Preserve trace arrivals in Frontier grid

runs/frontier-multicase-sufficiency-v0/best_effort/frontier_prefill_grid.py

@@ -319,6 +319,10 @@ def build_command(
         "frontier.main",
         "--simulation_mode",
         "offline",
+        # Frontier's offline default rewrites every request arrival to time 0.
+        # Preserve the trace arrival process so the simulator and serving run
+        # exercise the same queueing workload.
+        "--offline_use_generated_request_arrivals",
         "--sys_arch",
         "co-location",
         "--cluster_config_num_replicas",

runs/frontier-multicase-sufficiency-v0/best_effort/test_frontier_prefill_grid.py

@@ -64,3 +64,26 @@ def test_score_uses_trace_slo_and_request_id_mapping(tmp_path: Path) -> None:
     assert score["passed_request_count"] == 1
     assert score["slo_pass_rate"] == 0.5
     assert score["feasible"] is False
+
+
+def test_build_command_preserves_trace_arrivals(tmp_path: Path) -> None:
+    config = MODULE.GRID[0]
+    command = MODULE.build_command(
+        python=tmp_path / "python",
+        frontier_source=tmp_path / "frontier",
+        profile_root=tmp_path / "profiles",
+        profile_paths={
+            "linear": tmp_path / "linear.csv",
+            "attention": tmp_path / "attention.csv",
+            "moe": tmp_path / "moe.csv",
+            "all_reduce": tmp_path / "all_reduce.csv",
+        },
+        trace=tmp_path / "trace.csv",
+        config=config,
+        probe_dir=tmp_path / "probe",
+        run_id="arrival-contract",
+        cache_root=tmp_path / "cache",
+    )
+
+    assert command[command.index("--simulation_mode") + 1] == "offline"
+    assert "--offline_use_generated_request_arrivals" in command