fix(acp): suppress cancel interrupt sentinel

acp_adapter/server.py

@@ -88,6 +88,20 @@ _executor = ThreadPoolExecutor(max_workers=4, thread_name_prefix="acp-agent")
 # does not expose a client-side limit, so this is a fixed cap that clients
 # paginate against using `cursor` / `next_cursor`.
 _LIST_SESSIONS_PAGE_SIZE = 50
+_INTERRUPT_WAITING_FOR_MODEL_PREFIX = (
+    "Operation interrupted: waiting for model response ("
+)
+_INTERRUPT_WAITING_FOR_MODEL_SUFFIX = " elapsed)."
+
+
+def _is_interrupt_waiting_for_model_response(text: Any) -> bool:
+    """Return True for Hermes' local API-wait interruption status string."""
+    response = str(text or "").strip()
+    return (
+        response.startswith(_INTERRUPT_WAITING_FOR_MODEL_PREFIX)
+        and response.endswith(_INTERRUPT_WAITING_FOR_MODEL_SUFFIX)
+    )
+
 _MAX_ACP_RESOURCE_BYTES = 512 * 1024
 _TEXT_RESOURCE_MIME_PREFIXES = ("text/",)
 _TEXT_RESOURCE_MIME_TYPES = {
@@ -1513,7 +1527,12 @@ class HermesACPAgent(acp.Agent):
             self.session_manager.save_session(session_id)
 
         final_response = result.get("final_response", "")
-        if final_response:
+        cancelled = bool(state.cancel_event and state.cancel_event.is_set())
+        interrupted = bool(result.get("interrupted")) or cancelled
+        suppress_interrupt_response = (
+            interrupted and _is_interrupt_waiting_for_model_response(final_response)
+        )
+        if final_response and not suppress_interrupt_response:
             try:
                 from agent.title_generator import maybe_auto_title
 
@@ -1534,7 +1553,12 @@ class HermesACPAgent(acp.Agent):
                 )
             except Exception:
                 logger.debug("Failed to auto-title ACP session %s", session_id, exc_info=True)
-        if final_response and conn and (not streamed_message or result.get("response_transformed")):
+        if (
+            final_response
+            and conn
+            and not suppress_interrupt_response
+            and (not streamed_message or result.get("response_transformed"))
+        ):
             # Deliver the final response when streaming did not already send it,
             # or when a plugin hook transformed the response after streaming
             # finished (e.g. transform_llm_output) — otherwise the appended /
@@ -1576,7 +1600,7 @@ class HermesACPAgent(acp.Agent):
 
         await self._send_usage_update(state)
 
-        stop_reason = "cancelled" if state.cancel_event and state.cancel_event.is_set() else "end_turn"
+        stop_reason = "cancelled" if cancelled else "end_turn"
         return PromptResponse(stop_reason=stop_reason, usage=usage)
 
     # ---- Slash commands (headless) -------------------------------------------

tests/acp/test_server.py

@@ -1100,6 +1100,82 @@ class TestPrompt:
         ]
         assert any(update.session_update == "agent_message_chunk" for update in updates)
 
+    @pytest.mark.asyncio
+    async def test_prompt_suppresses_cancel_interrupt_sentinel(self, agent):
+        """ACP cancel status text should not be emitted as assistant output."""
+        new_resp = await agent.new_session(cwd=".")
+        state = agent.session_manager.get_session(new_resp.session_id)
+        sentinel = "Operation interrupted: waiting for model response (3.3s elapsed)."
+
+        def mock_run(*args, **kwargs):
+            state.cancel_event.set()
+            return {
+                "final_response": sentinel,
+                "messages": list(state.history),
+                "interrupted": True,
+                "completed": False,
+            }
+
+        state.agent.run_conversation = mock_run
+
+        mock_conn = MagicMock(spec=acp.Client)
+        mock_conn.session_update = AsyncMock()
+        agent._conn = mock_conn
+
+        with patch("agent.title_generator.maybe_auto_title") as mock_title:
+            prompt = [TextContentBlock(type="text", text="please do a long task")]
+            resp = await agent.prompt(prompt=prompt, session_id=new_resp.session_id)
+
+        updates = [
+            call.kwargs.get("update") or call.args[1]
+            for call in mock_conn.session_update.call_args_list
+        ]
+        agent_texts = [
+            update.content.text
+            for update in updates
+            if update.session_update == "agent_message_chunk"
+        ]
+        assert resp.stop_reason == "cancelled"
+        assert sentinel not in agent_texts
+        assert not any(text.startswith("Operation interrupted:") for text in agent_texts)
+        mock_title.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_prompt_keeps_real_final_response_on_cancelled_turn(self, agent):
+        """A cancel flag must not suppress actual assistant/model text."""
+        new_resp = await agent.new_session(cwd=".")
+        state = agent.session_manager.get_session(new_resp.session_id)
+        final_text = "The actual model answer arrived before cancellation settled."
+
+        def mock_run(*args, **kwargs):
+            state.cancel_event.set()
+            return {
+                "final_response": final_text,
+                "messages": [],
+                "interrupted": True,
+            }
+
+        state.agent.run_conversation = mock_run
+
+        mock_conn = MagicMock(spec=acp.Client)
+        mock_conn.session_update = AsyncMock()
+        agent._conn = mock_conn
+
+        prompt = [TextContentBlock(type="text", text="finish if you can")]
+        resp = await agent.prompt(prompt=prompt, session_id=new_resp.session_id)
+
+        updates = [
+            call.kwargs.get("update") or call.args[1]
+            for call in mock_conn.session_update.call_args_list
+        ]
+        agent_texts = [
+            update.content.text
+            for update in updates
+            if update.session_update == "agent_message_chunk"
+        ]
+        assert resp.stop_reason == "cancelled"
+        assert final_text in agent_texts
+
     @pytest.mark.asyncio
     async def test_prompt_propagates_hermes_session_id_env(self, agent, monkeypatch):
         """ACP must propagate the originating session id to the agent loop