Harrison/arxiv fix (#5993)

Co-authored-by: Juanjo do Olmo <87780148+SimplyJuanjo@users.noreply.github.com>

langchain/tools/arxiv/tool.py

@@ -2,6 +2,8 @@
 
 from typing import Optional
 
+from pydantic import Field
+
 from langchain.callbacks.manager import (
     AsyncCallbackManagerForToolRun,
     CallbackManagerForToolRun,
@@ -22,7 +24,7 @@ class ArxivQueryRun(BaseTool):
         "from scientific articles on arxiv.org. "
         "Input should be a search query."
     )
-    api_wrapper: ArxivAPIWrapper
+    api_wrapper: ArxivAPIWrapper = Field(default_factory=ArxivAPIWrapper)
 
     def _run(
         self,