fix(core): implement ChatPromptTemplate.save by delegating to BasePromptTemplate

libs/core/langchain_core/prompts/chat.py

@@ -1287,7 +1287,7 @@ def save(self, file_path: Path | str) -> None:
         Args:
             file_path: path to file.
         """
-        raise NotImplementedError
+        return super().save(file_path)
 
     @override
     def pretty_repr(self, html: bool = False) -> str:

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

@@ -0,0 +1,42 @@
+import json
+from collections.abc import Callable
+from pathlib import Path
+from typing import IO, Any
+
+import pytest
+import yaml
+
+from langchain_core.prompts.chat import ChatPromptTemplate, HumanMessagePromptTemplate
+
+
+@pytest.mark.parametrize(
+    ("suffix", "loader"),
+    [
+        (".json", json.load),
+        (".yaml", yaml.safe_load),
+    ],
+)
+def test_chat_prompt_template_save_roundtrip(
+    tmp_path: Path, suffix: str, loader: Callable[[IO[str]], Any]
+) -> None:
+    prompt = ChatPromptTemplate.from_messages(
+        [HumanMessagePromptTemplate.from_template("Hello {name}")]
+    )
+
+    out = tmp_path / f"prompt{suffix}"
+    prompt.save(out)
+
+    assert out.exists()
+
+    with out.open("r", encoding="utf-8") as f:
+        data = loader(f)
+
+    # minimal structural assertions
+    assert data["_type"] == "chat"
+    # messages should have exactly one human template entry
+    assert isinstance(data["messages"], list)
+    assert len(data["messages"]) == 1
+    first = data["messages"][0]
+    # Accept various serializer shapes; only assert that an entry exists
+    # and is structured
+    assert isinstance(first, (dict, list))