diff --git a/libs/core/langchain_core/tools.py b/libs/core/langchain_core/tools.py
index fcd93025bc7..2fa65902092 100644
--- a/libs/core/langchain_core/tools.py
+++ b/libs/core/langchain_core/tools.py
@@ -1664,6 +1664,10 @@ def _get_all_basemodel_annotations(
     if isinstance(cls, type):
         annotations: Dict[str, Type] = {}
         for name, param in inspect.signature(cls).parameters.items():
+            # Exclude hidden init args added by pydantic Config. For example if
+            # BaseModel(extra="allow") then "extra_data" will part of init sig.
+            if (fields := getattr(cls, "__fields__", {})) and name not in fields:
+                continue
             annotations[name] = param.annotation
         orig_bases: Tuple = getattr(cls, "__orig_bases__", tuple())
     # cls has subscript: cls = FooBar[int]
diff --git a/libs/core/tests/unit_tests/test_tools.py b/libs/core/tests/unit_tests/test_tools.py
index a222bc01500..0828066d99a 100644
--- a/libs/core/tests/unit_tests/test_tools.py
+++ b/libs/core/tests/unit_tests/test_tools.py
@@ -1751,11 +1751,11 @@ def test__get_all_basemodel_annotations_v2(use_v1_namespace: bool) -> None:
 
     if use_v1_namespace:
 
-        class ModelA(BaseModel, Generic[A]):
+        class ModelA(BaseModel, Generic[A], extra="allow"):
             a: A
     else:
 
-        class ModelA(BaseModelProper, Generic[A]):  # type: ignore[no-redef]
+        class ModelA(BaseModelProper, Generic[A], extra="allow"):  # type: ignore[no-redef]
             a: A
 
     class ModelB(ModelA[str]):
@@ -1812,7 +1812,7 @@ def test__get_all_basemodel_annotations_v2(use_v1_namespace: bool) -> None:
 def test__get_all_basemodel_annotations_v1() -> None:
     A = TypeVar("A")
 
-    class ModelA(BaseModel, Generic[A]):
+    class ModelA(BaseModel, Generic[A], extra="allow"):
         a: A
 
     class ModelB(ModelA[str]):
diff --git a/libs/standard-tests/langchain_standard_tests/integration_tests/chat_models.py b/libs/standard-tests/langchain_standard_tests/integration_tests/chat_models.py
index 5c23376a917..32d7af5f5f7 100644
--- a/libs/standard-tests/langchain_standard_tests/integration_tests/chat_models.py
+++ b/libs/standard-tests/langchain_standard_tests/integration_tests/chat_models.py
@@ -285,10 +285,7 @@ class ChatModelIntegrationTests(ChatModelTests):
         assert isinstance(chunk, dict)  # for mypy
         assert set(chunk.keys()) == {"setup", "punchline"}
 
-    def test_tool_message_histories_string_content(
-        self,
-        model: BaseChatModel,
-    ) -> None:
+    def test_tool_message_histories_string_content(self, model: BaseChatModel) -> None:
         """
         Test that message histories are compatible with string tool contents
         (e.g. OpenAI).