diff --git a/tests/test_video_side_shell.py b/tests/test_video_side_shell.py
new file mode 100644
index 00000000..6bb8d491
--- /dev/null
+++ b/tests/test_video_side_shell.py
@@ -0,0 +1,86 @@
+"""Video side shell: the Music ↔ Video toggle + video sidebar (experimental branch).
+
+This is the first slice of the video side. It must be ADDITIVE and ISOLATED:
+the music sidebar/nav is untouched, the toggle + video nav are wired purely via
+data-attributes (no inline onclick — which would also break the script-split
+integrity contract), and the controller is a self-contained IIFE that adds no
+globals. These pin all of that so a regression can't silently couple the two
+sides or break the music shell.
+"""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+_ROOT = Path(__file__).resolve().parent.parent
+_INDEX = (_ROOT / "webui" / "index.html").read_text(encoding="utf-8", errors="replace")
+_JS = (_ROOT / "webui" / "static" / "video" / "video-side.js").read_text(encoding="utf-8")
+_CSS_PATH = _ROOT / "webui" / "static" / "video" / "video-side.css"
+
+EXPECTED_VIDEO_PAGES = {
+ "video-dashboard", "video-search", "video-discover", "video-library",
+ "video-calendar", "video-import", "video-settings", "video-issues", "video-help",
+}
+
+
+def _block(html: str, open_tag_re: str, close_tag: str) -> str:
+ """Return the substring from the first match of open_tag_re to the next close_tag."""
+ m = re.search(open_tag_re, html)
+ assert m, f"could not find {open_tag_re!r}"
+ end = html.index(close_tag, m.end())
+ return html[m.start():end + len(close_tag)]
+
+
+# --- the toggle ------------------------------------------------------------
+
+def test_side_toggle_has_music_and_video():
+ block = _block(_INDEX, r'
")
+ assert 'data-side-target="music"' in block
+ assert 'data-side-target="video"' in block
+ # Wired by JS, not inline handlers (isolation + integrity contract).
+ assert "onclick" not in block
+
+
+# --- the video nav ---------------------------------------------------------
+
+def test_video_nav_has_all_expected_pages():
+ block = _block(_INDEX, r'