diff --git a/report_generation/tests/test_core.py b/report_generation/tests/test_core.py
new file mode 100644
index 0000000..7a78f14
--- /dev/null
+++ b/report_generation/tests/test_core.py
@@ -0,0 +1,75 @@
+import json
+import os
+from pathlib import Path
+import pandas as pd
+import pytest
+
+import src.report_generation.core as core
+
+@pytest.fixture
+def sample_analysis_results():
+    data = [
+        {
+            "run_id": "run_34",
+            "step_id": 1,
+            "epoch_ms": 1700000000000,
+            "monotonic_ns": 100000000,
+            "tz_offset_minutes": 0,
+            "retry_tail_p99": 12.5,
+            "band_width": 100.0
+        },
+        {
+            "run_id": "run_35",
+            "step_id": 1,
+            "epoch_ms": 1700000001000,
+            "monotonic_ns": 200000000,
+            "tz_offset_minutes": 0,
+            "retry_tail_p99": 10.0,
+            "band_width": 110.0
+        },
+        {
+            "run_id": "run_36",
+            "step_id": 1,
+            "epoch_ms": 1700000002000,
+            "monotonic_ns": 300000000,
+            "tz_offset_minutes": 0,
+            "retry_tail_p99": 11.0,
+            "band_width": 95.0
+        }
+    ]
+    return data
+
+
+def test_generate_report_creates_json_file(tmp_path, sample_analysis_results):
+    output_file = tmp_path / "stability_report.json"
+
+    df = pd.DataFrame(sample_analysis_results)
+    result_path = core.generate_report(df)
+
+    assert isinstance(result_path, str), "Return value must be a string path."
+    assert os.path.exists(result_path), "Output file should be created."
+    with open(result_path, 'r', encoding='utf-8') as f:
+        report = json.load(f)
+
+    assert isinstance(report, dict), "Report must be a dictionary."
+    assert set(["config_summary", "findings", "metric_comparison", "timestamp"]).issubset(report.keys())
+    assert isinstance(report["metric_comparison"], dict)
+    assert isinstance(report["findings"], str)
+
+
+def test_generate_report_handles_dict_input(tmp_path, sample_analysis_results):
+    data_dict = {"results": sample_analysis_results}
+    result_path = core.generate_report(data_dict)
+
+    assert isinstance(result_path, str)
+    assert Path(result_path).exists()
+
+    with open(result_path, 'r', encoding='utf-8') as f:
+        parsed = json.load(f)
+
+    assert "metric_comparison" in parsed
+
+
+def test_generate_report_invalid_input_type():
+    with pytest.raises((TypeError, ValueError)):
+        core.generate_report("invalid_input")
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