Forward cache_control from message-level placement in ChatDatabricks

integrations/langchain/src/databricks_langchain/chat_models.py

@@ -1419,6 +1419,14 @@ def _convert_message_to_dict(message: BaseMessage) -> dict:
     # if (name := message.name or message.additional_kwargs.get("name")) is not None:
     #     message_dict["name"] = name
 
+    # Forward an Anthropic prompt-caching breakpoint placed at the message level.
+    # LangChain stores a message-level `cache_control` in `additional_kwargs`. Without
+    # forwarding it here, the breakpoint reaches the endpoint only when it is nested
+    # inside a typed content block, so prompt caching silently never activates for the
+    # other natural placements.
+    if (cache_control := message.additional_kwargs.get("cache_control")) is not None:
+        message_dict["cache_control"] = cache_control
+
     if isinstance(message, ChatMessage):
         return {"role": message.role, **message_dict}
     elif isinstance(message, HumanMessage):

integrations/langchain/tests/unit_tests/test_chat_models.py

@@ -713,6 +713,23 @@ def test_convert_message_not_propagate_id() -> None:
     assert "id" not in result
 
 
+def test_convert_message_forwards_message_level_cache_control() -> None:
+    # A message-level cache_control breakpoint (stored by LangChain in
+    # additional_kwargs) must be forwarded to the request payload, so Anthropic
+    # prompt caching activates without nesting the flag inside a content block.
+    cache_control = {"type": "ephemeral"}
+    message = SystemMessage(content="foo", additional_kwargs={"cache_control": cache_control})
+    result = _convert_message_to_dict(message)
+    assert result == {"role": "system", "content": "foo", "cache_control": cache_control}
+
+
+def test_convert_message_omits_cache_control_when_absent() -> None:
+    # No cache_control key should appear when the message does not carry one.
+    message = SystemMessage(content="foo")
+    result = _convert_message_to_dict(message)
+    assert "cache_control" not in result
+
+
 def test_convert_message_with_tool_calls() -> None:
     ID = "call_fb5f5e1a-bac0-4422-95e9-d06e6022ad12"
     tool_calls = [