diff --git a/libs/partners/ollama/langchain_ollama/chat_models.py b/libs/partners/ollama/langchain_ollama/chat_models.py
index ffe8766b802..4892e350f26 100644
--- a/libs/partners/ollama/langchain_ollama/chat_models.py
+++ b/libs/partners/ollama/langchain_ollama/chat_models.py
@@ -865,6 +865,7 @@ class ChatOllama(BaseChatModel):
         Returns:
             List of messages in Ollama format.
         """
+        messages = list(messages)  # shallow copy to avoid mutating caller's list
         for idx, message in enumerate(messages):
             # Handle message content written in v1 format
             if (