Fix: lock use_responses_api to match discovered api_flavor

LangChain's ChatOpenAI auto-infers use_responses_api from parameters
like reasoning={}, model names (gpt-5-pro), context_management, etc.
This can silently switch a chat-completions-only model to the Responses
API, sending an incompatible payload format.

When api_flavor is locked:
- chat-completions → use_responses_api = False (prevents auto-switch)
- responses → use_responses_api = True (ensures correct format)
- None → use_responses_api stays None (dynamic auto-detect)

Also removes redundant use_responses_api logic from the factory since
the model class now handles it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cosminacho

packages/uipath_langchain_client/src/uipath_langchain_client/clients/openai/chat_models.py

@@ -43,6 +43,14 @@ class UiPathChatOpenAI(UiPathBaseChatModel, ChatOpenAI):  # type: ignore[overrid
     def setup_uipath_client(self) -> Self:
         if self.api_flavor is not None:
             self.api_config.api_flavor = self.api_flavor
+            # Lock LangChain's use_responses_api to match the discovered flavor.
+            # Without this, features like reasoning={} or certain model names
+            # silently switch LangChain to the Responses API, which would fail
+            # if the model only supports chat-completions (or vice versa).
+            if self.api_flavor == ApiFlavor.CHAT_COMPLETIONS:
+                self.use_responses_api = False
+            elif self.api_flavor == ApiFlavor.RESPONSES:
+                self.use_responses_api = True
         base_url = str(self.uipath_sync_client.base_url).rstrip("/")
         locked_flavor = str(self.api_config.api_flavor) if self.api_config.api_flavor else None
 
@@ -91,6 +99,10 @@ class UiPathAzureChatOpenAI(UiPathBaseChatModel, AzureChatOpenAI):  # type: igno
     def setup_uipath_client(self) -> Self:
         if self.api_flavor is not None:
             self.api_config.api_flavor = self.api_flavor
+            if self.api_flavor == ApiFlavor.CHAT_COMPLETIONS:
+                self.use_responses_api = False
+            elif self.api_flavor == ApiFlavor.RESPONSES:
+                self.use_responses_api = True
         base_url = str(self.uipath_sync_client.base_url).rstrip("/")
         locked_flavor = str(self.api_config.api_flavor) if self.api_config.api_flavor else None
 

packages/uipath_langchain_client/src/uipath_langchain_client/factory.py

@@ -171,9 +171,6 @@ def get_chat_model(
 
     match discovered_vendor_type:
         case VendorType.OPENAI:
-            if api_flavor == ApiFlavor.RESPONSES:
-                model_kwargs["use_responses_api"] = True
-
             if is_uipath_owned:
                 from uipath_langchain_client.clients.openai.chat_models import (
                     UiPathAzureChatOpenAI,