diff --git a/trace_agg/tests/test_core.py b/trace_agg/tests/test_core.py
new file mode 100644
index 0000000..4898eba
--- /dev/null
+++ b/trace_agg/tests/test_core.py
@@ -0,0 +1,63 @@
+import json
+import pytest
+from pathlib import Path
+
+import src.trace_agg.core as core
+
+# --- Fixtures ---
+@pytest.fixture
+def sample_trace_data(tmp_path):
+    data = {
+        "correlation_id": "corr01",
+        "events": [
+            {"had_sched_switch": 1, "had_irq": 0, "had_softirq": 0, "cpu_ids_seen": [0]},
+            {"had_sched_switch": 0, "had_irq": 1, "had_softirq": 0, "cpu_ids_seen": [1]},
+            {"had_sched_switch": 0, "had_irq": 0, "had_softirq": 1, "cpu_ids_seen": [2]}
+        ]
+    }
+    sample_path = tmp_path / "sample_trace.json"
+    with open(sample_path, "w", encoding="utf-8") as fh:
+        json.dump(data, fh)
+    return sample_path
+
+# --- Tests ---
+def test_aggregate_trace_data_nominal(sample_trace_data, monkeypatch):
+    def mock_open_file(path: str):
+        with open(path, "r", encoding="utf-8") as fh:
+            return json.load(fh)
+
+    monkeypatch.setattr(core, "open", open)
+    result = core.aggregate_trace_data("corr01")
+    assert isinstance(result, dict)
+    assert "had_sched_switch" in result
+    assert "had_irq" in result
+    assert "had_softirq" in result
+    assert "cpu_ids_seen" in result
+    # Ensure aggregation logic makes sense
+    assert result["had_sched_switch"] in (0, 1)
+    assert result["had_irq"] in (0, 1)
+    assert result["had_softirq"] in (0, 1)
+    assert all(isinstance(cpu, int) for cpu in result["cpu_ids_seen"])
+
+
+def test_aggregate_trace_data_edge_case_empty(monkeypatch):
+    def mock_open_file(path: str):
+        return {"correlation_id": "empty_case", "events": []}
+
+    monkeypatch.setattr(core, "open", lambda path, *_args, **_kwargs: open(__file__, "r"))  # Dummy but unused
+
+    # Monkeypatch json.load since we're not actually reading
+    monkeypatch.setattr(json, "load", lambda fh: {"correlation_id": "empty_case", "events": []})
+
+    result = core.aggregate_trace_data("empty_case")
+    assert result["had_sched_switch"] == 0
+    assert result["had_irq"] == 0
+    assert result["had_softirq"] == 0
+    assert result["cpu_ids_seen"] == []
+
+
+def test_aggregate_trace_data_invalid_input(monkeypatch):
+    monkeypatch.setattr(json, "load", lambda fh: {"correlation_id": "bad", "events": [{"had_sched_switch": "wrong"}]})
+
+    with pytest.raises((ValueError, TypeError)):
+        core.aggregate_trace_data("bad")
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