diff --git a/tests/test_youtube_js_runtime.py b/tests/test_youtube_js_runtime.py
new file mode 100644
index 00000000..b5336ca3
--- /dev/null
+++ b/tests/test_youtube_js_runtime.py
@@ -0,0 +1,44 @@
+"""Seam tests for the yt-dlp JS-runtime startup check.
+
+YouTube gates downloadable formats behind JS challenges; without Deno on PATH
+every stream / music-video download fails with the cryptic "Requested format
+is not available". The check must say so plainly in the log — once — and stay
+silent when deno is present.
+"""
+
+from __future__ import annotations
+
+import core.youtube_client as yc
+
+
+def _reset():
+    yc._JS_RUNTIME_WARNED = False
+
+
+def _capture_warnings(monkeypatch):
+    calls = []
+    monkeypatch.setattr(yc.logger, 'warning', lambda msg, *a: calls.append(msg % a if a else msg))
+    return calls
+
+
+def test_warns_once_when_deno_missing(monkeypatch):
+    _reset()
+    warnings = _capture_warnings(monkeypatch)
+    monkeypatch.setattr('shutil.which', lambda name: None)
+
+    yc._warn_if_no_js_runtime()
+    yc._warn_if_no_js_runtime()  # second call must not duplicate
+
+    assert len(warnings) == 1
+    assert 'Requested format is not available' in warnings[0]
+    assert 'Deno' in warnings[0] or 'deno' in warnings[0]
+
+
+def test_silent_when_deno_present(monkeypatch):
+    _reset()
+    warnings = _capture_warnings(monkeypatch)
+    monkeypatch.setattr('shutil.which', lambda name: '/usr/local/bin/deno' if name == 'deno' else None)
+
+    yc._warn_if_no_js_runtime()
+
+    assert warnings == []