From 6f3556a10ad56188e0fbbb534d68aee59c2d31fc Mon Sep 17 00:00:00 2001
From: arpan sahu <28574248+arpansahu@users.noreply.github.com>
Date: Thu, 27 Aug 2026 07:28:01 +0530
Subject: [PATCH] fix(prompt): preserve langchain placeholder messages

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: arpan sahu <28574248+arpansahu@users.noreply.github.com>
---
 langfuse/model.py                     | 67 ++++++++++++++++++++++-----
 tests/unit/test_prompt_compilation.py | 32 +++++++++++++
 2 files changed, 88 insertions(+), 11 deletions(-)

diff --git a/langfuse/model.py b/langfuse/model.py
index 69d721597..e5d01b87e 100644
--- a/langfuse/model.py
+++ b/langfuse/model.py
@@ -279,6 +279,15 @@ def get_langchain_prompt(self, **kwargs: Union[str, Any]) -> str:
 
 
 class ChatPromptClient(BasePromptClient):
+    _LANGCHAIN_MESSAGE_ROLE_MAP = {
+        "human": "user",
+        "ai": "assistant",
+        "system": "system",
+        "tool": "tool",
+        "function": "function",
+        "chat": "chat",
+    }
+
     def __init__(self, prompt: Prompt_Chat, is_fallback: bool = False):
         super().__init__(prompt, is_fallback)
         self.prompt: List[ChatMessageWithPlaceholdersDict] = []
@@ -346,17 +355,12 @@ def compile(
                     placeholder_value = kwargs[placeholder_name]
                     if isinstance(placeholder_value, list):
                         for msg in placeholder_value:
-                            if isinstance(msg, dict):
-                                # Preserve all fields from the original message, such as tool calls
-                                compiled_msg = dict(msg)  # type: ignore
-                                # Ensure role and content are always present
-                                compiled_msg["role"] = msg.get("role", "NOT_GIVEN")
-                                compiled_msg["content"] = (
-                                    TemplateParser.compile_template(
-                                        msg.get("content", ""),  # type: ignore
-                                        kwargs,
-                                    )
-                                )
+                            compiled_msg = self._compile_placeholder_message(
+                                msg=msg,
+                                kwargs=kwargs,
+                            )
+
+                            if compiled_msg is not None:
                                 compiled_messages.append(compiled_msg)
                             else:
                                 compiled_messages.append(
@@ -387,6 +391,47 @@ def compile(
 
         return compiled_messages  # type: ignore
 
+    def _compile_placeholder_message(
+        self, *, msg: Any, kwargs: Dict[str, Any]
+    ) -> Optional[Dict[str, Any]]:
+        if isinstance(msg, dict):
+            # Preserve all fields from the original message, such as tool calls
+            compiled_msg = dict(msg)
+            # Ensure role and content are always present
+            compiled_msg["role"] = msg.get("role", "NOT_GIVEN")
+            compiled_msg["content"] = TemplateParser.compile_template(
+                msg.get("content", ""),
+                kwargs,
+            )
+            return compiled_msg
+
+        if not hasattr(msg, "content"):
+            return None
+
+        role = getattr(msg, "role", None)
+        if role is None:
+            role = self._LANGCHAIN_MESSAGE_ROLE_MAP.get(
+                str(getattr(msg, "type", "")),
+            )
+
+        if role is None:
+            return None
+
+        content = getattr(msg, "content")
+        compiled_msg = {
+            "role": role,
+            "content": TemplateParser.compile_template(content, kwargs)
+            if isinstance(content, str)
+            else content,
+        }
+
+        for key in ("name", "tool_call_id", "tool_calls", "invalid_tool_calls"):
+            value = getattr(msg, key, None)
+            if value:
+                compiled_msg[key] = value
+
+        return compiled_msg
+
     @property
     def variables(self) -> List[str]:
         """Return all the variable names in the chat prompt template."""
diff --git a/tests/unit/test_prompt_compilation.py b/tests/unit/test_prompt_compilation.py
index 1b96a14dd..527225718 100644
--- a/tests/unit/test_prompt_compilation.py
+++ b/tests/unit/test_prompt_compilation.py
@@ -1,4 +1,5 @@
 import pytest
+from langchain_core.messages import AIMessage, HumanMessage
 from langchain_core.prompts import ChatPromptTemplate, PromptTemplate
 
 from langfuse.api import ChatMessage, Prompt_Chat
@@ -932,3 +933,34 @@ def test_tool_calls_preservation_in_message_placeholder():
     # Final user message with compiled variable
     assert compiled_messages[4]["role"] == "user"
     assert compiled_messages[4]["content"] == "Help me with weather inquiry"
+
+
+def test_langchain_messages_in_message_placeholder_are_preserved():
+    prompt_client = ChatPromptClient(
+        Prompt_Chat(
+            type="chat",
+            name="langchain_message_placeholder_test",
+            version=1,
+            config={},
+            tags=[],
+            labels=[],
+            prompt=[
+                {"type": "placeholder", "name": "history"},
+                {"role": "user", "content": "Question for {{name}}"},
+            ],
+        )
+    )
+
+    compiled_messages = prompt_client.compile(
+        name="Ada",
+        history=[
+            HumanMessage(content="Hello {{name}}"),
+            AIMessage(content="Hi {{name}}"),
+        ],
+    )
+
+    assert compiled_messages == [
+        {"role": "user", "content": "Hello Ada"},
+        {"role": "assistant", "content": "Hi Ada"},
+        {"role": "user", "content": "Question for Ada"},
+    ]