Remove trailing system→user conversion for cross-call stability

Perplexity appears to have statefulness within a conversation, so
converting a system message to "user" in one call and then back to
"system" in the next (after more messages are appended) causes API
errors. Remove the trailing system→user conversion entirely — if the
context only has system messages, the API call will fail but the
mistake will be caught right away.

src/pipecat/adapters/services/perplexity_adapter.py

@@ -80,15 +80,19 @@ class PerplexityLLMAdapter(OpenAILLMAdapter):
            (e.g. a converted system→user message adjacent to an existing user
            message gets merged).
 
-        3. **Ensure last message is user/tool** — If the last message is
+        3. **Remove trailing assistant messages** — If the last message is
            "assistant", remove it. OpenAI appears to silently ignore trailing
            assistant messages server-side, so removing them preserves equivalent
            behavior while satisfying Perplexity's "last message must be
-           user/tool" constraint. If the last message is "system", convert it
-           to "user". A trailing system message can only occur when the context
-           consists entirely of system messages (possibly followed by assistant
-           messages that were just removed), because step 1 converts any system
-           message that appears after a non-system message to "user".
+           user/tool" constraint.
+
+        Note: we intentionally do *not* convert a trailing system message to
+        "user". That would make the transformation unstable across calls —
+        Perplexity appears to have statefulness/caching within a conversation,
+        so a message that was sent as "user" in one call but becomes "system"
+        in the next (once more messages are appended) causes errors. If the
+        context consists entirely of system messages, the Perplexity API call
+        will fail, but that mistake will be caught right away.
 
         Args:
             messages: List of message dicts with "role" and "content" keys.
@@ -138,17 +142,11 @@ class PerplexityLLMAdapter(OpenAILLMAdapter):
             else:
                 i += 1
 
-        # Step 3: Handle trailing messages.
+        # Step 3: Remove trailing assistant messages.
         # Perplexity requires the last message to be "user" or "tool".
-        if messages:
-            # Remove trailing assistant messages. OpenAI appears to silently
-            # ignore trailing assistant messages server-side, so removing them
-            # preserves equivalent behavior.
-            while messages and messages[-1].get("role") == "assistant":
-                messages.pop()
-
-            # If the last message is "system", convert it to "user".
-            if messages and messages[-1].get("role") == "system":
-                messages[-1]["role"] = "user"
+        # OpenAI appears to silently ignore trailing assistant messages
+        # server-side, so removing them preserves equivalent behavior.
+        while messages and messages[-1].get("role") == "assistant":
+            messages.pop()
 
         return messages

tests/test_get_llm_invocation_params.py

@@ -1123,8 +1123,14 @@ class TestPerplexityGetLLMInvocationParams(unittest.TestCase):
         self.assertEqual(params["messages"][0]["role"], "user")
         self.assertEqual(params["messages"][0]["content"], "Hello")
 
-    def test_only_system_message_converted_to_user(self):
-        """Test that a single system message is converted to user role."""
+    def test_only_system_messages_preserved(self):
+        """Test that system-only contexts are left unchanged (no system→user conversion).
+
+        We intentionally do not convert trailing system messages to "user"
+        because that would make the transformation unstable across calls —
+        Perplexity has statefulness within a conversation, so a message that
+        was "user" in one call but becomes "system" in the next causes errors.
+        """
         messages: list[LLMStandardMessage] = [
             {"role": "system", "content": "You are a helpful assistant."},
         ]
@@ -1133,30 +1139,15 @@ class TestPerplexityGetLLMInvocationParams(unittest.TestCase):
         params = self.adapter.get_llm_invocation_params(context)
 
         self.assertEqual(len(params["messages"]), 1)
-        self.assertEqual(params["messages"][0]["role"], "user")
-        self.assertEqual(params["messages"][0]["content"], "You are a helpful assistant.")
-
-    def test_only_system_messages_last_converted_to_user(self):
-        """Test that when only system messages exist, the last one is converted to user."""
-        messages: list[LLMStandardMessage] = [
-            {"role": "system", "content": "You are a helpful assistant."},
-            {"role": "system", "content": "Always be polite."},
-        ]
-
-        context = LLMContext(messages=messages)
-        params = self.adapter.get_llm_invocation_params(context)
-
-        self.assertEqual(len(params["messages"]), 2)
         self.assertEqual(params["messages"][0]["role"], "system")
-        self.assertEqual(params["messages"][0]["content"], "You are a helpful assistant.")
-        self.assertEqual(params["messages"][1]["role"], "user")
-        self.assertEqual(params["messages"][1]["content"], "Always be polite.")
 
-    def test_trailing_assistant_removed_then_system_converted(self):
-        """Test that trailing assistant is removed, exposing a system message that becomes user.
+    def test_system_exposed_after_trailing_assistant_removed(self):
+        """Test that a system message exposed by trailing assistant removal stays system.
 
-        This exercises the ordering of step 3: strip trailing assistants first,
-        then convert a trailing system to user.
+        It's important that initial system messages are never converted to
+        "user", because Perplexity has statefulness within a conversation — if
+        a message was sent as "system" in one call and then becomes "user" in a
+        later call (after more messages are appended), the API rejects it.
         """
         messages: list[LLMStandardMessage] = [
             {"role": "system", "content": "You are helpful."},
@@ -1166,9 +1157,9 @@ class TestPerplexityGetLLMInvocationParams(unittest.TestCase):
         context = LLMContext(messages=messages)
         params = self.adapter.get_llm_invocation_params(context)
 
-        # Trailing assistant removed → [system] → system converted to user → [user]
+        # Trailing assistant removed → [system], system stays as-is
         self.assertEqual(len(params["messages"]), 1)
-        self.assertEqual(params["messages"][0]["role"], "user")
+        self.assertEqual(params["messages"][0]["role"], "system")
         self.assertEqual(params["messages"][0]["content"], "You are helpful.")
 
     def test_consecutive_assistants_merged_then_trailing_removed(self):