[eric] agents: a SIGKILLed CLI (-9) is an external kill too, and the shutdown line is stamped before the shutdown stops the turn

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012G8kyALnPjsA7aJFmMBq3R

backend/apps/agents/agents.py

@@ -34,6 +34,8 @@ async def agents_lifespan():
     pool_sweeper = start_pool_sweeper(agent_manager.client_pool)
     yield
     logger.info("Agents sub-app shutting down")
+    # Stamp before stopping: once stop_agent has run, a live chat is indistinguishable from one the user stopped.
+    agent_manager.note_shutdown_stops()
     for session_id in list(agent_manager.tasks.keys()):
         await agent_manager.stop_agent(session_id)
     await agent_manager.persist_all_sessions()

backend/apps/agents/core/error_classify.py

@@ -464,7 +464,8 @@ def is_stale_tool_schema_error(exc: BaseException, extra_text: str = "") -> bool
 # narrow. 401 stays out (a rotating token really does heal, which is why the reset-hint rule exists),
 # and so do 408/429. Matched only in status POSITION, so a "400" in a line number or a byte count
 # cannot promote itself into a verdict (ENG-365 learned that the hard way with "line 401,").
-P_KILLED_EXIT = re.compile(r"Command failed with exit code (143|137)\b")
+# 143/137 when the CLI re-raises the signal it caught; -15/-9 when it could not (SIGKILL is uncatchable, so a SIGKILL always arrives as -9).
+P_KILLED_EXIT = re.compile(r"Command failed with exit code (143|137|-15|-9)\b")
 
 
 @typechecked

backend/apps/agents/manager/session/SessionPersistence.py

@@ -134,16 +134,29 @@ class SessionPersistence(AgentManagerProtocol):
                 logger.warning(f"crash-resume: session {sid} failed to auto-resume; amber chip remains", exc_info=True)
         self.crash_resume_queue = []
 
+    @typechecked
+    def note_shutdown_stops(self) -> int:
+        """Stamp every chat with a live turn BEFORE the shutdown stops it. The lifespan stops the tasks
+        first and flushes second, so by flush time a running chat already reads "stopped" and the
+        note below never fired (dev kill matrix A9a, 2026-09-01). Returns how many were stamped."""
+        stamped = 0
+        for session_id in list(self.tasks.keys()):
+            session = self.sessions.get(session_id)
+            if session is None or session.status not in ("running", "waiting_approval"):
+                continue
+            session.messages.append(Message(role="system", content=SHUTDOWN_STOP_NOTE, branch_id=session.active_branch_id))
+            stamped += 1
+        return stamped
+
     @typechecked
     async def persist_all_sessions(self) -> None:
         """Flush every in-memory session to JSON files (for graceful shutdown)."""
         for session_id, session in list(self.sessions.items()):
             if session.status in ("running", "waiting_approval"):
                 session.status = "stopped"
-                # Say who stopped it. A chat flushed as plain "stopped" reads exactly like the user's own
-                # Stop, and when something else killed the backend (an agent's pkill, 2026-09-01) the
-                # user's running work vanished with nothing saying why: silent loss, row 1.
-                session.messages.append(Message(role="system", content=SHUTDOWN_STOP_NOTE, branch_id=session.active_branch_id))
+                # A chat that was never a task (restored mid-turn, never resumed) still gets the note here.
+                if not session.messages or str(session.messages[-1].content) != SHUTDOWN_STOP_NOTE:
+                    session.messages.append(Message(role="system", content=SHUTDOWN_STOP_NOTE, branch_id=session.active_branch_id))
             session.closed_at = None
             for req in list(session.pending_approvals):
                 ws_manager.resolve_approval(req.id, {"behavior": "deny", "message": "Server shutting down"})

backend/tests/test_external_kill_respawn.py

@@ -16,9 +16,12 @@ def p_session() -> AgentSession:
     return AgentSession(name="t", model="sonnet")
 
 
-def test_the_real_143_and_a_sigkill_137_are_recognised():
+def test_the_real_143_and_every_signal_spelling_are_recognised():
     assert is_external_kill_error(RuntimeError(REAL))
     assert is_external_kill_error(RuntimeError("Command failed with exit code 137 (exit code: 137)"))
+    # A SIGKILL cannot be caught, so the SDK reports it as a negative signal number, never 137 (live, dev kill matrix A3).
+    assert is_external_kill_error(RuntimeError("Command failed with exit code -9 (exit code: -9)\nError output: Check stderr output for details"))
+    assert is_external_kill_error(RuntimeError("Command failed with exit code -15 (exit code: -15)"))
 
 
 @pytest.mark.parametrize("innocent", [
@@ -26,6 +29,8 @@ def test_the_real_143_and_a_sigkill_137_are_recognised():
     ("Command failed with exit code 143 (exit code: 143)", "API Error: 401 authentication_error"),
     ("Command failed with exit code 143 (exit code: 143)", "Error: request blocked by Usage Policy"),
     ("Command failed with exit code 1430", ""),
+    ("Command failed with exit code -1 (exit code: -1)", ""),
+    ("Command failed with exit code -90", ""),
     ("Error code: 429 - rate limit", ""),
 ])
 def test_other_failures_never_claim_it(innocent):

backend/tests/test_shutdown_stop_note.py

@@ -36,3 +36,21 @@ def test_a_settled_chat_is_flushed_untouched(monkeypatch) -> None:
     doc = saved[s.id]
     assert doc["status"] == "completed"
     assert doc["messages"][-1]["role"] == "user", "no note on a chat that was not running"
+
+
+def test_the_lifespan_stamps_live_turns_before_it_stops_them(monkeypatch) -> None:
+    """Placement, not just behaviour: stop_agent flips running -> stopped, so a note keyed on
+    "running" at flush time never fires for a chat that was a task (dev kill matrix A9a)."""
+    import inspect
+    from backend.apps.agents import agents as agents_mod
+    src = inspect.getsource(agents_mod.agents_lifespan)
+    assert src.index("note_shutdown_stops()") < src.index("stop_agent(session_id)")
+    s = p_session("running")
+    agent_manager.sessions.clear(); agent_manager.sessions[s.id] = s
+    monkeypatch.setitem(agent_manager.tasks, s.id, object())
+    try:
+        assert agent_manager.note_shutdown_stops() == 1
+    finally:
+        agent_manager.tasks.pop(s.id, None)
+    assert s.messages[-1].role == "system" and "not your Stop" in str(s.messages[-1].content)
+    assert agent_manager.note_shutdown_stops() == 0, "a settled or already-stamped chat is not stamped twice"