diff --git a/tests/test_app.py b/tests/test_app.py
new file mode 100644
index 0000000..ad31adb
--- /dev/null
+++ b/tests/test_app.py
@@ -0,0 +1,134 @@
+"""Smoke tests for the Nepali Calendar API.
+
+These tests boot the FastAPI app against the live data/ tree and exercise
+the public endpoints plus input validation. They are not network-dependent
+and finish in well under a second.
+"""
+
+from unittest.mock import patch
+
+import pytest
+from fastapi.testclient import TestClient
+
+from app import app
+
+client = TestClient(app)
+
+
+def test_root():
+    resp = client.get("/")
+    assert resp.status_code == 200
+    body = resp.json()
+    assert "message" in body
+    assert isinstance(body["years_available"], int)
+
+
+def test_health():
+    resp = client.get("/health")
+    assert resp.status_code == 200
+    body = resp.json()
+    assert body["status"] == "ok"
+    assert body["years"] >= 0
+
+
+def test_get_monthly_calendar():
+    year, month = 2082, 1
+    resp = client.get(f"/calendar/{year}/{month}")
+    if resp.status_code == 200:
+        body = resp.json()
+        assert "metadata" in body
+        assert "days" in body
+        assert isinstance(body["days"], list)
+    else:
+        assert resp.status_code == 404
+
+
+def test_invalid_month_returns_422():
+    # FastAPI/Pydantic surfaces invalid path params as 422.
+    resp = client.get("/calendar/2082/13")
+    assert resp.status_code == 422
+
+
+def test_invalid_month_zero_returns_422():
+    resp = client.get("/calendar/2082/0")
+    assert resp.status_code == 422
+
+
+def test_invalid_year_too_low_returns_422():
+    resp = client.get("/calendar/1500")
+    assert resp.status_code == 422
+
+
+def test_unknown_year_returns_404():
+    # An in-range year that simply has no data should give 404, not 422.
+    resp = client.get("/calendar/2150")
+    assert resp.status_code == 404
+
+
+def test_etag_is_served_and_respected():
+    sample_year = 2082
+    first = client.get(f"/calendar/{sample_year}/1")
+    if first.status_code != 200:
+        return
+    etag = first.headers.get("ETag")
+    assert etag is not None
+    second = client.get(f"/calendar/{sample_year}/1", headers={"If-None-Match": etag})
+    assert second.status_code == 304
+
+
+def test_file_lock_serialises_writers(tmp_path):
+    """Two writers taking the same lock must run sequentially, not interleaved."""
+    from scraper import _file_lock
+
+    log_path = tmp_path / "order.log"
+    lock = tmp_path / "x.lock"
+
+    def _writer(tag: str, hold_ms: int) -> None:
+        with _file_lock(lock):
+            with log_path.open("a") as f:
+                f.write(f"{tag}:enter\n")
+                f.flush()
+            import time as _t
+
+            _t.sleep(hold_ms / 1000)
+            with log_path.open("a") as f:
+                f.write(f"{tag}:exit\n")
+
+    import threading
+
+    t1 = threading.Thread(target=_writer, args=("A", 200))
+    t2 = threading.Thread(target=_writer, args=("B", 50))
+    t1.start()
+    t2.start()
+    t1.join()
+    t2.join()
+
+    entries = log_path.read_text().splitlines()
+    # The only legal orderings are A-enter, A-exit, B-enter, B-exit OR
+    # B-enter, B-exit, A-enter, A-exit. Anything interleaved means the
+    # lock did not actually serialise.
+    assert entries in (
+        ["A:enter", "A:exit", "B:enter", "B:exit"],
+        ["B:enter", "B:exit", "A:enter", "A:exit"],
+    ), f"writers interleaved: {entries}"
+
+
+def test_atomic_write_is_visible_after_crash(tmp_path):
+    """A writer that raises must not leave a half-written file behind."""
+    from scraper import _atomic_write_json
+
+    target = tmp_path / "x.json"
+    try:
+        with (
+            pytest.raises(RuntimeError, match="boom"),
+            patch("scraper.orjson.dumps", side_effect=RuntimeError("boom")),
+        ):
+            _atomic_write_json(target, {"a": 1})
+    finally:
+        # Patch is module-level; restore the real orjson.dumps to avoid
+        # leaking the side effect into the rest of the test session.
+        pass
+
+    assert not target.exists(), f"partial file left behind: {target.read_bytes()!r}"
+    leftovers = list(tmp_path.glob(".x.json.*.tmp"))
+    assert leftovers == [], f"temp files leaked: {leftovers}"