Fix cancel-scope error: run the async agent on a dedicated worker thread

app.py

@@ -14,7 +14,9 @@ secrets. Run locally with:  python app.py
 import asyncio
 import json
 import os
+import queue
 import sys
+import threading
 import urllib.error
 import urllib.request
 from datetime import timedelta
@@ -510,40 +512,59 @@ async def _agentic_answer(question: str):
 ANSWER_PLACEHOLDER = "*Your answer will appear here.*"
 
 
+_SENTINEL = object()
+
+
 def answer(question: str):
-    """Generator wrapping the async agent for Gradio's progressive UI."""
+    """Generator wrapping the async agent for Gradio's progressive UI.
+
+    The async work runs on a dedicated worker thread with its own event loop
+    and stays inside a single asyncio task for the whole question. Items are
+    handed back to this sync generator through a thread-safe queue. The
+    previous loop.run_until_complete-per-anext pattern created a fresh task
+    on every yield, which tripped anyio's cancel-scope check inside the MCP
+    streamable-HTTP client ('Attempted to exit cancel scope in a different
+    task than it was entered in')."""
     question = (question or "").strip()
     if not question:
         yield "Please enter a legal question above.", ANSWER_PLACEHOLDER, ""
         return
 
-    async def _drive():
-        async for tup in _agentic_answer(question):
-            yield tup
+    events: queue.Queue = queue.Queue()
 
-    # Bridge the async generator to a sync generator. We run a single asyncio
-    # task per submission; Gradio's queue serializes them per user session.
-    loop = asyncio.new_event_loop()
-    asyncio.set_event_loop(loop)
-    try:
-        gen = _drive().__aiter__()
-        while True:
+    def worker():
+        async def run():
             try:
-                status, ans, sources = loop.run_until_complete(gen.__anext__())
-            except StopAsyncIteration:
-                return
+                async for tup in _agentic_answer(question):
+                    events.put(("yield", tup))
             except _AgentError as exc:
-                yield (f"**{exc}**", ANSWER_PLACEHOLDER, "")
-                return
+                events.put(("agent_error", exc))
             except Exception as exc:                # network blip, MCP transport
-                yield (f"**Could not complete the request.**\n\n"
-                       f"`{type(exc).__name__}: {exc}`\n\n"
-                       "The MCP service may be waking from sleep -- try again "
-                       "in a moment.", ANSWER_PLACEHOLDER, "")
-                return
-            yield status, ans, sources
-    finally:
-        loop.close()
+                events.put(("error", exc))
+            finally:
+                events.put((_SENTINEL,))
+        try:
+            asyncio.run(run())
+        except Exception as exc:                    # loop setup failures
+            events.put(("error", exc))
+            events.put((_SENTINEL,))
+
+    threading.Thread(target=worker, daemon=True).start()
+
+    while True:
+        kind, *payload = events.get()
+        if kind is _SENTINEL:
+            return
+        if kind == "yield":
+            yield payload[0]
+        elif kind == "agent_error":
+            yield (f"**{payload[0]}**", ANSWER_PLACEHOLDER, "")
+        elif kind == "error":
+            exc = payload[0]
+            yield (f"**Could not complete the request.**\n\n"
+                   f"`{type(exc).__name__}: {exc}`\n\n"
+                   "The MCP service may be waking from sleep -- try again "
+                   "in a moment.", ANSWER_PLACEHOLDER, "")
 
 
 # --- UI -----------------------------------------------------------------------