diff --git a/sanity_check_tool/tests/test_core.py b/sanity_check_tool/tests/test_core.py
new file mode 100644
index 0000000..d59da17
--- /dev/null
+++ b/sanity_check_tool/tests/test_core.py
@@ -0,0 +1,66 @@
+import pytest
+from sanity_check_tool import core
+
+
+@pytest.fixture
+def valid_run_data():
+    return {
+        "runs": [
+            {"id": 1, "corr_id": "abc", "write_pre": True, "write_post": True, "clock_event": "stable", "core_field": 42},
+            {"id": 2, "corr_id": "bcd", "write_pre": True, "write_post": True, "clock_event": "stable", "core_field": 24}
+        ],
+        "sanity_checks": ["missing_pairs", "broken_ids", "empty_fields", "clock_switch_count"]
+    }
+
+
+@pytest.fixture
+def invalid_run_data():
+    return {
+        "runs": [
+            {"id": 1, "corr_id": "abc", "write_pre": True, "write_post": False, "clock_event": "switch", "core_field": None},
+            {"id": 2, "corr_id": None, "write_pre": True, "write_post": False, "clock_event": "switch", "core_field": ""}
+        ],
+        "sanity_checks": ["missing_pairs", "broken_ids", "empty_fields", "clock_switch_count"]
+    }
+
+
+def test_perform_sanity_checks_valid(valid_run_data):
+    result = core.perform_sanity_checks(valid_run_data)
+    assert isinstance(result, dict)
+    assert set(result.keys()) == {"missing_pairs", "broken_ids", "empty_fields", "clock_switch_count"}
+    assert all(isinstance(v, int) for v in result.values())
+    assert result["missing_pairs"] == 0
+    assert result["broken_ids"] == 0
+    assert result["empty_fields"] == 0
+    assert result["clock_switch_count"] == 0
+
+
+def test_perform_sanity_checks_invalid(invalid_run_data):
+    result = core.perform_sanity_checks(invalid_run_data)
+    assert isinstance(result, dict)
+    assert result["missing_pairs"] > 0
+    assert result["broken_ids"] >= 0
+    assert result["empty_fields"] > 0
+    assert result["clock_switch_count"] >= 0
+
+
+def test_perform_sanity_checks_input_type():
+    with pytest.raises((TypeError, ValueError)):
+        core.perform_sanity_checks(None)
+    with pytest.raises((TypeError, ValueError)):
+        core.perform_sanity_checks([])
+
+
+def test_perform_sanity_checks_edge_case_empty():
+    run_data = {"runs": [], "sanity_checks": []}
+    result = core.perform_sanity_checks(run_data)
+    assert isinstance(result, dict)
+    for k, v in result.items():
+        assert v == 0
+
+
+def test_perform_sanity_checks_missing_keys():
+    bad_data = {"runs": [{"foo": "bar"}], "sanity_checks": ["missing_pairs"]}
+    result = core.perform_sanity_checks(bad_data)
+    assert "missing_pairs" in result
+    assert isinstance(result["missing_pairs"], int)
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