diff --git a/artifact.metrics_analysis/tests/test_core.py b/artifact.metrics_analysis/tests/test_core.py
new file mode 100644
index 0000000..2166111
--- /dev/null
+++ b/artifact.metrics_analysis/tests/test_core.py
@@ -0,0 +1,58 @@
+import pytest
+from artifact_metrics_analysis import core
+
+@pytest.fixture
+def card_data_40():
+    return {
+        'retry_tail_p99': 120.5,
+        'band_width': 45.0
+    }
+
+
+@pytest.fixture
+def card_data_42():
+    return {
+        'retry_tail_p99': 130.2,
+        'band_width': 50.5
+    }
+
+
+def test_analyze_metrics_basic(card_data_40, card_data_42):
+    result = core.analyze_metrics(card_data_40, card_data_42)
+    assert isinstance(result, core.MetricResults)
+
+    result_dict = result.to_dict()
+    assert pytest.approx(result_dict['retry_tail_p99'], rel=1e-5) == 130.2
+    assert pytest.approx(result_dict['band_width'], rel=1e-5) == 50.5
+    assert pytest.approx(result_dict['delta_band_width'], rel=1e-5) == 5.5
+
+
+def test_analyze_metrics_delta_negative(card_data_40, card_data_42):
+    data42 = {**card_data_42, 'band_width': 40.0}
+    result = core.analyze_metrics(card_data_40, data42)
+    result_dict = result.to_dict()
+    # delta_band_width = 40.0 - 45.0 = -5.0
+    assert result_dict['delta_band_width'] == pytest.approx(-5.0)
+
+
+def test_metricresults_to_dict_structure():
+    m = core.MetricResults(100.0, 40.0, -2.0)
+    d = m.to_dict()
+    assert set(d.keys()) == {'retry_tail_p99', 'band_width', 'delta_band_width'}
+    assert d['retry_tail_p99'] == 100.0
+    assert d['band_width'] == 40.0
+    assert d['delta_band_width'] == -2.0
+
+
+def test_invalid_input_handling():
+    # Missing keys should raise KeyError or ValueError
+    bad_40 = {'retry_tail_p99': 50.0}
+    bad_42 = {'band_width': 10.0}
+    with pytest.raises((KeyError, ValueError, TypeError)):
+        core.analyze_metrics(bad_40, bad_42)
+
+
+def test_type_integrity(card_data_40, card_data_42):
+    result = core.analyze_metrics(card_data_40, card_data_42)
+    d = result.to_dict()
+    assert all(isinstance(v, (float, int)) for v in d.values())