From e36e25fe2fb7bc694d560220b9daf5c8ce110e54 Mon Sep 17 00:00:00 2001
From: Christophe Bornet <cbornet@hotmail.com>
Date: Tue, 9 Sep 2025 16:11:12 +0200
Subject: [PATCH] feat(langchain): support PEP604 ( `|` union) in tool node
 error handlers (#32861)

This allows to use PEP604 syntax for `ToolNode` error handlers
```python
def error_handler(e: ValueError | ToolException) -> str:
    return "error"

ToolNode(my_tool, handle_tool_errors=error_handler).invoke(...)
```
Without this change, this fails with `AttributeError: 'types.UnionType'
object has no attribute '__mro__'`
---
 .../langchain_v1/langchain/agents/tool_node.py |  3 ++-
 .../unit_tests/agents/test_react_agent.py      | 18 +++++++++++++-----
 .../tests/unit_tests/agents/test_tool_node.py  |  2 +-
 3 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/libs/langchain_v1/langchain/agents/tool_node.py b/libs/langchain_v1/langchain/agents/tool_node.py
index aea11721681..263867bf758 100644
--- a/libs/langchain_v1/langchain/agents/tool_node.py
+++ b/libs/langchain_v1/langchain/agents/tool_node.py
@@ -40,6 +40,7 @@ import inspect
 import json
 from copy import copy, deepcopy
 from dataclasses import replace
+from types import UnionType
 from typing import (
     TYPE_CHECKING,
     Annotated,
@@ -246,7 +247,7 @@ def _infer_handled_types(handler: Callable[..., str]) -> tuple[type[Exception],
         type_hints = get_type_hints(handler)
         if first_param.name in type_hints:
             origin = get_origin(first_param.annotation)
-            if origin is Union:
+            if origin in [Union, UnionType]:
                 args = get_args(first_param.annotation)
                 if all(issubclass(arg, Exception) for arg in args):
                     return tuple(args)
diff --git a/libs/langchain_v1/tests/unit_tests/agents/test_react_agent.py b/libs/langchain_v1/tests/unit_tests/agents/test_react_agent.py
index d6a948f3185..1b2dd9eeb4f 100644
--- a/libs/langchain_v1/tests/unit_tests/agents/test_react_agent.py
+++ b/libs/langchain_v1/tests/unit_tests/agents/test_react_agent.py
@@ -1,5 +1,6 @@
 import dataclasses
 import inspect
+from types import UnionType
 from typing import (
     Annotated,
     Union,
@@ -343,16 +344,19 @@ def test__infer_handled_types() -> None:
     def handle2(e: Exception) -> str:
         return ""
 
-    def handle3(e: Union[ValueError, ToolException]) -> str:
+    def handle3(e: ValueError | ToolException) -> str:
+        return ""
+
+    def handle4(e: Union[ValueError, ToolException]) -> str:
         return ""
 
     class Handler:
         def handle(self, e: ValueError) -> str:
             return ""
 
-    handle4 = Handler().handle
+    handle5 = Handler().handle
 
-    def handle5(e: Union[Union[TypeError, ValueError], ToolException]) -> str:
+    def handle6(e: Union[Union[TypeError, ValueError], ToolException]) -> str:
         return ""
 
     expected: tuple = (Exception,)
@@ -367,14 +371,18 @@ def test__infer_handled_types() -> None:
     actual = _infer_handled_types(handle3)
     assert expected == actual
 
-    expected = (ValueError,)
+    expected = (ValueError, ToolException)
     actual = _infer_handled_types(handle4)
     assert expected == actual
 
-    expected = (TypeError, ValueError, ToolException)
+    expected = (ValueError,)
     actual = _infer_handled_types(handle5)
     assert expected == actual
 
+    expected = (TypeError, ValueError, ToolException)
+    actual = _infer_handled_types(handle6)
+    assert expected == actual
+
     with pytest.raises(ValueError):
 
         def handler(e: str) -> str:
diff --git a/libs/langchain_v1/tests/unit_tests/agents/test_tool_node.py b/libs/langchain_v1/tests/unit_tests/agents/test_tool_node.py
index 8581cf3c7cf..e843788d13a 100644
--- a/libs/langchain_v1/tests/unit_tests/agents/test_tool_node.py
+++ b/libs/langchain_v1/tests/unit_tests/agents/test_tool_node.py
@@ -272,7 +272,7 @@ def test_tool_node_error_handling_default_exception() -> None:
 
 
 async def test_tool_node_error_handling() -> None:
-    def handle_all(e: Union[ValueError, ToolException, ToolInvocationError]):
+    def handle_all(e: ValueError | ToolException | ToolInvocationError):
         return TOOL_CALL_ERROR_TEMPLATE.format(error=repr(e))
 
     # test catching all exceptions, via: