core: update load chat prompt template multi mesages

libs/core/langchain_core/prompts/loading.py

@@ -186,15 +186,21 @@ def _load_prompt_from_file(
 def _load_chat_prompt(config: dict) -> ChatPromptTemplate:
     """Load chat prompt from config"""
 
-    messages = config.pop("messages")
-    template = messages[0]["prompt"].pop("template") if messages else None
-    config.pop("input_variables")
+    template = config.pop("template")
 
     if not template:
         msg = "Can't load chat prompt without template"
         raise ValueError(msg)
 
-    return ChatPromptTemplate.from_template(template=template, **config)
+    messages = []
+    if isinstance(template, str):
+        messages.append(("human", template))
+
+    elif isinstance(template, list):
+        for item in template:
+            messages.append((item["role"], item["content"]))
+
+    return ChatPromptTemplate(messages=messages, **config)
 
 
 type_to_loader_dict: dict[str, Callable[[dict], BasePromptTemplate]] = {

libs/core/tests/unit_tests/examples/simple_chat_prompt.json

@@ -0,0 +1,19 @@
+{
+    "_type": "chat",
+    "input_variables": [
+        "adjective"
+    ],
+    "partial_variables": {
+        "content": "dogs"
+    },
+    "template": [
+        {
+            "role": "system",
+            "content": "You are a comedian"
+        },
+        {
+            "role": "human",
+            "content": "Tell me a {adjective} joke about {content}."
+        }
+    ]
+}

libs/core/tests/unit_tests/examples/simple_chat_prompt.yaml

@@ -0,0 +1,10 @@
+_type: chat
+input_variables:
+    ["adjective"]
+partial_variables:
+    content: dogs
+template:
+    -   role: system
+        content: "You are a comedian"
+    -   role: human
+        content: "Tell me a {adjective} joke about {content}."

libs/core/tests/unit_tests/prompts/test_loading.py

@@ -7,6 +7,7 @@
 
 import pytest
 
+from langchain_core.prompts.chat import ChatPromptTemplate
 from langchain_core.prompts.few_shot import FewShotPromptTemplate
 from langchain_core.prompts.loading import load_prompt
 from langchain_core.prompts.prompt import PromptTemplate
@@ -36,6 +37,20 @@ def test_loading_from_yaml() -> None:
     assert prompt == expected_prompt
 
 
+def test_loading_chat_from_yaml() -> None:
+    """Test loading from yaml file."""
+    prompt = load_prompt(EXAMPLE_DIR / "simple_chat_prompt.yaml")
+    expected_prompt = ChatPromptTemplate(
+        input_variables=["adjective"],
+        partial_variables={"content": "dogs"},
+        messages=[
+            ("system", "You are a comedian"),
+            ("human", "Tell me a {adjective} joke about {content}."),
+        ],
+    )
+    assert prompt == expected_prompt
+
+
 def test_loading_from_json() -> None:
     """Test loading from json file."""
     prompt = load_prompt(EXAMPLE_DIR / "simple_prompt.json")
@@ -46,6 +61,20 @@ def test_loading_from_json() -> None:
     assert prompt == expected_prompt
 
 
+def test_loading_chat_from_json() -> None:
+    """Test loading from json file."""
+    prompt = load_prompt(EXAMPLE_DIR / "simple_chat_prompt.json")
+    expected_prompt = ChatPromptTemplate(
+        input_variables=["adjective"],
+        partial_variables={"content": "dogs"},
+        messages=[
+            ("system", "You are a comedian"),
+            ("human", "Tell me a {adjective} joke about {content}."),
+        ],
+    )
+    assert prompt == expected_prompt
+
+
 def test_loading_jinja_from_json() -> None:
     """Test that loading jinja2 format prompts from JSON raises ValueError."""
     prompt_path = EXAMPLE_DIR / "jinja_injection_prompt.json"