fix(prompt-placeholders): don't strip tool calls from placeholder messages (#1346)

* add test that tool calls aren't stripped from msg placehodlers

* fix: allow arbitrary values to be inserted as placeholder fill-in

* fix formatting

Author: nimarb

langfuse/model.py

@@ -165,7 +165,13 @@ def compile(
         self, **kwargs: Union[str, Any]
     ) -> Union[
         str,
-        Sequence[Union[ChatMessageDict, ChatMessageWithPlaceholdersDict_Placeholder]],
+        Sequence[
+            Union[
+                Dict[str, Any],
+                ChatMessageDict,
+                ChatMessageWithPlaceholdersDict_Placeholder,
+            ]
+        ],
     ]:
         pass
 
@@ -327,7 +333,11 @@ def __init__(self, prompt: Prompt_Chat, is_fallback: bool = False):
     def compile(
         self,
         **kwargs: Union[str, Any],
-    ) -> Sequence[Union[ChatMessageDict, ChatMessageWithPlaceholdersDict_Placeholder]]:
+    ) -> Sequence[
+        Union[
+            Dict[str, Any], ChatMessageDict, ChatMessageWithPlaceholdersDict_Placeholder
+        ]
+    ]:
         """Compile the prompt with placeholders and variables.
 
         Args:
@@ -338,7 +348,11 @@ def compile(
             List of compiled chat messages as plain dictionaries, with unresolved placeholders kept as-is.
         """
         compiled_messages: List[
-            Union[ChatMessageDict, ChatMessageWithPlaceholdersDict_Placeholder]
+            Union[
+                Dict[str, Any],
+                ChatMessageDict,
+                ChatMessageWithPlaceholdersDict_Placeholder,
+            ]
         ] = []
         unresolved_placeholders: List[ChatMessageWithPlaceholdersDict_Placeholder] = []
 
@@ -361,20 +375,18 @@ def compile(
                     placeholder_value = kwargs[placeholder_name]
                     if isinstance(placeholder_value, list):
                         for msg in placeholder_value:
-                            if (
-                                isinstance(msg, dict)
-                                and "role" in msg
-                                and "content" in msg
-                            ):
-                                compiled_messages.append(
-                                    ChatMessageDict(
-                                        role=msg["role"],  # type: ignore
-                                        content=TemplateParser.compile_template(
-                                            msg["content"],  # type: ignore
-                                            kwargs,
-                                        ),
-                                    ),
+                            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_messages.append(compiled_msg)
                             else:
                                 compiled_messages.append(
                                     ChatMessageDict(

tests/test_prompt_compilation.py

@@ -850,3 +850,85 @@ def test_get_langchain_prompt_with_unresolved_placeholders(self):
 
         # Third message should be the user message
         assert langchain_messages[2] == ("user", "Help me with coding")
+
+
+def test_tool_calls_preservation_in_message_placeholder():
+    """Test that tool calls are preserved when compiling message placeholders."""
+    from langfuse.api.resources.prompts import Prompt_Chat
+
+    chat_messages = [
+        {"role": "system", "content": "You are a helpful assistant."},
+        {"type": "placeholder", "name": "message_history"},
+        {"role": "user", "content": "Help me with {{task}}"},
+    ]
+
+    prompt_client = ChatPromptClient(
+        Prompt_Chat(
+            type="chat",
+            name="tool_calls_test",
+            version=1,
+            config={},
+            tags=[],
+            labels=[],
+            prompt=chat_messages,
+        )
+    )
+
+    # Message history with tool calls - exactly like the bug report describes
+    message_history_with_tool_calls = [
+        {"role": "user", "content": "What's the weather like?"},
+        {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [
+                {
+                    "id": "call_123",
+                    "type": "function",
+                    "function": {
+                        "name": "get_weather",
+                        "arguments": '{"location": "San Francisco"}',
+                    },
+                }
+            ],
+        },
+        {
+            "role": "tool",
+            "content": "It's sunny, 72°F",
+            "tool_call_id": "call_123",
+            "name": "get_weather",
+        },
+    ]
+
+    # Compile with message history and variables
+    compiled_messages = prompt_client.compile(
+        task="weather inquiry", message_history=message_history_with_tool_calls
+    )
+
+    # Should have 5 messages: system + 3 from history + user
+    assert len(compiled_messages) == 5
+
+    # System message
+    assert compiled_messages[0]["role"] == "system"
+    assert compiled_messages[0]["content"] == "You are a helpful assistant."
+
+    # User message from history
+    assert compiled_messages[1]["role"] == "user"
+    assert compiled_messages[1]["content"] == "What's the weather like?"
+
+    # Assistant message with TOOL CALLS
+    assert compiled_messages[2]["role"] == "assistant"
+    assert compiled_messages[2]["content"] == ""
+    assert "tool_calls" in compiled_messages[2]
+    assert len(compiled_messages[2]["tool_calls"]) == 1
+    assert compiled_messages[2]["tool_calls"][0]["id"] == "call_123"
+    assert compiled_messages[2]["tool_calls"][0]["function"]["name"] == "get_weather"
+
+    # TOOL CALL results message
+    assert compiled_messages[3]["role"] == "tool"
+    assert compiled_messages[3]["content"] == "It's sunny, 72°F"
+    assert compiled_messages[3]["tool_call_id"] == "call_123"
+    assert compiled_messages[3]["name"] == "get_weather"
+
+    # Final user message with compiled variable
+    assert compiled_messages[4]["role"] == "user"
+    assert compiled_messages[4]["content"] == "Help me with weather inquiry"