Fix: warn on system_instruction conflict even with single system message

When the only message in context was a system message,
_extract_initial_system_or_developer would convert it to "user" (to
prevent empty history) without warning about the conflict with
system_instruction. Now warns inline before converting, with a message
explaining both the conflict and the user-role conversion.

src/pipecat/adapters/base_llm_adapter.py

@@ -188,6 +188,15 @@ class BaseLLMAdapter(ABC, Generic[TLLMInvocationParams]):
 
         # Would extracting empty the list? Convert to "user" instead.
         if len(messages) == 1:
+            if role == "system" and system_instruction:
+                if not self._warned_system_instruction:
+                    self._warned_system_instruction = True
+                    logger.warning(
+                        "Both system_instruction and a system message in context are set."
+                        " Using system_instruction. The system message in context is being"
+                        " converted to a user message to avoid sending an empty conversation"
+                        " history."
+                    )
             messages[0]["role"] = "user"
             return None, None
 

tests/test_get_llm_invocation_params.py

@@ -2002,6 +2002,23 @@ class TestBaseLLMAdapterHelpers(unittest.TestCase):
         self.assertEqual(len(messages), 1)  # not popped
         self.assertEqual(messages[0]["role"], "user")
 
+    def test_single_system_message_with_system_instruction_warns(self):
+        """Single system message + system_instruction still warns even though content isn't extracted."""
+        messages = [
+            {"role": "system", "content": "Be helpful."},
+        ]
+
+        adapter = OpenAILLMAdapter()
+        with patch("pipecat.adapters.base_llm_adapter.logger") as mock_logger:
+            content, role = adapter._extract_initial_system_or_developer(
+                messages, system_instruction="Be concise."
+            )
+            mock_logger.warning.assert_called_once()
+
+        self.assertIsNone(content)
+        self.assertIsNone(role)
+        self.assertEqual(messages[0]["role"], "user")
+
     def test_non_system_message_ignored(self):
         """Non-system/developer first message is ignored."""
         messages = [