fix(mcp): 파이프라인 호출을 전용 단일 워커 스레드에 고정

src/trawl_mcp/server.py

@@ -1,23 +1,34 @@
 """Stdio/HTTP MCP server exposing trawl's fetch_page and profile_page tools.
 
 The pipeline uses sync_playwright internally, which can't run inside an
-asyncio event loop on its own. We run each pipeline invocation in a worker
-thread via `asyncio.to_thread`, keeping the MCP server responsive.
+asyncio event loop on its own. We run every pipeline invocation on a
+single dedicated worker thread so the process-wide sync_playwright
+greenlet dispatcher — which is pinned to the thread that first called
+sync_playwright() — always sees the same thread. Using
+`asyncio.to_thread` (default executor) instead causes intermittent
+"Cannot switch to a different thread" greenlet errors whenever a call
+is dispatched to a different worker thread.
 """
 
 from __future__ import annotations
 
 import asyncio
+import functools
 import json
 import logging
 import os
+from concurrent.futures import ThreadPoolExecutor
 
 from mcp.server import Server
 from mcp.server.stdio import stdio_server
 from mcp.types import TextContent, Tool
 
 from trawl import fetch_relevant, to_dict
 
+_pipeline_executor = ThreadPoolExecutor(
+    max_workers=1, thread_name_prefix="trawl-pipeline"
+)
+
 logger = logging.getLogger("trawl_mcp")
 
 server: Server = Server("trawl")
@@ -166,13 +177,17 @@ async def _call_fetch_page(arguments: dict) -> list[TextContent]:
         use_hyde,
         use_rerank,
     )
-    result = await asyncio.to_thread(
-        fetch_relevant,
-        url,
-        query,
-        k=k,
-        use_hyde=use_hyde,
-        use_rerank=use_rerank,
+    loop = asyncio.get_running_loop()
+    result = await loop.run_in_executor(
+        _pipeline_executor,
+        functools.partial(
+            fetch_relevant,
+            url,
+            query,
+            k=k,
+            use_hyde=use_hyde,
+            use_rerank=use_rerank,
+        ),
     )
     payload = to_dict(result)
     payload["ok"] = not bool(payload.get("error"))
@@ -195,10 +210,14 @@ async def _call_profile_page(arguments: dict) -> list[TextContent]:
     # when the tool is actually called.
     from trawl.profiles import generate_profile
 
-    payload = await asyncio.to_thread(
-        generate_profile,
-        url,
-        force_refresh=force_refresh,
+    loop = asyncio.get_running_loop()
+    payload = await loop.run_in_executor(
+        _pipeline_executor,
+        functools.partial(
+            generate_profile,
+            url,
+            force_refresh=force_refresh,
+        ),
     )
     return [TextContent(type="text", text=json.dumps(payload, ensure_ascii=False))]