diff --git a/libs/partners/anthropic/langchain_anthropic/chat_models.py b/libs/partners/anthropic/langchain_anthropic/chat_models.py
index e5deebd6b7..5b2c715029 100644
--- a/libs/partners/anthropic/langchain_anthropic/chat_models.py
+++ b/libs/partners/anthropic/langchain_anthropic/chat_models.py
@@ -95,11 +95,12 @@ def _format_image(image_url: str) -> Dict:
 
 
 def _merge_messages(
-    messages: List[BaseMessage],
+    messages: Sequence[BaseMessage],
 ) -> List[Union[SystemMessage, AIMessage, HumanMessage]]:
     """Merge runs of human/tool messages into single human messages with content blocks."""  # noqa: E501
     merged: list = []
     for curr in messages:
+        curr = curr.copy(deep=True)
         if isinstance(curr, ToolMessage):
             if isinstance(curr.content, str):
                 curr = HumanMessage(
diff --git a/libs/partners/anthropic/pyproject.toml b/libs/partners/anthropic/pyproject.toml
index ba8a44ad54..6f15ec65cf 100644
--- a/libs/partners/anthropic/pyproject.toml
+++ b/libs/partners/anthropic/pyproject.toml
@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "langchain-anthropic"
-version = "0.1.10"
+version = "0.1.11"
 description = "An integration package connecting AnthropicMessages and LangChain"
 authors = []
 readme = "README.md"
diff --git a/libs/partners/anthropic/tests/unit_tests/test_chat_models.py b/libs/partners/anthropic/tests/unit_tests/test_chat_models.py
index 9373b0a1ea..7b5c1624df 100644
--- a/libs/partners/anthropic/tests/unit_tests/test_chat_models.py
+++ b/libs/partners/anthropic/tests/unit_tests/test_chat_models.py
@@ -165,6 +165,25 @@ def test__merge_messages() -> None:
     assert expected == actual
 
 
+def test__merge_messages_mutation() -> None:
+    original_messages = [
+        HumanMessage([{"type": "text", "text": "bar"}]),
+        HumanMessage("next thing"),
+    ]
+    messages = [
+        HumanMessage([{"type": "text", "text": "bar"}]),
+        HumanMessage("next thing"),
+    ]
+    expected = [
+        HumanMessage(
+            [{"type": "text", "text": "bar"}, {"type": "text", "text": "next thing"}]
+        ),
+    ]
+    actual = _merge_messages(messages)
+    assert expected == actual
+    assert messages == original_messages
+
+
 @pytest.fixture()
 def pydantic() -> Type[BaseModel]:
     class dummy_function(BaseModel):