settle child agents before closing sessions at wind-down

Every error path cancels the agent subtree before returning, so children unwind
while their sessions are still open. The success path did not: when root called
finish_scan with children still mid-turn, the finally closed their sessions
underneath them, the in-flight save raised "SQLiteSession is closed", and those
children were marked crashed - failing a scan whose report was already written.

Cancelling the descendants moves into the finally, ahead of the session close,
so it runs on every path - clean finish included. The per-branch cancels are
dropped since the finally now covers them; each branch keeps only its own root
status. cancel_descendants targets child tasks (root runs inline with no task),
and the finally already awaits snapshot and cleanup under cancellation, so the
added await is safe there.

Fixes #1012.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

strix/core/runner.py

@@ -430,7 +430,6 @@ async def run_strix_scan(
     except BudgetExceededError as exc:
         logger.info("Scan %s stopped: %s", scan_id, exc)
         if root_id is not None:
-            await coordinator.cancel_descendants(root_id)
             with contextlib.suppress(Exception):
                 await coordinator.set_status(root_id, "stopped")
         return None
@@ -443,26 +442,28 @@ async def run_strix_scan(
             scan_id,
         )
         if root_id is not None:
-            await coordinator.cancel_descendants(root_id)
             with contextlib.suppress(Exception):
                 await coordinator.set_status(root_id, "stopped")
         return None
     except (asyncio.CancelledError, KeyboardInterrupt):
         logger.info("Scan %s interrupted by the user", scan_id)
         if root_id is not None:
-            await coordinator.cancel_descendants(root_id)
             with contextlib.suppress(Exception):
                 await coordinator.set_status(root_id, "running")
         raise
     except BaseException:
         logger.exception("Strix scan %s failed", scan_id)
         if root_id is not None:
-            await coordinator.cancel_descendants(root_id)
             with contextlib.suppress(Exception):
                 await coordinator.set_status(root_id, "failed")
         raise
     finally:
         configure_spill_writer(None)
+        # Settle descendants before closing sessions: on a clean finish a child
+        # can still be mid-turn, and closing its session underneath it crashes it.
+        if root_id is not None:
+            with contextlib.suppress(Exception):
+                await coordinator.cancel_descendants(root_id)
         for s in sessions_to_close:
             with contextlib.suppress(Exception):
                 s.close()

tests/test_runner_teardown.py

@@ -0,0 +1,93 @@
+from __future__ import annotations
+
+import asyncio
+import types
+from typing import Any
+
+import pytest
+from agents import ModelSettings
+
+import strix.tools.notes.tools as notes_tools
+import strix.tools.todo.tools as todo_tools
+from strix.core import runner
+from strix.core.agents import AgentCoordinator
+from strix.runtime import session_manager
+
+
+def _wire_runner(monkeypatch: pytest.MonkeyPatch, tmp_path: Any) -> None:
+    monkeypatch.setattr(runner, "run_dir_for", lambda _scan_id: tmp_path)
+    monkeypatch.setattr(runner, "runtime_state_dir", lambda _run_dir: tmp_path)
+    monkeypatch.setattr(runner, "setup_scan_logging", lambda _run_dir: lambda: None)
+    monkeypatch.setattr(runner, "set_scan_id", lambda _scan_id: None)
+
+    settings = _settings()
+    monkeypatch.setattr(runner, "load_settings", lambda: settings)
+    monkeypatch.setattr(runner, "configure_sdk_model_defaults", lambda _s: None)
+    monkeypatch.setattr(runner, "uses_chat_completions_tool_schema", lambda _m, _s: False)
+    monkeypatch.setattr(todo_tools, "hydrate_todos_from_disk", lambda _d: None)
+    monkeypatch.setattr(notes_tools, "hydrate_notes_from_disk", lambda _d: None)
+
+    async def _create_or_reuse(*_a: Any, **_k: Any) -> dict[str, Any]:
+        return {"client": object(), "session": object(), "caido_client": None}
+
+    async def _cleanup(*_a: Any, **_k: Any) -> None:
+        return None
+
+    monkeypatch.setattr(session_manager, "create_or_reuse", _create_or_reuse)
+    monkeypatch.setattr(session_manager, "cleanup", _cleanup)
+    monkeypatch.setattr(runner, "build_root_task", lambda _c: "task")
+    monkeypatch.setattr(runner, "build_scope_context", lambda _c: "")
+    monkeypatch.setattr(runner, "make_model_settings", lambda *_a, **_k: ModelSettings())
+    monkeypatch.setattr(runner, "build_strix_agent", lambda **_k: object())
+    monkeypatch.setattr(runner, "make_child_factory", lambda **_k: lambda **_kk: object())
+    monkeypatch.setattr(runner, "open_agent_session", lambda _root_id, _db: object())
+
+
+def _settings() -> Any:
+    return types.SimpleNamespace(
+        llm=types.SimpleNamespace(
+            model="openai/gpt-4o",
+            reasoning_effort="high",
+            force_required_tool_choice=False,
+            timeout=300,
+            prompt_cache=True,
+            extra_headers=None,
+        ),
+        runtime=types.SimpleNamespace(max_context_images=3),
+    )
+
+
+@pytest.mark.asyncio
+async def test_a_live_child_is_settled_before_sessions_close(
+    monkeypatch: pytest.MonkeyPatch, tmp_path: Any
+) -> None:
+    _wire_runner(monkeypatch, tmp_path)
+    coordinator = AgentCoordinator()
+    child_started = asyncio.Event()
+    child_task: dict[str, asyncio.Task[None]] = {}
+
+    async def _root_finishes(**kwargs: Any) -> None:
+        root_id = kwargs["agent_id"]
+
+        async def _child_mid_turn() -> None:
+            child_started.set()
+            await asyncio.sleep(3600)
+
+        await coordinator.register("child", "Child", parent_id=root_id)
+        task = asyncio.create_task(_child_mid_turn())
+        child_task["t"] = task
+        await coordinator.attach_runtime("child", task=task)
+        await child_started.wait()
+
+    monkeypatch.setattr(runner, "run_agent_loop", _root_finishes)
+
+    await runner.run_strix_scan(
+        scan_config={"targets": [], "scan_mode": "deep"},
+        scan_id="scan-test",
+        image="img",
+        coordinator=coordinator,
+    )
+
+    task = child_task["t"]
+    assert task.done(), "the child task was left running past scan teardown"
+    assert task.cancelled(), "the child was not cancelled cleanly on a finish"