diff --git a/services/agent/e2e_u03_workflow.py b/services/agent/e2e_u03_workflow.py
index ee3b123b9..39346935a 100644
--- a/services/agent/e2e_u03_workflow.py
+++ b/services/agent/e2e_u03_workflow.py
@@ -1270,7 +1270,7 @@ async def _cancel_old_run_fence(
         current_code, current = _envelope(current_response)
         if current_response.status_code != 200 or current_code != 0:
             analysis_task.cancel()
-            raise ValueError("cancel_fence_state_unavailable")
+            raise ValueError(f"cancel_fence_state_unavailable:http={current_response.status_code}:code={current_code}")
         if current.get("status") == "analyzing":
             analysis_run_id = current.get("runId")
             if not analysis_run_id:
diff --git a/services/agent/tests/test_delivery_gates.py b/services/agent/tests/test_delivery_gates.py
index b42f00c95..f01955558 100644
--- a/services/agent/tests/test_delivery_gates.py
+++ b/services/agent/tests/test_delivery_gates.py
@@ -17,6 +17,29 @@
 u03 = importlib.util.module_from_spec(_SPEC)
 _SPEC.loader.exec_module(u03)
 
+@pytest.mark.parametrize("status,code", [(502, 50000), (200, 40900)])
+def test_cancel_fence_preserves_safe_failed_read_diagnostics(status, code):
+    import asyncio
+    import httpx
+
+    async def scenario():
+        async def respond(request):
+            if request.method == "POST":
+                data = {"threadId": str(uuid.uuid4()), "draft": {}}
+                return httpx.Response(200, json={"code": 0, "message": "success", "data": data,
+                                              "traceId": "private-trace"})
+            return httpx.Response(status, json={"code": code, "message": "private-message",
+                                               "data": {"reason": "private-reason"},
+                                               "traceId": "private-trace"})
+
+        async with httpx.AsyncClient(transport=httpx.MockTransport(respond),
+                                    base_url="http://localhost") as session:
+            with pytest.raises(ValueError) as failure:
+                await u03._cancel_old_run_fence(session, str(uuid.uuid4()))
+        assert str(failure.value) == f"cancel_fence_state_unavailable:http={status}:code={code}"
+
+    asyncio.run(scenario())
+
 def _prior_five_fixture(tmp_path, monkeypatch, *, unsupported_rejected=True, raw_records=None):
     import agent_service.gate as gate