[MCP] reinject JSON parse & runtime tool errors back into the chat history (#3137)

* re-inject tool parsing errors into the conversation

* style

Author: hanouticelina

src/huggingface_hub/inference/_mcp/mcp_client.py

@@ -310,7 +310,19 @@ async def process_single_turn_with_tools(
         # Process tool calls one by one
         for tool_call in final_tool_calls.values():
             function_name = tool_call.function.name
-            function_args = json.loads(tool_call.function.arguments or "{}")
+            try:
+                function_args = json.loads(tool_call.function.arguments or "{}")
+            except json.JSONDecodeError as err:
+                tool_message = {
+                    "role": "tool",
+                    "tool_call_id": tool_call.id,
+                    "name": function_name,
+                    "content": f"Invalid JSON generated by the model: {err}",
+                }
+                tool_message_as_obj = ChatCompletionInputMessage.parse_obj_as_instance(tool_message)
+                messages.append(tool_message_as_obj)
+                yield tool_message_as_obj
+                continue  # move to next tool call
 
             tool_message = {"role": "tool", "tool_call_id": tool_call.id, "content": "", "name": function_name}
 
@@ -324,11 +336,13 @@ async def process_single_turn_with_tools(
             # Execute tool call with the appropriate session
             session = self.sessions.get(function_name)
             if session is not None:
-                result = await session.call_tool(function_name, function_args)
-                tool_message["content"] = format_result(result)
+                try:
+                    result = await session.call_tool(function_name, function_args)
+                    tool_message["content"] = format_result(result)
+                except Exception as err:
+                    tool_message["content"] = f"Error: MCP tool call failed with error message: {err}"
             else:
-                error_msg = f"Error: No session found for tool: {function_name}"
-                tool_message["content"] = error_msg
+                tool_message["content"] = f"Error: No session found for tool: {function_name}"
 
             # Yield tool message
             tool_message_as_obj = ChatCompletionInputMessage.parse_obj_as_instance(tool_message)