> litellm.types.utils.ModelResponse | tuple[Response, AsyncStream[ChatCompletionChunk]]:
         converted_messages = Converter.items_to_messages(input)
+
+        # Fix for thinking models: Handle assistant messages with tool calls properly
+        if model_settings.reasoning:
+            converted_messages = self._fix_thinking_model_messages(converted_messages)
 
         if system_instructions:
             converted_messages.insert(
@@ -330,6 +334,48 @@ def _remove_not_given(self, value: Any) -> Any:
             return None
         return value
 
+    def _fix_thinking_model_messages(self, messages: list[dict]) -> list[dict]:
+        """
+        Fix assistant messages for LiteLLM thinking models.
+
+        When reasoning is enabled, assistant messages with tool calls should not have
+        content - LiteLLM will handle the thinking blocks automatically for supported
+        thinking models that also support function calling.
+
+        Verified working with:
+        - Anthropic Claude Sonnet 4
+        - OpenAI o4-mini
+
+        Note: Some thinking models like OpenAI o1-mini/o1-preview don't support function calling yet.
+
+        This fixes issue #765: https://github.com/openai/openai-agents-python/issues/765
+        """
+        logger.debug(f"Fixing messages for LiteLLM thinking model: {self.model}")
+        modified_messages = []
+
+        for i, message in enumerate(messages):
+            if (
+                message.get("role") == "assistant" 
+                and message.get("tool_calls") 
+                and len(message.get("tool_calls", [])) > 0
+            ):
+                logger.debug(f"Message {i}: Removing content from assistant message with tool calls")
+                # This assistant message has tool calls, remove content for thinking models
+                modified_message = message.copy()
+
+                # Remove content entirely - let LiteLLM handle thinking blocks automatically
+                if "content" in modified_message:
+                    del modified_message["content"]
+
+                logger.debug(f"Message {i}: Removed content, message now: {modified_message}")
+                modified_messages.append(modified_message)
+            else:
+                logger.debug(f"Message {i}: {message.get('role')} message, no modification needed")
+                modified_messages.append(message)
+
+        return modified_messages
+
+
 
 class LitellmConverter:
     @classmethod