diff --git a/rollup_rollout/tests/test_core.py b/rollup_rollout/tests/test_core.py
new file mode 100644
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+++ b/rollup_rollout/tests/test_core.py
@@ -0,0 +1,73 @@
+import json
+import csv
+import io
+import os
+from pathlib import Path
+import pytest
+
+import src.rollup_rollout.core as core
+
+@pytest.fixture
+def tmp_json_file(tmp_path):
+    data = [
+        {
+            "policy_hash": "abc123",
+            "outcome": "PASS",
+            "unknown_rate": 0.0,
+            "top_reasons": "[]"
+        },
+        {
+            "policy_hash": "def456",
+            "outcome": "FAIL",
+            "unknown_rate": 0.25,
+            "top_reasons": "[\"timeout\"]"
+        }
+    ]
+    path = tmp_path / "gate_result.json"
+    with open(path, 'w', encoding='utf-8') as f:
+        json.dump(data, f)
+    return path
+
+
+def test_generate_rollout_series_creates_csv(tmp_json_file, tmp_path):
+    output_path = tmp_path / "rollout_series.csv"
+    core.generate_rollout_series(str(tmp_json_file), str(output_path))
+
+    assert output_path.exists(), "Output CSV file should be created."
+    content = output_path.read_text(encoding='utf-8').strip().splitlines()
+    assert content[0].split(',') == ["policy_hash", "outcome", "unknown_rate", "top_reasons"]
+    assert any("abc123" in line for line in content)
+    assert any("def456" in line for line in content)
+
+
+def test_generate_rollout_series_invalid_json(tmp_path):
+    bad_json_file = tmp_path / "bad.json"
+    bad_json_file.write_text('{invalid json}', encoding='utf-8')
+    output_path = tmp_path / "rollout.csv"
+
+    with pytest.raises(Exception):
+        core.generate_rollout_series(str(bad_json_file), str(output_path))
+
+
+def test_generate_rollout_series_empty_input(tmp_path):
+    empty_json = tmp_path / "empty.json"
+    with open(empty_json, 'w', encoding='utf-8') as f:
+        json.dump([], f)
+    output_csv = tmp_path / "out.csv"
+
+    core.generate_rollout_series(str(empty_json), str(output_csv))
+    lines = output_csv.read_text(encoding='utf-8').splitlines()
+    assert len(lines) == 1, "Only header should exist for empty input."
+
+
+def test_rolloutdata_fields():
+    data = core.RolloutData(
+        policy_hash="xyz000",
+        outcome="WARN",
+        unknown_rate=0.1,
+        top_reasons="[\"unstable\"]"
+    )
+    assert data.policy_hash == "xyz000"
+    assert isinstance(data.outcome, str)
+    assert isinstance(data.unknown_rate, float)
+    assert "unstable" in data.top_reasons
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