fix(gateway): keep empty gateway responses silent

gateway/run.py

@@ -1079,6 +1079,7 @@ def _normalize_empty_agent_response(
     response: str,
     *,
     history_len: int = 0,
+    allow_user_visible_diagnostics: bool = True,
 ) -> str:
     """Normalize empty/None agent responses into user-facing messages.
 
@@ -1089,6 +1090,9 @@ def _normalize_empty_agent_response(
     if response:
         return response
 
+    if not allow_user_visible_diagnostics:
+        return response
+
     if agent_result.get("failed"):
         error_detail = agent_result.get("error", "unknown error")
         error_str = str(error_detail).lower()
@@ -7552,17 +7556,11 @@ async def _handle_message_with_agent(self, event, source, _quick_key: str, run_g
 
             response = agent_result.get("final_response") or ""
 
-            # Convert the agent's internal "(empty)" sentinel into a
-            # user-friendly message.  "(empty)" means the model failed to
-            # produce visible content after exhausting all retries (nudge,
-            # prefill, empty-retry, fallback).  Sending the raw sentinel
-            # looks like a bug; a short explanation is more helpful.
+            # Chat gateways treat silent outcomes as a valid no-op. Keep the
+            # agent's internal empty sentinel out of user-visible channels and
+            # let the adapter no-op on the empty final response.
             if response == "(empty)":
-                response = (
-                    "⚠️ The model returned no response after processing tool "
-                    "results. This can happen with some models — try again or "
-                    "rephrase your question."
-                )
+                response = ""
             agent_messages = agent_result.get("messages", [])
             _response_time = time.time() - _msg_start_time
             _api_calls = agent_result.get("api_calls", 0)
@@ -7594,7 +7592,10 @@ async def _handle_message_with_agent(self, event, source, _quick_key: str, run_g
             # Normalize empty responses: surface errors, partial failures, and
             # the case where agent did work but returned no text. Fix for #18765.
             response = _normalize_empty_agent_response(
-                agent_result, response, history_len=len(history),
+                agent_result,
+                response,
+                history_len=len(history),
+                allow_user_visible_diagnostics=False,
             )
 
             # If the agent's session_id changed during compression, update

tests/test_lazy_session_regressions.py

@@ -405,6 +405,49 @@ def test_failed_generic_error(self):
         assert "500 Internal Server Error" in response
         assert "/reset" in response
 
+    def test_gateway_mode_keeps_failed_empty_response_silent(self):
+        """Gateway turns should not leak backend failures into chat channels."""
+        from gateway.run import _normalize_empty_agent_response
+
+        agent_result = {
+            "final_response": None,
+            "api_calls": 0,
+            "failed": True,
+            "error": "500 Internal Server Error",
+        }
+
+        response = agent_result.get("final_response") or ""
+        response = _normalize_empty_agent_response(
+            agent_result,
+            response,
+            history_len=5,
+            allow_user_visible_diagnostics=False,
+        )
+
+        assert response == ""
+
+    def test_gateway_mode_keeps_partial_empty_response_silent(self):
+        """Gateway turns should stay silent for empty partial/tool failures."""
+        from gateway.run import _normalize_empty_agent_response
+
+        agent_result = {
+            "final_response": None,
+            "api_calls": 5,
+            "partial": True,
+            "interrupted": False,
+            "error": "Model generated invalid tool call: nonexistent_tool",
+        }
+
+        response = agent_result.get("final_response") or ""
+        response = _normalize_empty_agent_response(
+            agent_result,
+            response,
+            history_len=10,
+            allow_user_visible_diagnostics=False,
+        )
+
+        assert response == ""
+
     def test_nonempty_response_passes_through(self):
         """Non-empty response is returned unchanged."""
         from gateway.run import _normalize_empty_agent_response