Core[Patch] Parse tool input after on_start (#16430)

For tracing, if a validation error occurs, currently it is attributed to
the previous step of the chain. It would be nice to have the on_start
and on_error callbacks called for tools when there is a validation error
that occurs to more easily attribute the root-cause

libs/core/langchain_core/tools.py

@@ -312,7 +312,6 @@ class ChildTool(BaseTool):
         **kwargs: Any,
     ) -> Any:
         """Run the tool."""
-        parsed_input = self._parse_input(tool_input)
         if not self.verbose and verbose is not None:
             verbose_ = verbose
         else:
@@ -341,6 +340,7 @@ class ChildTool(BaseTool):
             **kwargs,
         )
         try:
+            parsed_input = self._parse_input(tool_input)
             tool_args, tool_kwargs = self._to_args_and_kwargs(parsed_input)
             observation = (
                 self._run(*tool_args, run_manager=run_manager, **tool_kwargs)
@@ -392,7 +392,6 @@ class ChildTool(BaseTool):
         **kwargs: Any,
     ) -> Any:
         """Run the tool asynchronously."""
-        parsed_input = self._parse_input(tool_input)
         if not self.verbose and verbose is not None:
             verbose_ = verbose
         else:
@@ -416,6 +415,7 @@ class ChildTool(BaseTool):
             **kwargs,
         )
         try:
+            parsed_input = self._parse_input(tool_input)
             # We then call the tool on the tool input to get an observation
             tool_args, tool_kwargs = self._to_args_and_kwargs(parsed_input)
             observation = (