diff --git a/artifact_3_retry_tail_analysis/tests/test_core.py b/artifact_3_retry_tail_analysis/tests/test_core.py
new file mode 100644
index 0000000..6fe07bd
--- /dev/null
+++ b/artifact_3_retry_tail_analysis/tests/test_core.py
@@ -0,0 +1,49 @@
+import pytest
+from artifact_3_retry_tail_analysis import core
+
+
+def test_analyze_retry_tails_basic_case():
+    data_enforced = {"retry_tail_p99": 120.0}
+    data_randomized = {"retry_tail_p99": 150.0}
+
+    result = core.analyze_retry_tails(data_enforced, data_randomized)
+
+    assert isinstance(result, dict)
+    assert "retry_tail_p99" in result
+    assert "difference" in result
+    assert result["retry_tail_p99"] == 120.0
+    assert pytest.approx(result["difference"]) == -30.0
+
+
+def test_analyze_retry_tails_swapped_values():
+    data_enforced = {"retry_tail_p99": 200.0}
+    data_randomized = {"retry_tail_p99": 100.0}
+
+    result = core.analyze_retry_tails(data_enforced, data_randomized)
+
+    assert pytest.approx(result["difference"]) == 100.0
+    assert result["retry_tail_p99"] == 200.0
+
+
+def test_analyze_retry_tails_input_validation():
+    # Missing required field
+    with pytest.raises((KeyError, ValueError, AssertionError)):
+        core.analyze_retry_tails({}, {"retry_tail_p99": 10.0})
+
+    # Invalid data type
+    with pytest.raises((TypeError, AssertionError)):
+        core.analyze_retry_tails({"retry_tail_p99": "not_a_number"}, {"retry_tail_p99": 10.0})
+
+
+def test_analyze_retry_tails_edges():
+    # Equal values => difference = 0.0
+    data_enforced = {"retry_tail_p99": 100.0}
+    data_randomized = {"retry_tail_p99": 100.0}
+    result = core.analyze_retry_tails(data_enforced, data_randomized)
+    assert pytest.approx(result["difference"]) == 0.0
+
+    # Negative values (simulated edge)
+    data_enforced = {"retry_tail_p99": -50.0}
+    data_randomized = {"retry_tail_p99": -30.0}
+    result = core.analyze_retry_tails(data_enforced, data_randomized)
+    assert pytest.approx(result["difference"]) == -20.0
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