diff --git a/libs/partners/anthropic/langchain_anthropic/chat_models.py b/libs/partners/anthropic/langchain_anthropic/chat_models.py
index d7f2643f0f7..5f69b62228a 100644
--- a/libs/partners/anthropic/langchain_anthropic/chat_models.py
+++ b/libs/partners/anthropic/langchain_anthropic/chat_models.py
@@ -1,3 +1,4 @@
+import copy
 import re
 import warnings
 from operator import itemgetter
@@ -116,7 +117,6 @@ def _merge_messages(
     """Merge runs of human/tool messages into single human messages with content blocks."""  # noqa: E501
     merged: list = []
     for curr in messages:
-        curr = curr.model_copy(deep=True)
         if isinstance(curr, ToolMessage):
             if (
                 isinstance(curr.content, list)
@@ -143,12 +143,12 @@ def _merge_messages(
             if isinstance(last.content, str):
                 new_content: List = [{"type": "text", "text": last.content}]
             else:
-                new_content = last.content
+                new_content = copy.copy(last.content)
             if isinstance(curr.content, str):
                 new_content.append({"type": "text", "text": curr.content})
             else:
                 new_content.extend(curr.content)
-            last.content = new_content
+            merged[-1] = curr.model_copy(update={"content": new_content}, deep=False)
         else:
             merged.append(curr)
     return merged
@@ -178,9 +178,11 @@ def _format_messages(
                 raise ValueError("System message must be at beginning of message list.")
             if isinstance(message.content, list):
                 system = [
-                    block
-                    if isinstance(block, dict)
-                    else {"type": "text", "text": "block"}
+                    (
+                        block
+                        if isinstance(block, dict)
+                        else {"type": "text", "text": "block"}
+                    )
                     for block in message.content
                 ]
             else: