diff --git a/python/scripts/nvbench_compare.py b/python/scripts/nvbench_compare.py
index 9d54889..c96a4d4 100644
--- a/python/scripts/nvbench_compare.py
+++ b/python/scripts/nvbench_compare.py
@@ -86,6 +86,19 @@ def read_nvbench_json_root(filename: str) -> Mapping[str, Any]:
             f"NVBench JSON file {filename!r} is missing required root key(s): {missing}"
         )
 
+    for key in ("devices", "benchmarks"):
+        value = root[key]
+        if not isinstance(value, list):
+            raise ValueError(
+                f"NVBench JSON file {filename!r} root key {key!r} must be an array"
+            )
+        for index, entry in enumerate(value):
+            if not isinstance(entry, Mapping):
+                raise ValueError(
+                    f"NVBench JSON file {filename!r} root key {key!r} entry "
+                    f"{index} must be an object"
+                )
+
     return root
 
 
@@ -131,7 +144,7 @@ COMPARISON_THRESHOLD_PRESET_VALUES = {
         "same_center_relative": 0.01,
         "same_overlap_fraction": 0.25,
         "same_relative_dispersion_ceiling": 0.05,
-        "bulk_same_sample_coverage": 0.98,
+        "bulk_same_sample_coverage": 0.90,
         "bulk_same_support_coverage": 0.60,
         "bulk_support_rare_sample_fraction": 0.001,
         "bulk_support_max_removed_sample_fraction": 0.02,
diff --git a/python/test/test_nvbench_compare.py b/python/test/test_nvbench_compare.py
index 99a3498..8ef9a05 100644
--- a/python/test/test_nvbench_compare.py
+++ b/python/test/test_nvbench_compare.py
@@ -2234,6 +2234,37 @@ def test_main_reports_missing_required_root_keys(monkeypatch, capsys, nvbench_co
     assert "'benchmarks'" in output
 
 
+def test_main_rejects_non_array_root_keys(monkeypatch, capsys, nvbench_compare):
+    monkeypatch.setattr(
+        nvbench_compare.reader,
+        "read_file",
+        lambda _: {"devices": {}, "benchmarks": []},
+    )
+    monkeypatch.setattr(sys, "argv", ["nvbench_compare", "ref.json", "cmp.json"])
+
+    assert nvbench_compare.main() == 1
+    output = capsys.readouterr().out
+    assert "NVBench JSON file 'ref.json' root key 'devices' must be an array" in output
+
+
+def test_main_rejects_non_object_root_array_entries(
+    monkeypatch, capsys, nvbench_compare
+):
+    monkeypatch.setattr(
+        nvbench_compare.reader,
+        "read_file",
+        lambda _: {"devices": [None], "benchmarks": []},
+    )
+    monkeypatch.setattr(sys, "argv", ["nvbench_compare", "ref.json", "cmp.json"])
+
+    assert nvbench_compare.main() == 1
+    output = capsys.readouterr().out
+    assert (
+        "NVBench JSON file 'ref.json' root key 'devices' entry 0 must be an object"
+        in output
+    )
+
+
 def test_main_prints_bulk_debug_python_to_stdout(monkeypatch, capsys, nvbench_compare):
     devices = [{"id": 0, "name": "Test GPU"}]
     root = {