core[major]: On Tool End Observation Casting Fix (#18798)

This PR updates the on_tool_end handlers to return the raw output from the tool instead of casting it to a string. 

This is technically a breaking change, though it's impact is expected to be somewhat minimal. It will fix behavior in `astream_events` as well.

Fixes the following issue #18760 raised by @eyurtsev

---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/core/langchain_core/callbacks/base.py

@@ -133,7 +133,7 @@ class ToolManagerMixin:
 
     def on_tool_end(
         self,
-        output: str,
+        output: Any,
         *,
         run_id: UUID,
         parent_run_id: Optional[UUID] = None,
@@ -440,7 +440,7 @@ class AsyncCallbackHandler(BaseCallbackHandler):
 
     async def on_tool_end(
         self,
-        output: str,
+        output: Any,
         *,
         run_id: UUID,
         parent_run_id: Optional[UUID] = None,

libs/core/langchain_core/callbacks/manager.py

@@ -976,14 +976,15 @@ class CallbackManagerForToolRun(ParentRunManager, ToolManagerMixin):
 
     def on_tool_end(
         self,
-        output: str,
+        output: Any,
         **kwargs: Any,
     ) -> None:
         """Run when tool ends running.
 
         Args:
-            output (str): The output of the tool.
+            output (Any): The output of the tool.
         """
+        output = str(output)
         handle_event(
             self.handlers,
             "on_tool_end",
@@ -1038,12 +1039,13 @@ class AsyncCallbackManagerForToolRun(AsyncParentRunManager, ToolManagerMixin):
         )
 
     @shielded
-    async def on_tool_end(self, output: str, **kwargs: Any) -> None:
+    async def on_tool_end(self, output: Any, **kwargs: Any) -> None:
         """Run when tool ends running.
 
         Args:
-            output (str): The output of the tool.
+            output (Any): The output of the tool.
         """
+        output = str(output)
         await ahandle_event(
             self.handlers,
             "on_tool_end",

libs/core/langchain_core/callbacks/stdout.py

@@ -37,13 +37,14 @@ class StdOutCallbackHandler(BaseCallbackHandler):
 
     def on_tool_end(
         self,
-        output: str,
+        output: Any,
         color: Optional[str] = None,
         observation_prefix: Optional[str] = None,
         llm_prefix: Optional[str] = None,
         **kwargs: Any,
     ) -> None:
         """If not the final action, print out observation."""
+        output = str(output)
         if observation_prefix is not None:
             print_text(f"\n{observation_prefix}")
         print_text(output, color=color or self.color)

libs/core/langchain_core/callbacks/streaming_stdout.py

@@ -59,7 +59,7 @@ class StreamingStdOutCallbackHandler(BaseCallbackHandler):
         """Run on agent action."""
         pass
 
-    def on_tool_end(self, output: str, **kwargs: Any) -> None:
+    def on_tool_end(self, output: Any, **kwargs: Any) -> None:
         """Run when tool ends running."""
 
     def on_tool_error(self, error: BaseException, **kwargs: Any) -> None:

libs/core/langchain_core/tools.py

@@ -410,17 +410,13 @@ class ChildTool(BaseTool):
                     f"Got unexpected type of `handle_tool_error`. Expected bool, str "
                     f"or callable. Received: {self.handle_tool_error}"
                 )
-            run_manager.on_tool_end(
-                str(observation), color="red", name=self.name, **kwargs
-            )
+            run_manager.on_tool_end(observation, color="red", name=self.name, **kwargs)
             return observation
         except (Exception, KeyboardInterrupt) as e:
             run_manager.on_tool_error(e)
             raise e
         else:
-            run_manager.on_tool_end(
-                str(observation), color=color, name=self.name, **kwargs
-            )
+            run_manager.on_tool_end(observation, color=color, name=self.name, **kwargs)
             return observation
 
     async def arun(
@@ -502,7 +498,7 @@ class ChildTool(BaseTool):
                     f"or callable. Received: {self.handle_tool_error}"
                 )
             await run_manager.on_tool_end(
-                str(observation), color="red", name=self.name, **kwargs
+                observation, color="red", name=self.name, **kwargs
             )
             return observation
         except (Exception, KeyboardInterrupt) as e:
@@ -510,7 +506,7 @@ class ChildTool(BaseTool):
             raise e
         else:
             await run_manager.on_tool_end(
-                str(observation), color=color, name=self.name, **kwargs
+                observation, color=color, name=self.name, **kwargs
             )
             return observation
 

libs/core/langchain_core/tracers/base.py

@@ -504,8 +504,9 @@ class BaseTracer(BaseCallbackHandler, ABC):
         self._on_tool_start(tool_run)
         return tool_run
 
-    def on_tool_end(self, output: str, *, run_id: UUID, **kwargs: Any) -> Run:
+    def on_tool_end(self, output: Any, *, run_id: UUID, **kwargs: Any) -> Run:
         """End a trace for a tool run."""
+        output = str(output)
         tool_run = self._get_run(run_id, run_type="tool")
         tool_run.outputs = {"output": output}
         tool_run.end_time = datetime.now(timezone.utc)