diff --git a/retry_mechanism/tests/test_core.py b/retry_mechanism/tests/test_core.py
new file mode 100644
index 0000000..aca9a43
--- /dev/null
+++ b/retry_mechanism/tests/test_core.py
@@ -0,0 +1,55 @@
+import pytest
+import time
+from types import SimpleNamespace
+
+from src.retry_mechanism import core
+
+
+@pytest.fixture(autouse=True)
+def patch_sleep(monkeypatch):
+    # Patch time.sleep to avoid delay
+    monkeypatch.setattr(time, 'sleep', lambda s: None)
+
+
+def test_retry_if_negative_dt_no_retry():
+    dt = 0.5
+    result = core.retry_if_negative_dt(dt)
+    assert isinstance(result, float)
+    assert result == pytest.approx(0.5)
+
+
+def test_retry_if_negative_dt_with_retry(monkeypatch):
+    calls = SimpleNamespace(count=0)
+
+    def fake_measure():
+        calls.count += 1
+        # First call negative, second call positive
+        return -1.0 if calls.count == 1 else 0.2
+
+    monkeypatch.setattr(core, '_measure_dt', fake_measure)
+
+    val = core.retry_if_negative_dt(-0.5)
+    assert isinstance(val, float)
+    assert val >= 0.0
+    assert calls.count == 2
+
+
+def test_retry_if_negative_dt_second_negative(monkeypatch):
+    calls = SimpleNamespace(count=0)
+
+    def always_negative():
+        calls.count += 1
+        return -0.5
+
+    monkeypatch.setattr(core, '_measure_dt', always_negative)
+
+    val = core.retry_if_negative_dt(-1.0)
+    assert isinstance(val, float)
+    # even if still negative after retry, returns that value
+    assert val < 0
+    assert calls.count == 2
+
+
+def test_input_type_validation():
+    with pytest.raises(TypeError):
+        _ = core.retry_if_negative_dt('invalid')