From 13b2f8d727f6d78daa4f38f0cc078898582377ce Mon Sep 17 00:00:00 2001
From: Hunter Lovell <40191806+hntrl@users.noreply.github.com>
Date: Wed, 12 Aug 2026 12:49:24 -0700
Subject: [PATCH] fix(core): handle v1 base model validation in async path
 (#39576)

---
 libs/core/langchain_core/tools/base.py   |  2 +-
 libs/core/tests/unit_tests/test_tools.py | 24 ++++++++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/libs/core/langchain_core/tools/base.py b/libs/core/langchain_core/tools/base.py
index 56ea214518..89fe3c4509 100644
--- a/libs/core/langchain_core/tools/base.py
+++ b/libs/core/langchain_core/tools/base.py
@@ -1238,7 +1238,7 @@ class ChildTool(BaseTool):
                         error_to_raise = ValueError(msg)
             else:
                 content = response
-        except ValidationError as e:
+        except (ValidationError, ValidationErrorV1) as e:
             if not self.handle_validation_error:
                 error_to_raise = e
             else:
diff --git a/libs/core/tests/unit_tests/test_tools.py b/libs/core/tests/unit_tests/test_tools.py
index 28ec11af51..2aaf509c5a 100644
--- a/libs/core/tests/unit_tests/test_tools.py
+++ b/libs/core/tests/unit_tests/test_tools.py
@@ -1148,6 +1148,30 @@ async def test_async_validation_error_handling_callable() -> None:
     assert expected == actual
 
 
+@pytest.mark.skipif(
+    sys.version_info >= (3, 14),
+    reason="pydantic.v1 namespace not supported with Python 3.14+",
+)
+async def test_async_validation_error_handling_pydantic_v1_schema() -> None:
+    """Test async validation error handling for Pydantic V1 schemas."""
+
+    class Args(BaseModelV1):
+        x: int
+
+    def foo(x: int) -> str:
+        """Return x as text."""
+        return str(x)
+
+    tool_ = StructuredTool.from_function(
+        foo,
+        args_schema=cast("ArgsSchema", Args),
+        handle_validation_error=True,
+    )
+
+    assert tool_.run({"x": "not-an-integer"}) == "Tool input validation error"
+    assert await tool_.arun({"x": "not-an-integer"}) == "Tool input validation error"
+
+
 @pytest.mark.parametrize(
     "handler",
     [