fix anthropic chat model mutating input list (#6457)

<!--
Thank you for contributing to LangChain! Your PR will appear in our
release under the title you set. Please make sure it highlights your
valuable contribution.

Replace this with a description of the change, the issue it fixes (if
applicable), and relevant context. List any dependencies required for
this change.

After you're done, someone will review your PR. They may suggest
improvements. If no one reviews your PR within a few days, feel free to
@-mention the same people again, as notifications can get lost.

Finally, we'd love to show appreciation for your contribution - if you'd
like us to shout you out on Twitter, please also include your handle!
-->

<!-- Remove if not applicable -->

Fixes: ChatAnthropic was mutating the input message list during
formatting which isn't ideal bc you could be changing the behavior for
other chat models when using the same input

#### Before submitting

<!-- If you're adding a new integration, please include:

1. a test for the integration - favor unit tests that does not rely on
network access.
2. an example notebook showing its use


See contribution guidelines for more information on how to write tests,
lint
etc:


https://github.com/hwchase17/langchain/blob/master/.github/CONTRIBUTING.md
-->

#### Who can review?

Tag maintainers/contributors who might be interested:

langchain/chat_models/anthropic.py

@@ -76,6 +76,8 @@ def _convert_messages_to_prompt(self, messages: List[BaseMessage]) -> str:
         Returns:
             str: Combined string with necessary HUMAN_PROMPT and AI_PROMPT tags.
         """
+        messages = messages.copy()  # don't mutate the original list
+
         if not self.AI_PROMPT:
             raise NameError("Please ensure the anthropic package is loaded")
 

tests/integration_tests/chat_models/test_anthropic.py

@@ -24,6 +24,22 @@ def test_anthropic_call() -> None:
     assert isinstance(response.content, str)
 
 
+def test_anthropic_generate() -> None:
+    """Test generate method of anthropic."""
+    chat = ChatAnthropic(model="test")
+    chat_messages: List[List[BaseMessage]] = [
+        [HumanMessage(content="How many toes do dogs have?")]
+    ]
+    messages_copy = [messages.copy() for messages in chat_messages]
+    result: LLMResult = chat.generate(chat_messages)
+    assert isinstance(result, LLMResult)
+    for response in result.generations[0]:
+        assert isinstance(response, ChatGeneration)
+        assert isinstance(response.text, str)
+        assert response.text == response.message.content
+    assert chat_messages == messages_copy
+
+
 def test_anthropic_streaming() -> None:
     """Test streaming tokens from anthropic."""
     chat = ChatAnthropic(model="test", streaming=True)