diff --git a/ci_report_hook/tests/test_core.py b/ci_report_hook/tests/test_core.py
new file mode 100644
index 0000000..14dfe85
--- /dev/null
+++ b/ci_report_hook/tests/test_core.py
@@ -0,0 +1,104 @@
+import json
+import os
+from pathlib import Path
+from datetime import datetime
+import pytest
+
+import src.ci_report_hook.core as core
+
+
+def make_sample_report(tmp_path: Path):
+    run_id = "RUN123"
+    path = tmp_path / f"{run_id}.json"
+    report = core.CIReport(
+        timestamp=datetime.utcnow().isoformat(),
+        run_id=run_id,
+        decision="PASS",
+        margin=0.95,
+        flaky_flag=False,
+        subset_flip_count=1,
+        mischfenster_p95=0.87,
+    )
+    return report, path
+
+
+def test_cireport_to_json_types(tmp_path):
+    report, _ = make_sample_report(tmp_path)
+    data = report.to_json()
+    expected_fields = [
+        "timestamp",
+        "run_id",
+        "decision",
+        "margin",
+        "flaky_flag",
+        "subset_flip_count",
+        "mischfenster_p95",
+    ]
+    for field in expected_fields:
+        assert field in data, f"Missing field {field}"
+
+    assert isinstance(data["timestamp"], str)
+    assert isinstance(data["run_id"], str)
+    assert data["decision"] in {"PASS", "WARN", "FAIL"}
+    assert isinstance(data["margin"], float)
+    assert isinstance(data["flaky_flag"], bool)
+    assert isinstance(data["subset_flip_count"], int)
+    assert isinstance(data["mischfenster_p95"], float)
+
+
+def test_record_ci_run_creates_valid_json(tmp_path, monkeypatch):
+    # prepare directories
+    reports_dir = tmp_path / "reports"
+    reports_dir.mkdir()
+    monkeypatch.chdir(tmp_path)
+
+    run_id = "RUN456"
+    core.record_ci_run(
+        run_id=run_id,
+        decision="WARN",
+        margin=0.12,
+        flaky_flag=True,
+        subset_flip_count=2,
+        mischfenster_p95=0.83,
+    )
+
+    target = reports_dir / f"{run_id}.json"
+    assert target.exists(), "Report file should be created."
+
+    with target.open("r", encoding="utf-8") as f:
+        data = json.load(f)
+
+    # simple field checks
+    assert data["run_id"] == run_id
+    assert data["decision"] == "WARN"
+    assert pytest.approx(data["margin"], 0.12)
+    assert data["flaky_flag"] is True
+    assert data["subset_flip_count"] == 2
+    assert pytest.approx(data["mischfenster_p95"], 0.83)
+    # timestamp field sanity
+    assert isinstance(data["timestamp"], str) and len(data["timestamp"]) >= 19
+
+
+def test_record_ci_run_overwrites_existing(tmp_path, monkeypatch):
+    reports_dir = tmp_path / "reports"
+    reports_dir.mkdir()
+    monkeypatch.chdir(tmp_path)
+
+    run_id = "RUN789"
+    path = reports_dir / f"{run_id}.json"
+    path.write_text(json.dumps({"dummy": 1}))
+
+    core.record_ci_run(
+        run_id=run_id,
+        decision="FAIL",
+        margin=0.0,
+        flaky_flag=False,
+        subset_flip_count=0,
+        mischfenster_p95=0.0,
+    )
+
+    with open(path, "r", encoding="utf-8") as f:
+        data = json.load(f)
+
+    assert data["decision"] == "FAIL"
+    assert "dummy" not in data
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