disable tool server initialization if no tool in request

Summary:
First part of fixing #25697

we can first fix this error
```
RuntimeError: Attempted to exit cancel scope in a different task than it was entered in
```
by not initializing the tool servers if the request doesn't need tools.

Test Plan:
- non stream request with tools still gets tools
- stream without tools no longer outputs the error
- stream with tools still works (but it still outputs the error)

Differential Revision: D83387320

Signed-off-by: Andrew Xia <[email protected]>

Author: qandrew

vllm/entrypoints/openai/serving_responses.py

@@ -442,6 +442,19 @@ def _make_request_with_harmony(
 
         return messages, [prompt_token_ids], [engine_prompt]
 
+    async def _initialize_tool_sessions(self, request: ResponsesRequest,
+                                        context: ConversationContext,
+                                        exit_stack: AsyncExitStack):
+        # we should only initialize the tool session if the request needs tools
+        if len(request.tools) == 0:
+            return
+        mcp_tools = {
+            tool.server_label: tool
+            for tool in request.tools if tool.type == "mcp"
+        }
+        await context.init_tool_sessions(self.tool_server, exit_stack,
+                                         request.request_id, mcp_tools)
+
     async def responses_full_generator(
         self,
         request: ResponsesRequest,
@@ -458,12 +471,8 @@ async def responses_full_generator(
 
         async with AsyncExitStack() as exit_stack:
             try:
-                mcp_tools = {
-                    tool.server_label: tool
-                    for tool in request.tools if tool.type == "mcp"
-                }
-                await context.init_tool_sessions(self.tool_server, exit_stack,
-                                                 request.request_id, mcp_tools)
+                await self._initialize_tool_sessions(request, context,
+                                                     exit_stack)
                 async for _ in result_generator:
                     pass
             except asyncio.CancelledError:
@@ -1647,12 +1656,10 @@ def _increment_sequence_number_and_return(
         async with AsyncExitStack() as exit_stack:
             processer = None
             if self.use_harmony:
-                mcp_tools = {
-                    tool.server_label: tool
-                    for tool in request.tools if tool.type == "mcp"
-                }
-                await context.init_tool_sessions(self.tool_server, exit_stack,
-                                                 request.request_id, mcp_tools)
+                # TODO: in streaming, we noticed this bug:
+                # https://github.com/vllm-project/vllm/issues/25697
+                await self._initialize_tool_sessions(request, context,
+                                                     exit_stack)
                 processer = self._process_harmony_streaming_events
             else:
                 processer = self._process_simple_streaming_events