fix(deepseek): Tool Choice to required for Azure Deployment in case specific function dict is given (#34848)

libs/partners/deepseek/langchain_deepseek/chat_models.py

@@ -194,6 +194,11 @@ class ChatDeepSeek(BaseChatOpenAI):
 
     model_config = ConfigDict(populate_by_name=True)
 
+    @property
+    def _is_azure_endpoint(self) -> bool:
+        """Check if the configured endpoint is an Azure deployment."""
+        return "azure.com" in (self.api_base or "").lower()
+
     @property
     def _llm_type(self) -> str:
         """Return type of chat model."""
@@ -276,6 +281,17 @@ class ChatDeepSeek(BaseChatOpenAI):
                     if isinstance(block, dict) and block.get("type") == "text"
                 ]
                 message["content"] = "".join(text_parts) if text_parts else ""
+
+        # Azure-hosted DeepSeek does not support the dict/object form of
+        # tool_choice (e.g. {"type": "function", "function": {"name": "..."}}).
+        # It only accepts string values: "none", "auto", or "required".
+        # Convert the unsupported dict form to "required", which is the closest
+        # string equivalent — it forces the model to call a tool without
+        # constraining which one. In the common with_structured_output() case
+        # only a single tool is bound, so the behavior is effectively identical.
+        if self._is_azure_endpoint and isinstance(payload.get("tool_choice"), dict):
+            payload["tool_choice"] = "required"
+
         return payload
 
     def _create_chat_result(