fix(litellm): convert message content to strings for providers requiring plain text format

src/strands/models/litellm.py

@@ -14,7 +14,7 @@
 
 from ..types.content import ContentBlock, Messages
 from ..types.streaming import StreamEvent
-from ..types.tools import ToolSpec
+from ..types.tools import ToolResult, ToolSpec
 from .openai import OpenAIModel
 
 logger = logging.getLogger(__name__)
@@ -103,6 +103,93 @@ def format_request_message_content(cls, content: ContentBlock) -> dict[str, Any]
 
         return super().format_request_message_content(content)
 
+    @override
+    @classmethod
+    def format_request_tool_message(cls, tool_result: ToolResult) -> dict[str, Any]:
+        """Format a LiteLLM compatible tool message.
+
+        Args:
+            tool_result: Tool result collected from a tool execution.
+
+        Returns:
+            LiteLLM compatible tool message with plain string content.
+        """
+        contents = cast(
+            list[ContentBlock],
+            [
+                {"text": json.dumps(content["json"])} if "json" in content else content
+                for content in tool_result["content"]
+            ],
+        )
+
+        # Extract plain text content for LiteLLM/OpenRouter compatibility
+        content_text = ""
+        if contents:
+            if "text" in contents[0]:
+                content_text = contents[0]["text"]
+            elif "json" in contents[0]:
+                content_text = json.dumps(contents[0]["json"])
+
+        return {
+            "role": "tool",
+            "tool_call_id": tool_result["toolUseId"],
+            "content": content_text,  # Plain string instead of content blocks
+        }
+
+    @override
+    @classmethod
+    def format_request_messages(cls, messages: Messages, system_prompt: Optional[str] = None) -> list[dict[str, Any]]:
+        """Format a LiteLLM compatible messages array.
+
+        Args:
+            messages: List of message objects to be processed by the model.
+            system_prompt: System prompt to provide context to the model.
+
+        Returns:
+            A LiteLLM compatible messages array.
+        """
+        formatted_messages: list[dict[str, Any]]
+        formatted_messages = [{"role": "system", "content": system_prompt}] if system_prompt else []
+
+        for message in messages:
+            contents = message["content"]
+
+            formatted_contents = [
+                cls.format_request_message_content(content)
+                for content in contents
+                if not any(block_type in content for block_type in ["toolResult", "toolUse"])
+            ]
+            formatted_tool_calls = [
+                cls.format_request_message_tool_call(content["toolUse"]) for content in contents if "toolUse" in content
+            ]
+            formatted_tool_messages = [
+                cls.format_request_tool_message(content["toolResult"])
+                for content in contents
+                if "toolResult" in content
+            ]
+
+            # Handle content format for messages with tool_calls
+            if formatted_tool_calls:
+                # Extract text from first content block if available
+                content_value = None
+                if formatted_contents and isinstance(formatted_contents, list):
+                    if formatted_contents[0].get("text"):
+                        content_value = formatted_contents[0]["text"]
+                formatted_message = {
+                    "role": message["role"],
+                    "content": content_value,  # String/None for messages with tool_calls
+                    "tool_calls": formatted_tool_calls,
+                }
+            else:
+                formatted_message = {
+                    "role": message["role"],
+                    "content": formatted_contents,
+                }
+            formatted_messages.append(formatted_message)
+            formatted_messages.extend(formatted_tool_messages)
+
+        return [message for message in formatted_messages if message["content"] or "tool_calls" in message]
+
     @override
     async def stream(
         self,