diff --git a/backend/app/core/config.py b/backend/app/core/config.py
index 458cc14..a4e7019 100644
--- a/backend/app/core/config.py
+++ b/backend/app/core/config.py
@@ -9,6 +9,7 @@ class Settings(BaseSettings):
     llm_base_url: str = "https://openrouter.ai/api/v1"
     llm_api_key: str = ""
     llm_model_name: str = "qwen/qwen3.5-35b-a3b"
+    llm_enable_thinking: bool = False
 
     # Embeddings
     embedding_model: str = "qwen/qwen3-embedding-4b"
diff --git a/backend/app/services/llm_client.py b/backend/app/services/llm_client.py
index f91c6d7..ef8a30b 100644
--- a/backend/app/services/llm_client.py
+++ b/backend/app/services/llm_client.py
@@ -1,8 +1,8 @@
-import asyncio
 import logging
 from typing import Optional
 
 import httpx
+from openai import AsyncOpenAI, APIError, APITimeoutError
 
 from app.core.config import Settings
 
@@ -12,38 +12,59 @@ class LLMClientError(Exception):
 
 
 class LLMClient:
-    """Asynchronous LLM HTTP client with connection pooling."""
+    """Asynchronous LLM client using OpenAI SDK with provider-agnostic config."""
 
     def __init__(self, settings: Settings):
         self.settings = settings
-        self.base_url = settings.llm_base_url.rstrip("/")
-        self.api_key = settings.llm_api_key
         self.model = settings.llm_model_name
+        self.enable_thinking = settings.llm_enable_thinking
         self.logger = logging.getLogger(__name__)
-        # Use a single shared AsyncClient for all requests
-        self._client = httpx.AsyncClient(
-            base_url=self.base_url,
+        self._client = AsyncOpenAI(
+            base_url=settings.llm_base_url.rstrip("/"),
+            api_key=settings.llm_api_key,
             timeout=settings.llm_timeout,
-            headers={"Authorization": f"Bearer {self.api_key}", "Content-Type": "application/json"},
+            http_client=httpx.AsyncClient(
+                headers={"Content-Type": "application/json"},
+            ),
         )
 
     async def complete(self, prompt: str, temperature: float = 0.7) -> str:
+        """Send a chat completion request with optional thinking control."""
+        messages = [{"role": "user", "content": prompt}]
+        extra_body = self._build_extra_body()
+
         try:
-            resp = await self._client.post(
-                "/chat/completions",
-                json={
-                    "model": self.model,
-                    "messages": [{"role": "user", "content": prompt}],
-                    "temperature": temperature,
-                },
+            response = await self._client.chat.completions.create(
+                model=self.model,
+                messages=messages,
+                temperature=temperature,
+                extra_body=extra_body if extra_body else None,
             )
-            resp.raise_for_status()
-            data = resp.json()
-            return data["choices"][0]["message"]["content"]
-        except (httpx.TimeoutException, httpx.HTTPStatusError, httpx.RequestError) as exc:
+            return response.choices[0].message.content or ""
+        except (APITimeoutError, APIError) as exc:
             self.logger.error("LLM API error: %s", exc)
             raise LLMClientError from exc
+        except Exception as exc:
+            self.logger.error("Unexpected LLM error: %s", exc)
+            raise LLMClientError from exc
+
+    def _build_extra_body(self) -> dict:
+        """Build extra_body for provider-specific parameters.
+
+        For Qwen3.5 models, disables thinking content via chat_template_kwargs
+        (vLLM/SGLang convention). When thinking is enabled, no extra params
+        are passed and the model uses its default thinking mode.
+        """
+        if self.enable_thinking:
+            return {}
+
+        # Non-thinking mode for Qwen3.5
+        # Uses chat_template_kwargs for vLLM/SGLang compatibility.
+        # For Alibaba Cloud Model Studio, use top-level enable_thinking instead.
+        return {
+            "chat_template_kwargs": {"enable_thinking": False},
+            "top_k": 20,
+        }
 
     async def close(self):
-        if self._client:
-            await self._client.aclose()
+        await self._client.close()
diff --git a/backend/requirements.txt b/backend/requirements.txt
index c5599fd..7a5e7b5 100644
--- a/backend/requirements.txt
+++ b/backend/requirements.txt
@@ -7,6 +7,7 @@ python-docx==1.1.0
 pypdf==4.0.2
 python-dotenv==1.0.0
 httpx==0.26.0
+openai==1.12.0
 pytest==7.4.4
 pytest-asyncio==0.23.4
 tiktoken==0.5.2