diff --git a/nanobot/agent/loop.py b/nanobot/agent/loop.py
index 7c4c7b205..61de18fb2 100644
--- a/nanobot/agent/loop.py
+++ b/nanobot/agent/loop.py
@@ -760,7 +760,7 @@ class AgentLoop:
 
         Returns the total number of cancelled tasks + subagents.
         """
-        tasks = self._active_tasks.pop(key, set())
+        tasks = tuple(self._active_tasks.pop(key, set()))
         cancelled = sum(1 for t in tasks if not t.done() and t.cancel())
         for t in tasks:
             with suppress(asyncio.CancelledError, Exception):
diff --git a/tests/agent/test_task_cancel.py b/tests/agent/test_task_cancel.py
index 8e51091b7..6acb41ff7 100644
--- a/tests/agent/test_task_cancel.py
+++ b/tests/agent/test_task_cancel.py
@@ -73,7 +73,9 @@ class TestHandleStop:
 
         task = asyncio.create_task(slow_task())
         await asyncio.sleep(0)
-        loop._active_tasks["test:c1"] = {task}
+        active_tasks = {task}
+        loop._active_tasks["test:c1"] = active_tasks
+        task.add_done_callback(active_tasks.discard)
 
         msg = InboundMessage(channel="test", sender_id="u1", chat_id="c1", content="/stop")
         ctx = CommandContext(msg=msg, session=None, key=msg.session_key, raw="/stop", loop=loop)