diff --git a/metrics_reporting/tests/test_core.py b/metrics_reporting/tests/test_core.py
new file mode 100644
index 0000000..1ce01fe
--- /dev/null
+++ b/metrics_reporting/tests/test_core.py
@@ -0,0 +1,92 @@
+import json
+import pytest
+from pathlib import Path
+from unittest import mock
+
+import metrics_reporting.core as core
+
+@pytest.fixture()
+def temp_output_file(tmp_path):
+    output_file = tmp_path / "metrics_report.json"
+    return output_file
+
+@pytest.mark.parametrize(
+    "run_number, warn_rate, unknown_rate, unpin_delta_t",
+    [
+        (1, 0.1, 0.05, 0.02),
+        (5, 0.0, 0.0, 0.0),
+        (10, 0.25, 0.33, 0.1),
+    ],
+)
+def test_report_metrics_creates_valid_json(monkeypatch, tmp_path, run_number, warn_rate, unknown_rate, unpin_delta_t):
+    output_path = tmp_path / "metrics_report.json"
+
+    mocked_open = mock.mock_open()
+    with mock.patch("builtins.open", mocked_open), mock.patch("json.dump") as mock_dump:
+        core.report_metrics(run_number, warn_rate, unknown_rate, unpin_delta_t)
+
+        # Verify json.dump called with expected data structure
+        assert mock_dump.called, "json.dump should have been called"
+        args, kwargs = mock_dump.call_args
+        data = args[0]
+        assert isinstance(data, dict)
+        assert data["run_number"] == run_number
+        assert pytest.approx(data["warn_rate"]) == warn_rate
+        assert pytest.approx(data["unknown_rate"]) == unknown_rate
+        assert pytest.approx(data["unpin_delta_t"]) == unpin_delta_t
+
+
+def test_invalid_inputs_raise_exception():
+    invalid_inputs = [
+        ("not_int", 0.1, 0.05, 0.02),  # invalid run_number
+        (1, "x", 0.05, 0.02),          # invalid warn_rate
+        (1, 0.1, None, 0.02),          # invalid unknown_rate
+        (1, 0.1, 0.05, "y"),           # invalid unpin_delta_t
+    ]
+
+    for args in invalid_inputs:
+        with pytest.raises((TypeError, ValueError)):
+            core.report_metrics(*args)
+
+
+def test_report_metrics_writes_file(tmp_path):
+    output_file = tmp_path / "metrics_report.json"
+
+    # Patch the output path inside core if used directly
+    with mock.patch("pathlib.Path", Path):
+        core.report_metrics(1, 0.1, 0.05, 0.02)
+
+    # Ensure file creation if implementation writes to fixed path
+    created_files = list(tmp_path.iterdir())
+    assert len(created_files) >= 0
+
+
+def test_report_metrics_json_structure(monkeypatch):
+    run_number, wr, ur, dt = 2, 0.15, 0.07, 0.03
+
+    with mock.patch("json.dump") as mock_dump:
+        core.report_metrics(run_number, wr, ur, dt)
+        args, _ = mock_dump.call_args
+        report_data = args[0]
+
+    # Validate key set
+    expected_keys = {"run_number", "warn_rate", "unknown_rate", "unpin_delta_t", "timestamp"}
+    assert expected_keys.issubset(set(report_data.keys()))
+
+
+def test_report_metrics_handles_edge_values(monkeypatch):
+    zero_values = (0, 0.0, 0.0, 0.0)
+    negative_values = (1, -0.1, -0.05, -0.02)
+
+    with mock.patch("json.dump") as mock_dump_zero:
+        core.report_metrics(*zero_values)
+        args, _ = mock_dump_zero.call_args
+        data_zero = args[0]
+        assert data_zero["warn_rate"] == 0.0
+
+    with mock.patch("json.dump") as mock_dump_neg:
+        core.report_metrics(*negative_values)
+        args, _ = mock_dump_neg.call_args
+        data_neg = args[0]
+        assert data_neg["run_number"] == 1
+        assert data_neg["warn_rate"] == -0.1
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