Update session.py

Author: ochafik

src/mcp/server/session.py

@@ -248,37 +248,29 @@ async def create_message(
             The sampling result from the client.
 
         Raises:
-            McpError: If tool_use or tool_result blocks are misused
+            McpError: If tool_use or tool_result blocks are misused when tools are provided.
         """
 
         if messages and tools:
-            last_msg_content = messages[-1].content
-            last_content = last_msg_content if isinstance(last_msg_content, list) else [last_msg_content]
+            last_content = messages[-1].content_as_list
             has_tool_results = any(c.type == "tool_result" for c in last_content)
 
-            previous_content: list[types.SamplingMessageContentBlock] | None = None
-            if len(messages) >= 2:
-                prev_msg_content = messages[-2].content
-                previous_content = prev_msg_content if isinstance(prev_msg_content, list) else [prev_msg_content]
+            previous_content = messages[-2].content_as_list if len(messages) >= 2 else None
             has_previous_tool_use = previous_content and any(c.type == "tool_use" for c in previous_content)
 
             if has_tool_results:
                 if any(c.type != "tool_result" for c in last_content):
                     raise ValueError("The last message must contain only tool_result content if any is present")
-                if len(messages) == 1:
-                    raise ValueError(
-                        "The last message cannot contain tool_result content if there is no previous message"
-                    )
                 if not has_previous_tool_use:
                     raise ValueError(
-                        "The previous message must contain tool_use content if the last message contains tool_result content"
+                        "tool_result blocks are not matching any tool_use from the previous message"
                     )
             if has_previous_tool_use and previous_content:
                 tool_use_ids = {c.id for c in previous_content if c.type == "tool_use"}
                 tool_result_ids = {c.toolUseId for c in last_content if c.type == "tool_result"}
                 if tool_use_ids != tool_result_ids:
                     raise ValueError(
-                        "The tool_result content in the last message must correspond to all tool_use content in the previous message"
+                        "ids of tool_result blocks should match all tool_use blocks from previous message"
                     )
 
         return await self.send_request(