diff --git a/changelog/4009.added.md b/changelog/4009.added.md
new file mode 100644
index 0000000000..9ebbec7dd9
--- /dev/null
+++ b/changelog/4009.added.md
@@ -0,0 +1 @@
+- Added `PerplexityLLMAdapter` that automatically transforms conversation messages to satisfy Perplexity's stricter API constraints (strict role alternation, no non-initial system messages, last message must be user/tool). Previously, certain conversation histories could cause Perplexity API errors that didn't occur with OpenAI (`PerplexityLLMService` subclasses `OpenAILLMService` since Perplexity uses an OpenAI-compatible API).
diff --git a/src/pipecat/adapters/services/perplexity_adapter.py b/src/pipecat/adapters/services/perplexity_adapter.py
new file mode 100644
index 0000000000..a8fbe3c189
--- /dev/null
+++ b/src/pipecat/adapters/services/perplexity_adapter.py
@@ -0,0 +1,152 @@
+#
+# Copyright (c) 2024-2026, Daily
+#
+# SPDX-License-Identifier: BSD 2-Clause License
+#
+
+"""Perplexity LLM adapter for Pipecat.
+
+Perplexity's API uses an OpenAI-compatible interface but enforces stricter
+constraints on conversation history structure:
+
+1. **Strict role alternation** — Messages must alternate between "user"/"tool"
+   and "assistant" roles. Consecutive messages with the same role (e.g. two
+   "user" messages in a row) are rejected with:
+   ``"messages must be an alternating sequence of user/tool and assistant messages"``
+
+2. **No non-initial system messages** — "system" messages are only allowed at
+   the start of the conversation. A system message after a non-system message
+   causes:
+   ``"only the initial message can have the system role"``
+
+3. **Last message must be user/tool** — The final message in the conversation
+   must have role "user" or "tool". A trailing "assistant" message causes:
+   ``"the last message must have the user or tool role"``
+
+This adapter transforms the message list to satisfy all three constraints before
+the messages are sent to Perplexity's API.
+"""
+
+import copy
+from typing import List
+
+from openai.types.chat import ChatCompletionMessageParam
+
+from pipecat.adapters.services.open_ai_adapter import OpenAILLMAdapter, OpenAILLMInvocationParams
+from pipecat.processors.aggregators.llm_context import LLMContext
+
+
+class PerplexityLLMAdapter(OpenAILLMAdapter):
+    """Adapter that transforms messages to satisfy Perplexity's API constraints.
+
+    Perplexity's API is stricter than OpenAI about message structure. This
+    adapter extends ``OpenAILLMAdapter`` and applies message transformations
+    to ensure compliance with Perplexity's constraints (role alternation,
+    no non-initial system messages, last message must be user/tool).
+
+    The transformations are applied in ``get_llm_invocation_params`` after the
+    parent adapter extracts messages from the LLM context, and before
+    ``build_chat_completion_params`` prepends ``system_instruction``.
+    """
+
+    def get_llm_invocation_params(self, context: LLMContext) -> OpenAILLMInvocationParams:
+        """Get OpenAI-compatible invocation parameters with Perplexity message fixes applied.
+
+        Args:
+            context: The LLM context containing messages, tools, etc.
+
+        Returns:
+            Dictionary of parameters for Perplexity's ChatCompletion API, with
+            messages transformed to satisfy Perplexity's constraints.
+        """
+        params = super().get_llm_invocation_params(context)
+        params["messages"] = self._transform_messages(list(params["messages"]))
+        return params
+
+    def _transform_messages(
+        self, messages: List[ChatCompletionMessageParam]
+    ) -> List[ChatCompletionMessageParam]:
+        """Transform messages to satisfy Perplexity's API constraints.
+
+        Applies three transformation steps in order:
+
+        1. **Convert non-initial system messages to user** — Any system message
+           after the initial system message block is converted to role "user",
+           since Perplexity rejects system messages after a non-system message.
+
+        2. **Merge consecutive same-role messages** — After the above
+           conversions, adjacent messages with the same role are merged using
+           list-of-dicts content format. This ensures strict role alternation
+           (e.g. a converted system→user message adjacent to an existing user
+           message gets merged).
+
+        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.
+
+        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.
+
+        Returns:
+            Transformed list of message dicts satisfying Perplexity's constraints.
+        """
+        if not messages:
+            return messages
+
+        messages = copy.deepcopy(messages)
+
+        # Step 1: Convert non-initial system messages to "user".
+        # Perplexity allows system messages at the start, but rejects them
+        # after any non-system message.
+        in_initial_system_block = True
+        for i in range(len(messages)):
+            if messages[i].get("role") == "system":
+                if not in_initial_system_block:
+                    messages[i]["role"] = "user"
+            else:
+                in_initial_system_block = False
+
+        # Step 2: Merge consecutive same-role messages.
+        # After system→user conversions above, we may have adjacent same-role
+        # messages that violate Perplexity's strict alternation requirement.
+        # Skip consecutive system messages at the start — Perplexity allows those.
+        i = 0
+        while i < len(messages) - 1:
+            current = messages[i]
+            next_msg = messages[i + 1]
+            if current["role"] == next_msg["role"] == "system":
+                # Perplexity allows multiple initial system messages, don't merge
+                i += 1
+            elif current["role"] == next_msg["role"]:
+                # Convert string content to list-of-dicts format for merging
+                if isinstance(current.get("content"), str):
+                    current["content"] = [{"type": "text", "text": current["content"]}]
+                if isinstance(next_msg.get("content"), str):
+                    next_msg["content"] = [{"type": "text", "text": next_msg["content"]}]
+                # Merge content from next message into current
+                if isinstance(current.get("content"), list) and isinstance(
+                    next_msg.get("content"), list
+                ):
+                    current["content"].extend(next_msg["content"])
+                messages.pop(i + 1)
+            else:
+                i += 1
+
+        # Step 3: Remove trailing assistant messages.
+        # Perplexity requires the last message to be "user" or "tool".
+        # 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
diff --git a/src/pipecat/services/cerebras/llm.py b/src/pipecat/services/cerebras/llm.py
index 7c31a68573..dfb62baf82 100644
--- a/src/pipecat/services/cerebras/llm.py
+++ b/src/pipecat/services/cerebras/llm.py
@@ -117,6 +117,10 @@ def build_chat_completion_params(self, params_from_context: OpenAILLMInvocationP
         # Prepend system instruction if set
         if self._settings.system_instruction:
             messages = params.get("messages", [])
+            if messages and messages[0].get("role") == "system":
+                logger.warning(
+                    f"{self}: Both system_instruction and an initial system message in context are set. This may be unintended."
+                )
             params["messages"] = [
                 {"role": "system", "content": self._settings.system_instruction}
             ] + messages
diff --git a/src/pipecat/services/fireworks/llm.py b/src/pipecat/services/fireworks/llm.py
index bf141fac19..5efa607939 100644
--- a/src/pipecat/services/fireworks/llm.py
+++ b/src/pipecat/services/fireworks/llm.py
@@ -118,6 +118,10 @@ def build_chat_completion_params(self, params_from_context: OpenAILLMInvocationP
         # Prepend system instruction if set
         if self._settings.system_instruction:
             messages = params.get("messages", [])
+            if messages and messages[0].get("role") == "system":
+                logger.warning(
+                    f"{self}: Both system_instruction and an initial system message in context are set. This may be unintended."
+                )
             params["messages"] = [
                 {"role": "system", "content": self._settings.system_instruction}
             ] + messages
diff --git a/src/pipecat/services/mistral/llm.py b/src/pipecat/services/mistral/llm.py
index 3ee1b26230..063dac3aa7 100644
--- a/src/pipecat/services/mistral/llm.py
+++ b/src/pipecat/services/mistral/llm.py
@@ -236,6 +236,10 @@ def build_chat_completion_params(self, params_from_context: OpenAILLMInvocationP
         # Prepend system instruction if set
         if self._settings.system_instruction:
             messages = params.get("messages", [])
+            if messages and messages[0].get("role") == "system":
+                logger.warning(
+                    f"{self}: Both system_instruction and an initial system message in context are set. This may be unintended."
+                )
             params["messages"] = [
                 {"role": "system", "content": self._settings.system_instruction}
             ] + messages
diff --git a/src/pipecat/services/openai/base_llm.py b/src/pipecat/services/openai/base_llm.py
index 41b26bd206..eb8ce3cc66 100644
--- a/src/pipecat/services/openai/base_llm.py
+++ b/src/pipecat/services/openai/base_llm.py
@@ -332,8 +332,7 @@ def build_chat_completion_params(self, params_from_context: OpenAILLMInvocationP
             messages = params.get("messages", [])
             if messages and messages[0].get("role") == "system":
                 logger.warning(
-                    f"{self}: Both system_instruction and a system message in context are set."
-                    " Using system_instruction."
+                    f"{self}: Both system_instruction and an initial system message in context are set. This may be unintended."
                 )
             params["messages"] = [
                 {"role": "system", "content": self._settings.system_instruction}
@@ -381,8 +380,7 @@ async def run_inference(
             messages = params.get("messages", [])
             if messages and messages[0].get("role") == "system":
                 logger.warning(
-                    f"{self}: Both system_instruction and a system message in context are set."
-                    " Using system_instruction."
+                    f"{self}: Both system_instruction and an initial system message in context are set. This may be unintended."
                 )
             params["messages"] = [{"role": "system", "content": system_instruction}] + messages
 
diff --git a/src/pipecat/services/perplexity/llm.py b/src/pipecat/services/perplexity/llm.py
index 6c2ceba355..9ea323c5d5 100644
--- a/src/pipecat/services/perplexity/llm.py
+++ b/src/pipecat/services/perplexity/llm.py
@@ -14,7 +14,10 @@
 from dataclasses import dataclass
 from typing import Optional
 
+from loguru import logger
+
 from pipecat.adapters.services.open_ai_adapter import OpenAILLMInvocationParams
+from pipecat.adapters.services.perplexity_adapter import PerplexityLLMAdapter
 from pipecat.metrics.metrics import LLMTokenUsage
 from pipecat.processors.aggregators.llm_context import LLMContext
 from pipecat.processors.aggregators.openai_llm_context import OpenAILLMContext
@@ -37,6 +40,8 @@ class PerplexityLLMService(OpenAILLMService):
     in token usage reporting between Perplexity (incremental) and OpenAI (final summary).
     """
 
+    adapter_class = PerplexityLLMAdapter
+
     Settings = PerplexityLLMSettings
     _settings: Settings
 
@@ -119,6 +124,10 @@ def build_chat_completion_params(self, params_from_context: OpenAILLMInvocationP
         # Prepend system instruction if set
         if self._settings.system_instruction:
             messages = params.get("messages", [])
+            if messages and messages[0].get("role") == "system":
+                logger.warning(
+                    f"{self}: Both system_instruction and an initial system message in context are set. This may be unintended."
+                )
             params["messages"] = [
                 {"role": "system", "content": self._settings.system_instruction}
             ] + messages
diff --git a/src/pipecat/services/sambanova/llm.py b/src/pipecat/services/sambanova/llm.py
index 3c7d767371..710a22db21 100644
--- a/src/pipecat/services/sambanova/llm.py
+++ b/src/pipecat/services/sambanova/llm.py
@@ -134,6 +134,10 @@ def build_chat_completion_params(self, params_from_context: OpenAILLMInvocationP
         # Prepend system instruction if set
         if self._settings.system_instruction:
             messages = params.get("messages", [])
+            if messages and messages[0].get("role") == "system":
+                logger.warning(
+                    f"{self}: Both system_instruction and an initial system message in context are set. This may be unintended."
+                )
             params["messages"] = [
                 {"role": "system", "content": self._settings.system_instruction}
             ] + messages
diff --git a/tests/test_get_llm_invocation_params.py b/tests/test_get_llm_invocation_params.py
index c93275b67b..9cfeb89339 100644
--- a/tests/test_get_llm_invocation_params.py
+++ b/tests/test_get_llm_invocation_params.py
@@ -48,6 +48,7 @@
 from pipecat.adapters.services.bedrock_adapter import AWSBedrockLLMAdapter
 from pipecat.adapters.services.gemini_adapter import GeminiLLMAdapter
 from pipecat.adapters.services.open_ai_adapter import OpenAILLMAdapter
+from pipecat.adapters.services.perplexity_adapter import PerplexityLLMAdapter
 from pipecat.processors.aggregators.llm_context import (
     LLMContext,
     LLMStandardMessage,
@@ -992,5 +993,222 @@ def test_single_system_message_handling(self):
         self.assertEqual(len(params["messages"]), 0)
 
 
+class TestPerplexityGetLLMInvocationParams(unittest.TestCase):
+    def setUp(self) -> None:
+        """Sets up a common adapter instance for all tests."""
+        self.adapter = PerplexityLLMAdapter()
+
+    def test_standard_messages_pass_through(self):
+        """Test that a valid [user, assistant, user] sequence passes through unchanged."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "Hi there!"},
+            {"role": "user", "content": "How are you?"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 3)
+        self.assertEqual(params["messages"][0]["role"], "user")
+        self.assertEqual(params["messages"][0]["content"], "Hello")
+        self.assertEqual(params["messages"][1]["role"], "assistant")
+        self.assertEqual(params["messages"][1]["content"], "Hi there!")
+        self.assertEqual(params["messages"][2]["role"], "user")
+        self.assertEqual(params["messages"][2]["content"], "How are you?")
+
+    def test_initial_system_message_preserved(self):
+        """Test that a valid [system, user, assistant, user] sequence passes through unchanged."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "system", "content": "You are a helpful assistant."},
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "Hi!"},
+            {"role": "user", "content": "Bye"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 4)
+        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"][2]["role"], "assistant")
+        self.assertEqual(params["messages"][3]["role"], "user")
+
+    def test_consecutive_same_role_messages_merged(self):
+        """Test that consecutive user messages are merged into list-of-dicts content."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "user", "content": "First message"},
+            {"role": "user", "content": "Second message"},
+            {"role": "assistant", "content": "Response"},
+            {"role": "user", "content": "Third message"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 3)
+
+        # First message should be merged users
+        merged = params["messages"][0]
+        self.assertEqual(merged["role"], "user")
+        self.assertIsInstance(merged["content"], list)
+        self.assertEqual(len(merged["content"]), 2)
+        self.assertEqual(merged["content"][0]["type"], "text")
+        self.assertEqual(merged["content"][0]["text"], "First message")
+        self.assertEqual(merged["content"][1]["type"], "text")
+        self.assertEqual(merged["content"][1]["text"], "Second message")
+
+        self.assertEqual(params["messages"][1]["role"], "assistant")
+        self.assertEqual(params["messages"][2]["role"], "user")
+
+    def test_non_initial_system_converted_to_user(self):
+        """Test that non-initial system messages are converted to user and merged with adjacent user."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "system", "content": "You are helpful."},
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "Hi!"},
+            {"role": "system", "content": "Be concise."},
+            {"role": "user", "content": "Tell me about Python."},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        # system(initial), user, assistant, merged(system→user + user)
+        self.assertEqual(len(params["messages"]), 4)
+        self.assertEqual(params["messages"][0]["role"], "system")
+        self.assertEqual(params["messages"][1]["role"], "user")
+        self.assertEqual(params["messages"][2]["role"], "assistant")
+
+        # The converted system→user and the following user should be merged
+        merged = params["messages"][3]
+        self.assertEqual(merged["role"], "user")
+        self.assertIsInstance(merged["content"], list)
+        self.assertEqual(len(merged["content"]), 2)
+        self.assertEqual(merged["content"][0]["text"], "Be concise.")
+        self.assertEqual(merged["content"][1]["text"], "Tell me about Python.")
+
+    def test_multiple_system_messages_at_start_preserved(self):
+        """Test that multiple consecutive system messages at start pass through unchanged."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "system", "content": "You are a helpful assistant."},
+            {"role": "system", "content": "Always be polite."},
+            {"role": "user", "content": "Hello"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 3)
+        self.assertEqual(params["messages"][0]["role"], "system")
+        self.assertEqual(params["messages"][0]["content"], "You are a helpful assistant.")
+        self.assertEqual(params["messages"][1]["role"], "system")
+        self.assertEqual(params["messages"][1]["content"], "Always be polite.")
+        self.assertEqual(params["messages"][2]["role"], "user")
+        self.assertEqual(params["messages"][2]["content"], "Hello")
+
+    def test_trailing_assistant_removed(self):
+        """Test that a trailing assistant message is removed."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "Hi there!"},
+        ]
+
+        context = LLMContext(messages=messages)
+        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"], "Hello")
+
+    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."},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 1)
+        self.assertEqual(params["messages"][0]["role"], "system")
+
+    def test_system_exposed_after_trailing_assistant_removed(self):
+        """Test that a system message exposed by trailing assistant removal stays system.
+
+        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."},
+            {"role": "assistant", "content": "Sure thing."},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        # Trailing assistant removed → [system], system stays as-is
+        self.assertEqual(len(params["messages"]), 1)
+        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):
+        """Test that consecutive assistant messages are merged, then trailing assistant is removed."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "First response"},
+            {"role": "assistant", "content": "Second response"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        # After merging assistants we get [user, assistant(merged)], then trailing
+        # assistant is removed, leaving just [user]
+        self.assertEqual(len(params["messages"]), 1)
+        self.assertEqual(params["messages"][0]["role"], "user")
+        self.assertEqual(params["messages"][0]["content"], "Hello")
+
+    def test_tool_messages_preserved(self):
+        """Test that tool messages pass through without modification."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "user", "content": "What's the weather?"},
+            {
+                "role": "assistant",
+                "content": "Let me check.",
+                "tool_calls": [{"id": "1", "function": {"name": "get_weather", "arguments": "{}"}}],
+            },
+            {"role": "tool", "content": "Sunny, 72F", "tool_call_id": "1"},
+            {"role": "user", "content": "Thanks!"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 4)
+        self.assertEqual(params["messages"][0]["role"], "user")
+        self.assertEqual(params["messages"][1]["role"], "assistant")
+        self.assertEqual(params["messages"][2]["role"], "tool")
+        self.assertEqual(params["messages"][2]["content"], "Sunny, 72F")
+        self.assertEqual(params["messages"][3]["role"], "user")
+
+    def test_empty_messages(self):
+        """Test that empty messages list returns empty."""
+        context = LLMContext(messages=[])
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(params["messages"], [])
+
+
 if __name__ == "__main__":
     unittest.main()