Apply additional fixups to context messages to meet Mistral-specific requirements

CHANGELOG.md

@@ -12,6 +12,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added `OpenAIRealtimeLLMService` and `AzureRealtimeLLMService` which provide
   access to OpenAI Realtime.
 
+### Fixed
+
+- Add additional fixups to Mistral context messages to ensure they meet
+  Mistral-specific requirements, avoiding Mistral "invalid request" errors.
+
 ### Deprecated
 
 - `NoisereduceFilter` is now deprecated and will be removed in a future

src/pipecat/services/mistral/llm.py

@@ -57,16 +57,18 @@ class MistralLLMService(OpenAILLMService):
         logger.debug(f"Creating Mistral client with api {base_url}")
         return super().create_client(api_key, base_url, **kwargs)
 
-    def _apply_mistral_assistant_prefix(
+    def _apply_mistral_fixups(
         self, messages: List[ChatCompletionMessageParam]
     ) -> List[ChatCompletionMessageParam]:
-        """Apply Mistral's assistant message prefix requirement.
+        """Apply fixups to messages to meet Mistral-specific requirements.
 
-        Mistral requires assistant messages to have prefix=True when they
-        are the final message in a conversation. According to Mistral's API:
-        - Assistant messages with prefix=True MUST be the last message
-        - Only add prefix=True to the final assistant message when needed
-        - This allows assistant messages to be accepted as the last message
+        1. A "tool"-role message must be followed by an assistant message.
+
+        2. "system"-role messages must only appear at the start of a
+           conversation.
+
+        3. Assistant messages must have prefix=True when they are the final
+           message in a conversation (but at no other point).
 
         Args:
             messages: The original list of messages.
@@ -80,6 +82,23 @@ class MistralLLMService(OpenAILLMService):
         # Create a copy to avoid modifying the original
         fixed_messages = [dict(msg) for msg in messages]
 
+        # Ensure all tool responses are followed by an assistant message
+        assistant_insert_indices = []
+        for i, msg in enumerate(fixed_messages[:-1]):
+            if msg.get("role") == "tool" and not fixed_messages[i + 1].get("role") == "assistant":
+                assistant_insert_indices.append(i + 1)
+        for idx in reversed(assistant_insert_indices):
+            fixed_messages.insert(idx, {"role": "assistant", "content": " "})
+
+        # Convert any "system" messages that aren't at the start (i.e., after the initial contiguous block) to "user"
+        first_non_system_idx = next(
+            (i for i, msg in enumerate(fixed_messages) if msg.get("role") != "system"),
+            len(fixed_messages),
+        )
+        for i, msg in enumerate(fixed_messages):
+            if msg.get("role") == "system" and i >= first_non_system_idx:
+                msg["role"] = "user"
+
         # Get the last message
         last_message = fixed_messages[-1]
 
@@ -88,6 +107,8 @@ class MistralLLMService(OpenAILLMService):
         if last_message.get("role") == "assistant" and "prefix" not in last_message:
             last_message["prefix"] = True
 
+        print(f"Fixed messages for Mistral: {fixed_messages}")
+
         return fixed_messages
 
     async def run_function_calls(self, function_calls: Sequence[FunctionCallFromLLM]):
@@ -158,7 +179,7 @@ class MistralLLMService(OpenAILLMService):
         - Core completion settings
         """
         # Apply Mistral's assistant prefix requirement for API compatibility
-        fixed_messages = self._apply_mistral_assistant_prefix(params_from_context["messages"])
+        fixed_messages = self._apply_mistral_fixups(params_from_context["messages"])
 
         params = {
             "model": self.model_name,