diff --git a/unknown_case_counter/tests/test_core.py b/unknown_case_counter/tests/test_core.py
new file mode 100644
index 0000000..0488fa4
--- /dev/null
+++ b/unknown_case_counter/tests/test_core.py
@@ -0,0 +1,74 @@
+import csv
+import io
+import json
+from pathlib import Path
+import pytest
+
+from src.unknown_case_counter import core
+
+@pytest.fixture
+def sample_csv(tmp_path: Path) -> Path:
+    data = [
+        ["case_id", "previous_status", "new_status", "unknown_reason"],
+        ["1", "PASS", "Unknown", "timeout"],
+        ["2", "FAIL", "Unknown", "schema_error"],
+        ["3", "FAIL", "PASS", ""],
+        ["4", "PASS", "Unknown", "timeout"],
+        ["5", "Unknown", "Unknown", "corrupt_data"],
+        ["6", "PASS", "FAIL", ""],
+    ]
+    file_path = tmp_path / "delta_cases.csv"
+    with open(file_path, "w", newline="", encoding="utf-8") as f:
+        writer = csv.writer(f)
+        writer.writerows(data)
+    return file_path
+
+
+def test_count_unknown_reasons_basic(sample_csv):
+    result = core.count_unknown_reasons(str(sample_csv))
+    assert isinstance(result, dict)
+    expected_keys = {"timeout", "schema_error", "corrupt_data"}
+    assert expected_keys == set(result.keys())
+    assert result["timeout"] == 2
+    assert result["schema_error"] == 1
+    assert result["corrupt_data"] == 1
+
+
+def test_empty_file(tmp_path):
+    file_path = tmp_path / "empty.csv"
+    with open(file_path, "w", encoding="utf-8") as f:
+        f.write("case_id,previous_status,new_status,unknown_reason\n")
+    result = core.count_unknown_reasons(str(file_path))
+    assert result == {}
+
+
+def test_no_unknown_rows(tmp_path):
+    data = [
+        ["case_id", "previous_status", "new_status", "unknown_reason"],
+        ["1", "PASS", "FAIL", ""],
+        ["2", "FAIL", "PASS", ""],
+    ]
+    file_path = tmp_path / "cases.csv"
+    with open(file_path, "w", newline="", encoding="utf-8") as f:
+        writer = csv.writer(f)
+        writer.writerows(data)
+    result = core.count_unknown_reasons(str(file_path))
+    assert result == {}
+
+
+def test_invalid_file_path_raises(tmp_path):
+    invalid = tmp_path / "not_exists.csv"
+    with pytest.raises(FileNotFoundError):
+        core.count_unknown_reasons(str(invalid))
+
+
+def test_invalid_data_missing_columns(tmp_path):
+    file_path = tmp_path / "bad.csv"
+    with open(file_path, "w", newline="", encoding="utf-8") as f:
+        f.write("case_id,new_status\n1,Unknown\n")
+    # Should handle missing columns gracefully, returning empty dict or raising ValueError
+    try:
+        result = core.count_unknown_reasons(str(file_path))
+        assert isinstance(result, dict)
+    except ValueError:
+        pytest.skip("Function validates input strictly and raises on missing columns.")
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